Layout 1 INTRODUCTION The study of mucus secretion by cnidarians has se- duced scientists for nearly a century (Brown and Bythell, 2005). Mucus is produced to a greater or lesser extent by almost all cnidarians, in particular Anthozoans and Scyphozoans, and is essential for numerous life processes such as heterotrophic nutrition and sediment cleansing and as a defense against various environmental stressors and pathogens (Parisi et al., 2020). Much of the literature has examined the functions and composition of mucus in a limited number of cnidarians, investigating the trophic significance of mucus production, especially in coral reef species, but neglecting fundamental aspects of its ecolog- ical, physiological, and evolutionary role in a multitude of organisms belonging to this Phylum (Bythell and Wild, 2011; Myldlarz et al., 2016; Bakshani et al., 2018; Palmer and Traylor-Knowles, 2018). Furthermore, the prevalence of issues related to climate change has redirected the re- search on implications related to this phenomenon, reduc- ing the interest in the secretion of mucus in sensu stricto as a process underlying the adaptation and evolution of this ancient Phylum. Indeed, mucus is involved in several biological processes essential for almost all aquatic metazoan acting as the first contact stratum with surrounding media from Cnidaria to Chordata (Stabili et al., 2015; Alesci et al., 2022). The mucus layers provide a multifunctional and pro- tective hydrogel interface between the organisms’ epithelial cells and the external environment (Cabillon and Lazado, 2019; Kramar et al., 2019). Aquatic invertebrates and fish use a layer of mucus to protect body surfaces, gills, and in- testines. It effectively acts as a dynamic physical and bio- chemical barrier, showing numerous biological and ecological roles such as protection against abrasion, toxins, environmental pollutants and pathogens, and osmoregula- tion, chemical communication, parental nutrition and many others (Hanaoka et al., 2001; Ames et al., 2020). Mucus has exceptional properties including elasticity, changing rheology and the ability to self-repair, thus representing an ideal means of trapping and immobilizing pathogens (Brown and Bythell, 2005). It is a key feature of the innate immune system in most aquatic and terrestrial metazoic phyla, playing a vital role in the prevention of microbial disease. In addition, while acting as a protective barrier, mucus allows the exchange of oxygen, carbon dioxide and nutrients and metabolites, lubricating the surfaces, reducing damage due to lesions, reducing dehydration of the epithe- lia and providing the polymer matrix which allows the transport of ciliary mucus particles (Reverter et al., 2018; Alesci et al., 2022). The properties of the secreted mucus generally depend on its composition consisting of exudates, such lipids, and mucins, polymeric glycoproteins that give the mucus its ability to gel, both released by mucocytes of REVIEW Mucus secretions in Cnidarian, an ecological, adaptive and evolutive tool Serena Savoca1, Dario Di Fresco2, Alessio Alesci2, Gioele Capillo*3, Nunziacarla Spanò1 1Department of Biomedical, Dental and Morphological and Functional Imaging, University of Messina, Messina, Italy; 2Department of Chemical, Biological, Pharmaceutical and Environmental Sciences, University of Messina, Messina, Italy; 3Department of Veterinary Sciences, University of Messina, Messina, Italy ABSTRACT Mucus secretion provides an interface with unique and multifunctional properties between the epithelial cells of many aquatic organisms and their surrounding environment. Indeed, mucus is involved in various essential biological processes including feeding, reproduction, osmoregulation, competition for space, defense against pathogens, xenobiotics, and a multitude of environmental stressors. The ability to produce a functional mucus layer is an important evolutionary step, arising first in Cnidaria that allowed for the development of the mucus-lined digestive cavity seen in higher metazoans. Mucus secretion by cnidarians has been mod- erately investigated in both corals and jellyfish, which among cnidarians are the ones that have shown the highest secretion rates to date. However, although in corals the production of mucus has received more attention, especially in view of the important eco- logical role played in coral reefs, in medusozoans the topic is little considered. Although the mucus secreted by corals has innu- merable and important immunological, nutritional, and protective responsibilities, it should be remembered that jellyfish too represent a fundamental component of marine trophic web, playing numerous and important roles that are still unclear today. What is certain is that jellyfish are characterized (especially in the era of climate change) by large fluctuations in population density, the ecological implications of which are poorly understood. However, in both cases (Medusozoans and Anthozoans) to date some as- pects relating to mucous secretions seem completely obscure, such as the microbiome and its variations as a function of environ- mental conditions or ontogenetic development, its implications in the field of immunological ecology, the consequent energy costs and finally the role played by the mucus in evolutionary terms. This review summarizes the properties, functions, ecological im- plications and evolutionary importance of mucus, in cnidarians, mainly focusing its roles in corals and jellyfish. Understanding these aspects relating to the ecological and evolutionary importance played by mucus is of fundamental importance for the ecosys- tems functioning. Non -co mmerc ial us e o nly S. Savoca et al.72 the epithelium (Brown and Bythell, 2005). Aging and some physical factors, such as depth and irradiance levels (Cross- land, 1987; Zamzow, 2007), may influence the composition of the mucus, varying temporally within the same species and among different ones (Crossland et al., 1980; Meikle et al. 1988; Ferrier-Pages et al., 1998; Hadaidi, 2019). It would seem that the main biophysical characteristics of mucus, like the ability to form gels of different viscosity and elasticity that lubricate and protect the underlying ep- ithelia, depend on the type of mucins expressed (Bythell and Wild 2011). Many hard and soft corals, for instance, continually release mucus, with a species-specific compo- sition. Mucus produced by cnidarians, similarly to other phyla, represents a fundamental aspect of the biology of these or- ganisms and it is essential for several biological functions, including feeding, defending against pathogens, xenobi- otics, and a multitude of environmental stressors, warding off aggression, and acting as an attacking weapon (Baier et al., 1985; Rivera-Ortega and Thomè, 2018; Camacho- Pacheco et al., 2022;). Although continuous mucus releas- ing is physiological, several studies demonstrated that some environmental stressors, such as high particle concentra- tions or high sediment loads, and exposition to air due to low tide conditions, can increase mucus production (Schuh- macher, 1977; Rublee et al., 1980; Krupp, 1984; Wild et al., 2004; Wild et al., 2005; Liu et al., 2018;). Additionally, there is limited information in the litera- ture on mucus release as a consequence of ocean acidifica- tion and global warming (Bythell and Wild, 2011); however, during coral bleaching, caused by rising temper- atures, an increase in mucus production has been detected (Niggl et al., 2009; Fransolet et al., 2012), possibly result- ing in a mucus composition changes (Wooldridge, 2009). Anthozoan mucus has been studied for its biophysical characteristics, including rheology (activity under strain forces), and it has been discovered that it contains high mo- lecular weight glycoproteins with characteristics that are similar to those of vertebrate mucins (Jatkar et al., 2010), suggesting a similarity of mucus function between cnidar- ians and chordates (Bythell & Wild 2011). Effectively, the ability to produce a functional mucosa represents an im- portant evolutionary step, which first evolved in Cnidaria and which was conserved into higher organisms, including terrestrial and human mammals. Although cnidarians are widely used as model systems for evolutionary and biolog- ical investigations in metazoans, the importance of the evo- lution of a functional mucus layer in invertebrates is often overlooked (Bakshani et al., 2018). Another fundamental but little explored aspect is rep- resented by the ecological role of mucus in the life strategy of cnidarians, especially in jellyfish (e.g., habitat for micro- bial colonization, involvement in biogeochemical cycles, dispersal of larvae, reproduction, and fertilization) (Pitt et al., 2009; Liu et al., 2018). Zooxanthellae have been found to contribute substantially to the high gross primary pro- duction in coral reefs; however, corals release nearly half of the carbon assimilated by their zooxanthellae in the form of mucus (Tarrant, 2007; Bythell and Wild, 2011;). The re- leased mucus effectively traps organic matter from the water column and transports energy and nutrients to the sediment which acts as a biocatalytic mineralizing filter. In this way the mucus supplies energy to the heterotrophic reef community, thus establishing a recycling loop that supports benthic life, while reducing the loss of energy and nutrients from the reef ecosystem (Wagner et al., 2012; Goldberg, 2018). In this perspective, there is very little information on the ecological role of mucus in jellyfish. For example, up to 7% of the carbon assimilated by jellyfish is released into the environment in the form of mucus (Hansson and Norrman, 1995), which appears to play an important role in the carbon cycle (Hansson and Norrman, 1995; Condon et al., 2011; Tinta et al., 2021). However, jellyfish mucus has received little attention, resulting in a poor understand- ing of mucus excretion rates, composition, and its fate in the ecosystem. In the context of the expected increase in the jellyfish population, it is desirable to include jellyfish in energy flow models and ecosystem studies (Ramondenc et al., 2020). Lack of knowledge of the characteristics and functions of jellyfish mucus makes it more difficult to un- derstand and model its role in the marine ecosystem. Despite the multitude of ecological and physiological roles played by cnidarian mucus, and although some as- pects of mucus secretion, function and composition have been widely investigated in some cnidarians, there are still many gaps in the knowledge of its composition variability, production rate, inter- and intraspecific variability, energetic significance, and ecological and evolutionary roles. The purpose of this review is therefore to examine what is known about cnidarian mucus in sensu stricto, sites and mechanisms of secretion, composition, function and ecological role and fate, providing an “evolutionary key reading”. Cnidarian ecology: general features and evolution Cnidarians occupy a key evolutionary position as basal metazoans and are ecologically important as predators, prey, and structure builders (Goffredo and Dubinsky, 2016; Santos et al., 2016). The cnidarians are collectively an ex- tremely diverse group of highly successful ancient aquatic organisms, which represent animals with incredibly differ- ent morphologies and life-history traits, but united by the presence of a specialized cell type: the cnidocyte (Basso et al., 2019). Molecular data suggest that the first cnidarians appeared about 740 million years ago (Park et al., 2012). In 200 million years the phylum has undergone significant diversification (Park et al., 2012) and today it consists of about 14,866 accepted species (WoRMS, 2022), taxonom- Non -co mmerc ial us e o nly Mucus secretions in cnidarian, an ecological, adaptive and evolutive tool 73 ically divided into five classes Anthozoans, Cubozoans, Hydrozoans, Scyphozoa and Staurozoa, with an estimated number of species of ~11,000, including mainly marine in- vertebrates such as anemones, corals and jellyfish (Daly et al., 2007). Together, cnidarians have a global distribution and can occupy a wide range of both marine and freshwater environments. Cnidarians present two forms, the polyp and the jelly- fish, sessile and planktonic respectively. These are formed by three layers: an internal gastrodermis derived from the endoderm, surrounding the gastrovascular cavity, an inter- mediate matrix called mesoglea, and an external epidermis derived from ectoderm (Lommel et al., 2018; D’Ambra et al., 2021; Zhao et al., 2021). The oral opening of the polyp is surrounded by tentacles, suitable for capturing the prey since heterotrophy is required to adequately supply nutri- ents for some species when photosymbiotic partners do not sufficiently supply the nutritional requirement (Arai, 1997; Goldberg, 2018). Polyps can form colonies or live as soli- tary organisms. Apart from the polyp stage, Cubozoa, Hy- drozoa, Scyphozoa, and Staurozoa have a free-swimming planktonic stage, the jellyfish, which coexists with the polyp stage, but not all Scyphozoa exhibit a polypoid phase, such as Pelagia noctiluca. Cubozoa, Hydrozoa, and Scyphozoa classes have a metagenic life cycle: this consists of a pelagic jellyfish that reproduces sexually and a benthic polyp that reproduces asexually (Lucas et al., 2012). This suggests that they may have separated evolutionarily from Anthozoa since most of them have only the polyp stage. Cnidarians are phylogenetically basal aquatic animals be- longing to Metazoa, with radial symmetry and simple tissue organization. Evolutionarily they represent the first Meta- zoa; some of them can live for several centuries despite continuous exposure to different pathogens during their lifetime (Petralia et al., 2014). Since they possess only an innate immune system to protect themselves against pathogens, it is difficult to understand how cnidarians can achieve such high levels of longevity (Parisi et al., 2020). Phylogenetic analyses have confirmed that Anthozoa ap- peared earlier than the other classes in the evolutionary his- tory of the Metazoa as they have circular DNA, in contrast to Cubozoa, Hydrozoa, and Scyphozoa, which have linear DNA (Technau and Steele, 2011). Due to their key position in evolution, these organisms play an essential ecological role in marine ecosystems and trophic web (Tarrant, 2007). Since Cnidarians represent the most primitive meta- zoans, their evolution is of fundamental importance to study the origin of animal. The phylogeny of Cnidaria has been debated for decades, and still remains unresolved for some taxa (Kayal et al. 2018; McFadden et al., 2022). The origin and evolution of Cnidaria phylum posed the basis for their ecological success but the mechanisms lie at the origins and subsequent diversification of some characters, like symbiosis, colonial body plans, elaborate life histories, and cnidocytes, remain source of debate. However, a today accepted version of Cnidaria phylogeny, based on morphol- ogy, life-history traits, and molecular data, is presented below. It is accepted that the common ancestor of all Cnidaria is represented by a single polyp that lacked a medusoid stage. From this ancestor, around 15,100 species sorted in the following clades: Anthozoa, Endocnidozoa, and Medusozoa. Anthozoa comprise most of the species (~10,130) of Cnidaria sorted in two accepted subclasses Octocorallia and Hexacorallia (see McFadden et al., 2022 for a detailed phylogeny of Anthozoa). Endocnidozoa con- sist of around 798 species of endoparasites (Myxozoa) and the monospecific Polypodiozoa, Polypodium hydriforme Ussow, 1887 (a parasite of the oocytes of acipenseriform fishes (paddlefish and sturgeons). Medusozoa encompass Cubozoa (~48 species), Hydrozoa (~3,875 species), Scyphozoa (~241 species), and Staurozoa (~49 species) (Evans, et al. 2008; Chang et al., 2015; Zapata et al., 2015; Kayal et al., 2017; McFadden et al., 2022). The phylogenetic relationships of Cnidaria are still dis- cussed and in some cases far from being elucidated. Despite this, recent studies using morphological, transcriptome, genome (both mitochondrial and nuclear), single gene, data tried to disentangle the complicated question (Chang et al., 2015; Zapata et al., 2015; Kayal et al., 2017). Despite the divergencies between the studies of Chang ES et al., 2015 and Zapata et al., 2015, large congruence resulted in the evolutionary reconstructions of the cnidarian lineages, highlighting the importance of using large genomic datasets and increased taxon sampling in dealing evolutive ques- tions (Kayal et al., 2018). In this latter study, the authors, providing the most comprehensive taxon sampling of Cnidaria for phylogenomic analysis, supported the mono- phyly of Anthozoan, considering Ceriantharia as the sister group of Hexacoralia; and Zoantharia as the sister group of all other Hexacoralia. This, due to the presence in all the cited taxa of a single siphonoglyph (a ciliated groove lo- cated in the actinopharynx) shading light on the possible bilateral character of the Anthozoa, and obviously of Cnidaria, ancestor already proposed by Finnerty et al., 2004. Antipatharia were not included in the study by Kayal et al., 2018, for the unavailability of genomic data for this group, that still represent a gap in evolutionary useful data for the Cnidaria. For Medusozoa the phylogenetic back- ground is less entangled respect to Anthozoa. In fact, it is accepted that Scyphozoa, Staurozoa, Cubozoa, and Hydro- zoa, composing Medusozoa, clade as two distinct mono- phyletic groups: i) the first, encompassing Scyphozoa, Staurozoa, and Cubozoa, of which the earlier name is Acraspeda in Haeckel, 1880, that used the name with its present sense; and ii) the Hydrozoa composed of mono- phyletic Hydroidolina and Trachylinae. Finally, the sister relationship between Endocnidozoa (Myxozoa and Poly- podiozoa) and Medusozoa, is corroborate by genomic Non -co mmerc ial us e o nly S. Savoca et al.74 analyses too (Siddall et al., 1995; Jimenez-Guri et al., 2007; Collins, 2009; Chang et al., 2015; Okamura et al., 2015; Kayal et al., 2018; Novosolov et al., 2022). Depicting off an evolutionary processes conclusion it seems that all the Cnidaria evolved from an ancestral non- symbiotic, stinging, and medusa-lacking, solitary polyp with a planula larva (Kayal et al., 2018; Khalturin et al., 2019). Symbiosis, coloniality, medusa life-history stages (also comprising losses of medusa stage events) occurred independently during the evolution of Cnidaria. In addition to their key evolutionary position, cnidarians are also important as both predators and prey in aquatic ecosystems, playing a key role in biogeochemical cycles as recently suggested by Tinta et al. (2021). Gelatinous cnidar- ians are prey to several taxa including other cnidarians, fish and turtles. Although gelatinous cnidarians are sometimes considered to be relatively unsightly or of low nutritional value (e.g., Avent et al., 2001), in reality they are often un- derestimated as a food source because they tend to be di- gested quickly (Arai et al., 2003; Arai, 2005, Costa et al., 2018). As predators, cnidarians can dramatically alter plankton composition and even compromise fishing yield through dietary overlap and direct predation (Arai, 1988; Purcell and Arai, 2001; Purcell and Sturdevant, 2001). Sus- pension-fed benthic cnidarians also impact planktonic com- position and may play an important role in benthic-pelagic coupling. For example, a Mediterranean gorgonian has been found to consume a large number of invertebrate eggs and larvae, in particular larval bivalves, with unknown ef- fects on bivalve recruitment and consequently on popula- tion dynamics (Rossi et al., 2004). Particularly influential can be the aggregations or blooms of jellyfish (Hydrozoa, Scyphozoa or Cubozoa) which can have important ecolog- ical consequences. Recognition of the importance of jellyfish in marine ecosystems is growing steadily (Tinta et al., 2021). The clade Medusozoa includes four classes: Cubozoa (Caryb- deida and Chirodropida), Hydrozoa (Hydroidolina and Tra- chylina), Scyphozoa (Coronatae and Discomedusae), and Staurozoa (Cleistocarpida and Eleutherocarpida). Gelatinous cnidarians are found in all the world’s oceans and can occur in very high densities in large blooms; however, trends in jellyfish abundance appear to act on a local rather than a global scale (Richardson et al., 2009; Brotz et al., 2012; Condon et al., 2013; Schaub et al., 2018). For example, distinct coastal jellyfish hotspots that can span tens of square kilometres can occur in coastal waters (Houghton et al., 2006). Global jellyfish outbreaks appear to have become increasingly frequent and abundant in several coastal areas around the world and to last longer in recent years (Heaslip et al., 2012). It is not yet clear whether this is just an upward phase of a natural pattern of decadal oscillations or a real increase in gelatinous zooplankton blooms (McCanch and McCanch, 1996; Arai, 2005; Purcell, 2005). It has been hypothesized that jellyfish have benefited from human-caused environ- mental changes, such as: climate change, overfishing, eu- trophication, habitat modification (Cardona et al., 2012; Hamilton, 2016). Outbreaks of jellyfish have caused con- cern about their potential harm to human welfare, tourism, and fisheries (Graham et al., 2014). Indeed, it would ap- pear that due to their high abundance and spatial overlap, jellyfish could have a negative impact on fish of commer- cial interest through direct predation or through competi- tion for plankton prey (Decker et al., 2018). Considering the extremely negative perception of jellyfish by the gen- eral public, it is not surprising that these organisms are often considered as stressors affecting the viability of ecosystem services (as in the case of competition with commercial fish stocks or reduction of the quality of bathing). In this regard, the role of jellyfish as “providers” of ecosystem services in terms of regulation, support, and provisioning should be redefined and highlighted. Jellyfish have characteristics that place them in an in- fluential position to structure the flow of energy through pelagic food webs, such as high growth and reproduction rates, large planktivorous diets, and few apparent predators (Richardson et al., 2009). A wide range of taxa including other jellyfish, some molluscs, arthropods, fish, and birds regularly or opportunistically include jellyfish in their diet. Even though jellyfish can have a low nutritional value com- pared to other prey items (Doyle et al., 2007), consumption of sufficient quantities can sustain large predators. Jellyfish are also extremely important predators in pelagic marine systems (Pauly et al., 2009). The diversity in their feeding mechanisms and body size allows jellyfish to feed on a wide range of prey types and sizes (micro-het- erotrophs, zooplankton, other jellyfish, fish, fish eggs and larvae). Furthermore, most jellyfish are characterized by some sort of selectivity. When jellyfish blooms occur their rate of collective prey consumption can be so high as to di- rectly or indirectly control the population size of other zoo- planktonic organisms, including fish larvae (Purcell, 1989). In particular, intense jellyfish predation on some prey items can cause a shift in the trophic structure of marine commu- nities as a result of trophic cascades. Thus, large jellyfish populations that can influence pelagic food webs, exerting top-down control over zooplankton communities (Purcell, 2005; Sommer and Lengfellner, 2008; Canepa et al., 2014; Milisenda et al., 2018;). Jellyfish can therefore significantly redirect the flow of energy through food webs and nutrient recycling (Deason and Smayda, 1982; Suchman et al., 2008; Condon et al., 2012; Ruzicka et al., 2020). One of the most important services jellyfishes provide is climate regulation through the process of sequestering and trans- porting carbon through the water column. The sinking or accumulation of jellyfish carcasses on the seabed may also play an important role in the transfer of carbon from surface Non -co mmerc ial us e o nly Mucus secretions in cnidarian, an ecological, adaptive and evolutive tool 75 waters to the seabed (Lebrato et al., 2012) through a com- plex pelagic-benthic coupling process. Through the process of carbon sequestration, jellyfish provide regulating serv- ices (Doyle et al., 2014), nutrients that support primary pro- duction (West et al., 2009), and organic matter that stimulates microbes. Indeed, jellyfish can also provide small but significant amounts of nutrients to support pri- mary production (Pitt et al., 2009). Inorganic nutrients such as carbon, nitrogen and phosphorus are “regenerated” by jellyfish excretion or also through mucus production. Sim- ilarly, regenerated jellyfish products released into water are available to heterotrophic bacteria, and thus for respiration rather than primary production, creating a “jelly loop” in- volving the carbon cycle between jellyfish, bacteria, nano- flagellates heterotrophs, and ciliates (Condon et al., 2011). Numerous studies have investigated the role of bacte- ria during jellyfish bloom highlighting that variations in bacterial community structure associated with living or decaying jellyfish can have important consequences in trophic interactions and implications for carbon, nitrogen and phosphorus recycling (Titelman et al., 2006; Pitt et al., 2009; Tinta et al., 2010; Tinta et al., 2012; Blanchet et al., 2015; Tinta et al., 2016; Apprill, 2017). Bacteria as- sociated with the outer surfaces of cnidarian epithelia may play a number of crucial roles, such as nitrogen fixation (Lesser et al., 2004), antibiotic synthesis (Ritchie and Smith, 2004; Kelman et al., 2006) decomposition of or- ganic compounds (Di Salvo, 1969), primary defense against pathogens (Ritchie, 2006). The surfaces of cnidarians, as well as other marine or- ganisms, are a unique and favorable habitat for colonization by microorganisms and microbial communities often show- ing a marked variability (Apprill, 2017). In fact, jellyfish can also provide important habitats for various species of marine organisms. The relationships and uses of jellyfish are so varied and complex that they resemble those typi- cally described for tropical rainforests. Jellyfish can provide (1) pelagic refuges or shelters, (2) pelagic substrate, and (3) a host for algal symbiotic associations. One of the best-doc- umented biological interactions between jellyfish and ma- rine organisms is the interaction between jellyfish and juvenile fish, generally considered a facultative symbiotic relationship (Kramar et al., 2019). Gelatinous cnidarians can provide a pelagic substrate (habitat) for a range of taxa from microbes to inverte- brates, including crustaceans (Perissinotto and Pakhomov, 1998; Pagès, 2000), pycnogonids (Pagès et al., 2007), di- geneans (Martorelli, 2001) and protists (Moss et al., 2001). Studies on cnidarian jellyfish show the presence of endobiotic bacteria in tentacle jellyfish (Kramar et al., 2019), as in the case of the upside-down jellyfish Cassio- pea sp. This spends most of the time upside down resting on the seabed in order to maximize the photosynthetic ac- tivity of the zooxanthellae present in their oral arms; their activity can increase benthic oxygen production nearly 100-fold (Welsh et al., 2009). Another algal host is the pleustonic jellyfish Velella velella (Purcell et al., 2012), in which its symbiotic zooxanthellae are able to photo- synthesize and provide energy to the host. Cnidarian-microrganisms interactions are well docu- mented due to the extraordinary ecological importance of the mutualistic symbiosis between coralline anthozoans and cellular photosynthetic dinoflagellates (Davy et al., 2012). Cnidarians, mainly Anthozoans, provide a three-dimen- sional structure to benthic ecosystems, particularly tropical coral reefs. Coral reef ecosystems provide habitat for sev- eral taxa, protect coasts, and provide commercial and recre- ational resources for humans. Indeed, several species of anthozoans play a vital role as bio-constructors, creating habitats that serve as shelter for many other organisms and are home to great biodiver- sity. Anthozoans make up a class within this phylum and comprise a relatively well-known group consisting of hard and soft corals, gorgonians, sea feathers, black corals, and anemones (Palmer and Traylor-Knowles, 2018). Anthozoans are widely distributed throughout the seas of the world from the intertidal to the deep sea. Unlike other classes of cnidarians, anthozoans never produce jellyfish during their life cycle (Goffredo and Dubinsky, 2016). Within the anthozoans, the subclass Octocorallia refers to the colonial species with eight tentacles arranged in a single cycle and has three orders: Alcyonacea, Heliopo- racea and Pennatulacea. The other subclass, Hexacorallia, includes species of anthozoans that can be solitary, colonial or aggregated, and consists of 6 orders: Actiniaria, An- tipatharia, Ceriantharia, Corallimorpharia, Zoanthinaria and Scleractinia (Otero et al., 2017). The latter are mainly re- sponsible for the construction of coral reef structures, es- pecially in tropical and subtropical seas, as well as Cold-Water Corals (CWCs) that are among the main ecosystem engineering species in the deep sea worldwide and particularly in the Mediterranean Sea (Soetaert et al., 2016; Angiolillo et al., 2021). Anthozoans are the most specious class of the phylum Cnidaria, with an estimated 10,130 extant species (Daly et al., 2007). Anthozoans are phylogenetically basal, both within the Metazoans as a whole and probably within Cnidaria (Kayal et al. 2013), with Scleractinia already ap- pearing in the Middle Triassic (about 250 million years ago (Romano and Palumbi, 1996). Most existing anthozoans live in obligate endosymbiosis with dinoflagellates mem- bers of the genus Symbiodinium (Aranda et al., 2016), a re- lationship that underlies the ecological success of the class. In this association, Symbiodinium provides over 90% of the energy needs of the anthozoan (Muscatine and Porter, 1977) and, for hard corals, facilitates the calcification of the exoskeleton, allowing the formation of coral reef ecosystems. In return, the anthozoan host protects its algal Non -co mmerc ial us e o nly S. Savoca et al.76 partner useful for the purpose of recycling waste carbon and nitrogen (Jeong et al., 2012). This coral-scleractinian algae association is among the most investigated of the re- lationships that anthozoans have with the microbiota. This interaction could be closely related to the secretion of mucus in corals, with a preponderant role also in the ecology of the anthozoan immune system. Cnidarians, par- ticularly corals, possess a microbiota that is not only distinct from that of their surroundings, but also from other coral species (Cooney et al., 2002; Frias-Lopez, 2002; Ritchie and Smith, 2004; Brown and Bythell, 2005). It is therefore plausible to hypothesize that the evolution of the superficial mucosa as a barrier may be associated with the ability to exclude certain bacterial species from body tissues, with the exception of a selected central microbiome. With a layer of mucus, as in Cnidaria, non-commensal bacteria are es- sentially excluded from the tissues of the cnidarians. The activity of mucus in the exclusion of non-commensal bac- teria may have initiated the evolution of the alimentary canal and therefore the evolution of higher organisms, as well as representing an essential feature of the innate im- mune system. It is therefore increasingly evident that the microbiome associated with anthozoans is crucial for their health and is possibly partially modulated by the immune mechanisms of corals (Bourne et al., 2016). Deciphering the immunological complexities of coral-microbe symbio- sis is an ecologically key research field that could provide important insights into the establishment and functioning of symbiosis throughout the animal kingdom. This is particularly true when one considers that, despite their phylogenetic position and apparent morphological simplicity, anthozoans are immunologically complex (Miller et al., 2007; Shinzato et al., 2011), with large genomes and gene families that are comparable to those of Bilateria (Augustin and Bosch, 2010), which is why they represent a very interesting group for the study of the evo- lution of immunity and mutualism. Like all organisms, an- thozoans possess innate immune mechanisms (Palmer and Traylor-Knowles, 2012), but as invertebrates they lack the more complex adaptive component of immunity. Innate im- munity allows a non-specific and immediate response to both endogenous and exogenous threats with the aim of restoring homeostasis (Beutler, 2004; Medzhitov, 2008). However, at the same time, anthozoans utilize a large, com- plex, and diverse repertoire of immune receptors (Miller et al., 2007), signaling pathways (Wolenski et al., 2011) and “stress” responses (Palmer et al., 2008), which allow to eliminate pathogens, heal wounds, and defend oneself (Palmer and Traylor-Knowles, 2012; Mydlarz et al., 2016). Ecological immunology theory hypothesizes that variations between and within constituent immunity and immune re- sponses are due to energy trade-offs between costly func- tional traits such as reproduction, growth, and maintenance / immunity (Sheldon and Verhulst, 1996; Sadd and Schmid- Hempel, 2009). In this regard, one of the first signs of suf- fering of corals is the increase in mucus secretion (Brown and Bythell 2005), a function that requires huge energy in- vestments and that leads to the exhaustion of reserves, com- promising the organism’s immunity (Sheridan et al., 2014). Similarly, thermal bleaching events result in the loss of Symbiodinium which supplies energy to the coral causing it to die (Brandt and McManus, 2009). The interruption of this interaction results in a state of energy deficit and mal- nutrition, which causes a lowering of the defenses and an increase in energy expenditure in order to resist or tolerate the disease (Mayack and Naug, 2009). Site, mechanism of mucus secretion and composition In cnidarians mucus secretion is mainly attributed to specific cells, the mucocytes (Goldberg, 2002; Clarke et al., 2020), the only cells currently identified with mucus production in these organisms (Brown and Bythell 2005). However, other glandular cells have been identified in coral tissues (Kinchington, 1981, Le Tissier, 1987; Parker and Krumlauf, 2017). Mucocytes can occupy up to 90% of the ectoderm in some tissue areas: this has been confirmed by observation of histological sections in selected coral species (Brown and Bythell 2005). Several studies highlighted the pres- ence of mucocytes in three coral species: Galexea fascic- ularis (Linnaeus, 1767), Tubastrea faulkneri (Wells, 1982), Acropora muricata (Linnaeus, 1758) (Marshall and Wright, 1993). These three species belonging to class An- thozoa, seem to be evolutionarily separate from the other classes of Cnidarians, but they represent a good model for the description and mode of mucous secretion. In G. fas- cicularis, large mucocytes from 10 to 20 μm in size, were found on the external oral ectoderm, and in the oral gas- trodermis. These cells were characteristically club-shaped, with basal nuclei and large granular inclusions. In the oral gastrodermis, they surrounded the zooxanthellae and pre- sented granular inclusions larger than ectoderm muco- cytes. In contrast, the aboral gastrodermis (opposite to the oral gastrodermis) was characterized by fewer and smaller mucocytes, 5 to 10 μm in length, although structurally comparable. Few small mucocytes were also found in the calicoblastic layer, the portion of epidermis responsible for the formation of the skeletal structure. In T. faulkeri, mu- cocytes were very abundant in the calicoblastic layer and seem to be closely related to the skeletal structure. In A. muricata, no mucocytes were found in the apical portion of the oral ectoderm, although a thick layer of mucus cov- ering the ectoderm was observed; in contrast, numerous mucocytes were found in the basal portion oral ectoderm (Marshall & Wright, 1993). This demonstrates that there is no single pattern of mucocytes distribution and abun- dance valid for all Cnidarians, although such differences may depend on the physiological development/exchange Non -co mmerc ial us e o nly Mucus secretions in cnidarian, an ecological, adaptive and evolutive tool 77 of mucocytes (Brown & Bythell, 2005). It has been noted that due to induced stress the number of mucocytes increases. In a study by Fransolet et al. (2013), the sea anemone Exaiptasia diaphana (Rapp, 1829) was subjected to a transient increase in temperature combined with high illumination for 30 hours; this re- sulted in bleaching, as endosymbiont microalgae belong- ing to the genus Symbiondium disappeared. During the first day after the stress, a significant increase in cell pro- liferation was recorded at the gastrodermis and ectoderm levels, the ratio of ectodermal mucocytes increased sig- nificantly three weeks after the induced stress. The in- crease in mucocytes numbers may help restore symbiosis with endosymbiont dinoflagellates of the genus Sym- biondium (Fransolet et al., 2013). In general, the main mucus functions are to lubricate the epithelium and keep it hydrated and provide protection from pathogens (Mall, 2008). Mucus prevents the passage of bacteria due to its particularly high viscosity while main- taining its impermeability to water and gases (Cone, 2009). The ability of mucus to trap particles is closely related to its unique physicochemical properties as well as its phos- pholipid and glycolipid content (Murty et al., 1984). An important characteristic of mucus is its layered structure: the layers are cyclically removed and regenerated, which allows contaminants to be efficiently eliminated, thus not reaching the underlying tissues (Ducklow and Mitchell, 1979), in a process known as sloughing, that further than playing an essential role in defense against pathogens, it determines the thickness of the mucus layer (Cone, 2009). Furthermore, mucus has elasticity, provided by the mucin fibres, which are composed of alternating glycosylated (hy- drophilic) and bare (hydrophobic) protein regions; this con- formation allows them to trap particles using a large number of low-affinity bonds that form and break very eas- ily (Cone, 2009). The mucus of the anemone Actinia equina (Linnaeus, 1758) contains biomolecules that exhibit lysozyme-like antibacterial activity, and it has been shown that the efficiency of these compounds is influenced by some environmental parameters, such as temperature, pH and ionic strength (Stabili et al., 2015). However, these bioactive molecules have shown relevant antimicrobial ef- fects under controlled conditions (temperature of 37 °C) that rarely occur in the natural environment of A. equina; this highlights the limitations of these compounds in per- forming their antibacterial activity at lower temperatures and more basic pH (Stabili et al., 2015). Similarly happens in hard and soft corals (class Anthozoa) (Kelman et al., 2006). However, the efficacy of antimicrobial compounds in the absence of mucus and the efficacy of mucus in the absence of antimicrobial compounds have not been tested (Bakshani et al., 2018). In general, mucus in the Animalia kingdom is predom- inantly composed of water (approximately 95% of its total wet mass), while the remaining 5% is composed of mucins, phospholipids, cholesterol, salt, lipids, fatty acids and pro- teins with defensive purpose, such as lysozyme, im- munoglobulins and defensine (Marshall and Wright ,1991; Celli et al., 2005; Bansil & Turner 2006; Pearson et al., 2011;Hubot et al., 2022). Mucus of Cnidaria is composed principally, as for other Metazoa, of water at a percentage around 95%, with the remaining part composed by glycoproteins (�3%) and a mixture of other molecules (�2%), mostly represented by both inorganic salts (NaCl) and organic compounds like peptides, nucleic acids, lipids, and antibodies (Stabili et al., 2015; Bakshani et al., 2018; Hubot et al., 2022). These components, mainly glycoproteins, manage the mucus elasticity and viscosity (Bansil and Turner, 2018) useful in the defense (trough antibiotics and antimicrobial peptides), lubrification of underlying epithelia and act as a trap for food particles and transport to digestive system (Bakshani et al., 2018; Hubot et al., 2022). It has been re- ported the presence of nematocysts and toxins in the mucus of scyphomedusae (Shanks and Graham, 1988). Mucus, when released, is inevitably contaminated by sev- eral elements, such as sediment, and a wide range of mi- crorganisms: therefore, attempts have been made, in a controlled environment, to exploit artificial stresses such as dehydration and physical stimulation in order to achieve the release of mucus free of other components. However, any functional differences between mucus re- leased under natural conditions and mucus released under artificial stress have not been investigated (Brown and Bythell, 2005). In this regard, previous work suggests that mucus obtained through artificial stress induction had a higher organic content than mucus taken in the natural en- vironment (Gottfried and Roman, 1983). To our best knowledge, mucus macromolecular com- position (proteins, lipids, and carbohydrates) and the C/N ratio in cnidarian has been described for few species in- cluding mainly belonging to Anthozoa and Scyphozoa (Ducklow and Mitchell, 1979; Ducklow and Mitchell, 1991; Vacelet and Thomassin, 1991; Stabili et al., 2015; Hubot et al., 2022). Ducklow and Mitchell (1991), studied the composition variability of the mucus of different An- thozoa species, highlighting its utilization by microbial communities and the protective role of mucus especially related to water insolubility, chemical resistance to degra- dation, and inhibiting properties allowing a long degrada- tion time by bacteria. In Scyphozoa, Hubot et al., 2022, evaluated the mucus macromolecules in the species Au- relia aurita, Chrysaora fulgida, Chrysaora pacifica, Eu- pilema inexpectata, and Rhizostoma pulmo concluding a non-species-specific composition, highlighting the ab- sence of data that demonstrate the variation in mucus bio- chemical features, related to different conditions and situations as feeding, reproduction and stress proposed by Non -co mmerc ial us e o nly S. Savoca et al.78 Tinta et al., 2021. Biological and ecological function of cnidarian mucus As stated above, most cnidarian species secrete mucus more or less constantly, which seems to be involved in both physiological and non-physiological functions. Moreover, under certain stressful conditions, a considerable increase in mucus secretion rate can be observed (Camacho- Pacheco, 2022). From the literature is clear that the func- tions of mucus have been documented for some groups, especially in corals, where this seems to be closely related to their role as ecosystem engineers (Bythell and Wild, 2011). Instead, the information about the role and the eco- logical and evolutionary implications of the mucus secreted by medusozoans is less substantial. Basically, the secretion of mucus in invertebrates performs three essential func- tions, identified in defense, feeding and locomotion activ- ities (Brown and Bythell, 2005). However, particularly in cnidarians, this assumes much more important roles and closely connected to the well-being of the cnidarians them- selves and to the provision of ecosystem services. These include for example resistance to desiccation, sediment shedding, abrasion protection, defense against predators and pathogens, feeding, reproduction, settlement behaviour, spatial competition (Stabili et al., 2015; Camacho-Pacheco, 2022). Recently, some authors (Liu et al., 2018; Ames et al., 2020; Hubot et al., 2022) have reported for some jelly- fish species the mucus composition, excretion, and possible fate in marine ecosystems, which, considering the scarce information available in the literature, could represent an important piece in the understanding the biological and eco- logical function network claimed by mucosal products. Several authors have eviscerated, demonstrated, or hy- pothesized all the possible biological functions of the mucus secreted by cnidarians, with particular attention paid to corals. However, our feeling is that few studies or re- views, currently present in the literature, have paid attention to the implications and ecological effects of the mucus pro- duced by cnidarian (Tinta et al., 2021). Coral tissue is often heavily covered with cells that pro- duce copious amounts of mucus (e.g., Marshall and Wright, 1993). Although less well known, the epidermis and gas- trodermis of jellyfish, including A. aurita, also contain nu- merous types of single-celled, mucus-producing glandular cells, leading to the formation of thin mucus layers that are constantly renewed on the outer surface of the jellyfish. Under certain conditions such as stress, during reproduc- tion, feeding and digestion, and even during death, the amount of mucus released is even more pronounced (Rot- tini Sandrini and Avian, 1991; Doyle et al., 2014). Secretion of mucus sheets is known to be an important means of trapping and ingestion of small nutrient particles (e.g., Yonge, 1930) by scleractinian corals and gorgonians (Bessell-Browne et al., 2017). Lewis and Price (1975) showed that increased mucus secretion by corals within dif- ferent groups was a common response to the presence of food, which produced a dense net of filaments of various thicknesses, depending either on the species or water con- ditions. Several jellyfish taxa have been listed as mucus net filter feeders (salps) (Doyle et al., 2014). The production of mucus greatly facilitates the capture of prey by jellyfish due to the formation of lumps or conglomerates. Studies of other jellyfish have reported that the mucus aids in the transport of food through the oesophagus to the gastric pouches, even when prey items were small (Ames et al., 2020; Daly et al., 2007). Others can produce toxic mucus, such as Cassiopea spp, which release large amounts of toxic mucus into the water column (Ohdera et al., 2018; Ames et al., 2020). Cassiopeia mucus kills some fish species on contact (Ames et al., 2020). Motile cellular structures that trap and paralyze prey have been identified in the mucus of the jellyfish Cassiopea xamachana (Ames et al., 2020). In the order Rhizostomae, mucus can facilitate feeding (Arai, 1997), although its precise role and function are not yet fully understood (Camacho-Pacheco, 2022). The mucus is also involved in some reproductive processes, effectively becoming part of the reproductive or larval settlement strategy of some cnidarian species. A study conducted on the genus Eudendrium (class Hydro- zoa) reported that polyps release planulae which remain at- tached to a thread of mucus which detaches once it reaches the bottom. This strategy allows the species to occupy well- defined sites, maintaining a gregarious behavior (Wasserthal and Wasserthal, 1973). A similar strategy has been found in Nemertesia antennina (Linnaeus, 1758, class Hydrozoa), where the mucous filament reaches consider- able lengths (up to 50 m) and allows the planulae to reach areas further away from the mother colony (Hughes, 1975). In Pelagia noctiluca, mucus increases the probability of fertility (Lilley et al., 2014). First, each mature female medusa lays the oocytes in a mucus tape, holding the eggs together for several minutes before its dissolution. This pe- culiarity can favor the fertilization of the entire set of oocytes by the spermatozoa released by one or a few male partners (Aglieri et al., 2014). Only about 50% of the car- bon and nitrogen invested during reproduction goes into the eggs, while the rest it produces a thread of mucus that binds all the eggs together. The mucus presumably in- creases the chances of fertilization and may increase the buoyancy of the egg series in Pelagia (Lilley et al., 2014). In a study performed on S. meleagris the greatest num- ber of oocytes were found in the mucus, suggesting that it is also important in sexual reproduction, as previously re- ported for P. noctiluca (Nagata and Morandini, 2018) Mucocyte secretions have distinct functions including cleansing sediment, healing wounds, and protecting against invasive microbes, as well as shielding from des- iccation and UV damage (reviewed by Brown and Bythell 2005). Increased mucus production is observed in Non -co mmerc ial us e o nly Mucus secretions in cnidarian, an ecological, adaptive and evolutive tool 79 colonies that are in direct contact with sunlight (remaining fully exposed during low tides) as a strategy to avoid des- iccation (Santos et al., 2016; Sebens, 1982), as reported in the coral Orbicella annularis (Ellis & Solander, 1786) (Piggot et al., 2009). Briefly, in sessile cnidarians the mucus represents a strong protection against desiccation (Krupp, 1984), as its hydroscopic characteristics allow to keep the coral surface moist during exposure to air due to low tide conditions; however, the mechanisms that control these processes are still unclear. Although the mucous layer is not waterproof, it can greatly reduce the airflow and water exchange rate. In anemones, Shick (1991) observed that the mucus pres- ents few barriers to the diffusion of water, but can produce a boundary layer which, when dried, separates from the ex- ternal tissues (Griffiths, 1977). Another factor that may in- fluence mucus secretion rates in corals is sedimentation (Bak and Elghershuizen, 1976; Schumacher, 1977; Rogers, 1990; Stafford-Smith and Ormond 1992; Stafford-Smith, 1993). In some cases, repeated influxes of sediment can lead to depletion of mucus secretion causing the rejection of sediment to slow and cease (Schumacher, 1977). In other species, such as Gardinoseris planulata, on the other hand, mucus secretion continues long after the sediment rejection activity has ceased, suggesting that in this coral exhaustion of the ciliary mechanism rather than mucus secretion occurs (Stafford-Smith, 1993). Stabili and collaborators (2015) demonstrated that some properties of the mucus secreted by A. equina, such as osmolarity and viscosity, as well as the chemical composition, play a key role in the defense against sedimentation and desiccation, however it is still evident that the hydrodynamism has a relevant influence on these phenomena. During an experimental manipulation, Coffroth (1985) points out that the reduction of water movement also led to the formation of mucous sheets, thus the formation of the mucous sheet may be a secondary mechanism to cope with sediment stress, especially when currents are not strong enough to relieve environmental conditions. Several studies demonstrated an increase in mucus secretion in corals due to sedimentation (Bak and Elghershuizen 1976; Schumacher 1977; Rogers, 1990; Stafford-Smith and Ormond 1992; Stafford-Smith 1993; Bongaerts et al. 2012; Weber et al., 2012). This strategy turns out to be energetically quite expensive since (the car- bon requirement for mucus secretion is more than doubled) (Edmunds and Davies,1989; Riegl and Branch, 1995). A study conducted on several species belonging to the orders Scleractinia and Malacalcyonacea showed that, in absence of sediment, mucus production was around 35% of daily respiration; in the presence of sediment, production in- creased up to 65%. Therefore, sediments affect coral metabolise decreasing photosynthetic production, with greater carbon losses due to the greater amount of mucus produced (Riegl and Branch, 1995; Weber et al., 2012). As a barrier defense, mucus on jellyfish surfaces has also been found to play a role in surface cleaning and pred- ator defense (Patwa et al., 2015). For scyphozoans, includ- ing S. meleagris, the mucus was found to contain nematocysts and toxins that serve defensive purposes (Shanks and Graham, 1988). Shanks and Graham (1988) identified mucus secretion as an important chemical defence mechanism against pred- ators. In previous studies with Aurelia coerulea, mucus was shown to function as a “cleaner”, preventing the accumu- lation of bacteria and debris on their surfaces (Patwa et al., 2015). In addition to mechanical protection, the mucus of many cnidarians contains specific compounds to make the animal venomous, unpleasant, irritating or a combination of these characteristics. It is not surprising that these inver- tebrates have developed an innate immune system that pro- duces a considerable number of defense molecules, such as lytic compounds (Mayer et al., 2013), bioactive antimicro- bials (Aneiros and Garateix, 2004; Otero Gonzalez et al., 2010; Smith et al., 2010), toxins, and carbohydrate-based anti-adhesives (Bavington et al., 2004). Effectively the mucus acts as a physical barrier and the presence of cilia on the ectodermal cells favours the muco-ciliary transport of particles, including foreign cells, towards the mouth of the polyp where they can be di- gested or removed by the secretion of mucus. The first line of defense against invading pathogens in cnidarians is the superficial mucous layer. In reality, the superficial mucous layer represents the natural microbiota of the cnidarian surface, which may also include pathogens. The secreted mucus is an attractive niche for bacteria. Since jellyfish mucus is composed primarily of proteins, lipids, and a lower proportion of carbohydrates (Ducklow and Mitchell, 1979), it is a high-quality energy source that is readily utilized by bacteria. Cnidarians indeed host a va- riety of microbial species (Rohwer et al., 2001; Webster and Bourne, 2007). A large number of evidence (reviewed by Ainsworth et al., 2010) shows for example that “healthy” corals in their natural environment are associ- ated with microbial groups and that when they are stressed or the environment changes, the associated microbes change in their turn. The cnidarian’s ability to control the production and composition of the superficial mucus layer and its associated bacteria may represent an important portion of their immunity. Antibacterial compounds pro- duced by the cnidarian host, as well as its associated mi- crobial consortium, are released into the mucus and play an important role in the control of the mucus-associated microbial community (Brown and Bythell, 2005; Reshef et al. 2006; Ritchie 2006; Rosenberg et al., 2007; Shnit- Orland and Kushmaro, 2009; Rypien et al., 2010; Krediet et al., 2013). The identification of the role of the micro- biota in the development of the eukaryotic host and their response to environmental variations and/or stress, has led Non -co mmerc ial us e o nly S. Savoca et al.80 to the definition of a “holobiont”. There is now strong ev- idence indicating that the innate immune system of cnidarians is not only involved in the destruction of harm- ful microorganisms but is also crucial in the structuring of the microbial communities associated with the tissues that are essential components of the holobiont and with the health of the organism (Rivera et al., 2015). Cnidari- ans have many microorganisms associated with their tis- sues (Rivera et al., 2015; Tinta et al., 2019). Actually, it is not entirely clear whether the bacteria adhere directly to the outer cell layers of the cnidarians or are associated only in the thin mucus layer, however, the association be- tween bacteria and cnidarians is highly dynamic and com- plex. Recent studies have focused on the relationships between microbes and their host organisms, investigating the composition and ecological role of microbial commu- nities associated with cnidarians (Basso et al., 2019; Kra- mar et al., 2019; Stabili et al., 2020). These studies addressed questions relating to the ecological role and composition of cnidarian-associated microbial communi- ties, the mechanisms underlying these interactions, and the nature of the relationships that arise between cnidari- ans and their associated microbiome. Further investiga- tions involved bacteria associated with the outer surfaces epithelia in several cnidarian species and at different life stages, documenting their involvement in a multitude of important potential roles including antibiotic synthesis, nitrogen fixation, decomposition of organic compounds, the primary defence against pathogens or the modulation of contractile activities (Weiland-Bräuer et al., 2015; Kra- mar et al., 2019; Tinta et al., 2019). The microbial com- munities associated with the jellyfish species Aurelia aurita, Mastigias papua, Cotylorhiza tuberculata and Rhi- zostoma pulmo and Tripedalia cystophora analyzed in several studies, showed extremely variable bacterial as- sociations. The microbiome associated with different life stages of A. aurita (polyp, strobila, ephyra, juvenile and adult medusa) and with different compartments of adult specimens (exumbrella, mucus and gastric cavity), from different geographical areas was examined by Weiland- Bräuer et al. (2015). The results of this study showed that the A. aurita microbial community appears to be highly host-specific and distinct from bacterioplankton sus- pended in the surrounding water column. In adults, the microbiota showed significant differences between the various compartments, showing a more variable bacterial composition associated with the mucus than that present in the gastric cavity. Furthermore, Kramar et al. (2019) monitored the bacterial community associated with the moon jellyfish Aurelia solida. In particular, different parts of the body (umbrella surface, oral arms and gastric cav- ity) were analysed for bacterial community diversity. The authors reported that the microbiota associated with this species was different from the microbial assemblages in the surrounding seawater and differed significantly be- tween different body compartments. Furthermore, during the senescent phase, the bacterial community was found to mutate in structure with increased Gammaproteobac- teria (entirely Vibrio). Based on these results, it was hy- pothesized that the bacterial community associated with jellyfish may play an important role for the host. The com- positional analysis of the microbiome associated with jel- lyfish mucus was conducted by Tinta et al. (2019) to conclude that the Gammaproteobacteria (mainly Pseudoalteromonas and Vibrio) are the most abundant, followed, to some extent also by the Alphaproteobacteria (Phaeobacter, Rugeria and Roseovarius). These bacteria are known for their ability to synthesize antimicrobial compounds when attached to live or inert surfaces, and therefore involved in defense of the host against pathogens and encrusting organisms from the surrounding environment. The microorganisms associated with the mucus of cnidarians appear to be characterized by a great diversity and the mucus represents the compartment richest in bac- terial associates compared to the oral arms and the um- brella. In anthozoans, microbial agents perform key functions, including regulation of metabolism, immune de- fense, development, and behavior. Bacteria associated with anthozoan tissues can fix nitrogen, digest complex poly- saccharides, and produce antibiotics to prevent infection by pathogens. In turn, Symbiodinium produces dimethylsulfo- niopropionate (DMSP) as an osmolyte, an antioxidant agent (Lawson et al., 2018) and a nutrient source for asso- ciated bacteria (Rosenberg et al., 2009). A certain specificity has also been found in the Antho- zoa-bacteria association; Porporato et al. (2013), for exam- ple, described the bacterial communities associated with Pennatula phosphorea and Pteroeides spinosum, showing a high species-specificity. Moreover, since within the same species only a few phylotypes were shared between mucus and tissues, the authors also hypothesized that a partition of the microhabitat could exist between the associated mi- crobial communities. In the case of P. phosphorea, both tis- sue and mucus associated communities were characterized by the predominance of Alphaproteobacteria. Conversely, Alphaproteobacteria prevailed in the mucus layer of P. spinosum and the tissues were dominated by Gammapro- teobacteria. The isolates of strains belonging to Vibrio spp., mainly obtained from coral mucus, showed an antibacterial activity against some indicator organisms, indicating a pro- tective function of the bacterial communities associated with the coral as in the case of jellyfish. Based on this evidence, the ability of cnidarians to con- trol the production and composition of a mucous matrix and its associated bacteria may represent an important part of immunity (Ocampo and Cadavid, 2015). The ability of some bacteria to produce antimicrobial compounds is likely Non -co mmerc ial us e o nly Mucus secretions in cnidarian, an ecological, adaptive and evolutive tool 81 to contribute to competition for space and resources with potential pathogenic host bacteria. Due to these character- istics, mucus and its components have interesting implica- tions in molecular ecology and biotechnology. The specific interactions of microbial colonization of mucosal surfaces are still unknown. Calow (1979) highlighted that any differences in the biochemical composition of the mucus can influence the attack of microbial agents that use exoenzymes to degrade the mucoid polymers. The microbes themselves can trans- form dissolved and particulate matter into living matter, attracting other predatory organisms. It also appears that changes in the composition and abundance of bacterial communities may affect the health of the host, making it more vulnerable to disease (Reshef et al., 2006). Recent documentation on the succession of microbial communities associated with the developmental stages of Porites astreoides (Sharp et al., 2012) and the discovery of beneficial functions in favor of Alphaproteobacteria and Marinobacter strains (Lubbock, 1980) provide fun- damental support to the hypothesis of “hologenome evo- lution”, i.e. the idea that in symbiotic organisms that were colonial, the hologenome, and thus sprung from all mem- bers of the holobiont, actually may act as a single unit of evolution, with rapidly evolving microorganisms provid- ing the plasticity to adapt to the changing environment (Rosenberg and Zilber-Rosenberg, 2011). The highly diverse mucosal microbiome is therefore generally believed to perform vital services and to be in- volved in the flow of energy within marine ecosystems. In this prospective, after detachment from the surface of cnidarians, mucus can play several important roles in the functioning of the ecosystem services it provides. First of all, the mucus acts as an energy vector (Bythell and Wild, 2011). Some authors (Haas et al., 2010; Naumann et al., 2010; Wild et al., 2010) confirm and demonstrate that all examined scleractinian corals release organic matter in the form of mucus, which includes both a particulate frac- tion and a dissolved fraction (mainly organic carbon, DOC (Crossland, 1987; Naumann et al. 2010) and that mucus release by corals is largely independent of the car- bon acquisition mechanism (autotrophy versus heterotro- phy). Studies conducted by Naumann et al., 2010 suggest that photosynthates translocated by the endosymbiotic zooxanthellae of cnidarians represent the basis of mucus production (Naumann et al. 2010). Although the energy cost is quite high, (approximately 20-45% of the daily net photosynthate produced (Brown and Bythell, 2005), coral mucus is continuously released into the water column, ac- counting for half of the total mucus released by all benthic organisms (animals and plants) on coral reefs (Crossland, 1987; Naumann et al. 2010). Suffice it to say that Acrop- ora spp., the dominant scleractinian genus on the Great Barrier Reef, releases up to 4.8 L of mucus per square meter of coral reef surface per day (Wild et al., 2004). Zooxanthellae, endosymbiotic algae of reef-building corals, contribute substantially to the high gross primary production of coral reefs, supplying a substantial portion of their hosts’ energy needs by transferring photosynthet- ically fixed carbon to the coral. The high arabinose con- tents found in the carbohydrate fraction of coral mucus suggest that much of the fixed carbon is released in the form of mucus, since arabinose is not usually a constituent of animal cells (Wild et al., 2004). Between 56% and 80% of the released adhesive mucus dissolves in the reef water and effectively traps organic matter from the water col- umn, increasing its initial organic carbon and nitrogen content by three orders of magnitude in short time and rapidly transporting energy and nutrients to the sediment, which acts as a biocatalytic mineralizing filter. Coral mucus provides light energy transformed by zooxanthel- lae and trapped particles to the heterotrophic reef commu- nity, thus establishing a recycling cycle that supports benthic life, reducing energy and nutrient loss from the reef ecosystem (Wild et al., 2004). However, mucus release can vary between species and depending on conditions. Tanaka et al. (2009) found that only about 5% of the net daily photosynthetic production was released by Acropora pulchra. Muscatine et al. (1984) observed that in Stylophora pistillata, depending on the irradiance levels, the loss of the newly fixed carbon varied from 6 to 50%. Tremblay et al. (2012) found that in S. pistillata, the availability of heterotrophic foods and high light levels were required for the accumulation of au- totrophic carbon, subsequently released as dissolved or- ganic carbon. Mucus release appears to constitute a dominant form of organic matter generated in coral reef ecosystems (e.g., Fer- rier-Pagès et al., 1998; Hatcher, 1988; Bythell and Wild, 2011) and represents the major route by which primary pro- duction of coral enters the food web (Hatcher, 1988). Freshly released coral mucus differs between species due to variations in lipid, sugar, and amino acid composi- tion (Ducklow and Mitchell, 1979; Crossland, 1987; Meikle et al., 1988; Wild et al. 2010). Particulate mucus may also contain varying levels of phosphate or nitrogen, which may for example be influenced by planktonic food availability, colonization by picoplanktonic organisms and nitrogen-fixing bacteria (Hubot et al., 2022). An old view held that coral mucus was a negligible source of nutrients (e.g., Krupp, 1984; Coffroth, 1990). Today, however, it is known that mucus represents an energy substrate, rich in glucose (Wild et al., 2010), degradable by microbes and higher organisms (Benson and Muscatine, 1974; Grange, 1991; Rinkevich et al., 1991; Patton, 1994; Naumann et al., 2010). Several experimental studies (Ducklow and Non -co mmerc ial us e o nly S. Savoca et al.82 Mitchell, 1979; Ferrier-Pages et al., 2000; Wild et al., 2004; 2005; 2009; 2010) have shown a stimulation of planktonic or benthic microbial activity after the addition of coral mucus. Additional studies (Allers et al., 2008; Schöttner et al., 2009) have also shown that not only mi- crobial activity, but also its diversity can be influenced by the presence of coral mucus. The cycles of production, aging and elimination of mucus are accompanied by vari- ations in total microbial abundance, confirming that this colonization increases the nutritional value, in terms of carbon and nitrogen, of the released mucus (Ferrier-Pagès et al. 1998a; Nakajima et al. 2009; Grover et al. 2014; Bednarz et al., 2017 and therein references). This enriched exudate becomes a downstream pelagic food source for fish (Benson and Muscatine, 1974) and zooplankton (Got- tfried and Roman, 1983), and after sinking into the sedi- ment, it is recycled back to members of the benthic community (Wild et al., 2005; Huettel et al., 2006; Mayer and Wild, 2010; Tanaka et al. 2011; Naumann et al., 2012). This results in a mechanism by which the pelagic food supply is coupled to the benthos (Naumann et al., 2009; Bythell and Wild, 2011). In addition to its energy carrier function, the released adhesive mucus also func- tions, for obvious reasons, as a particle trap, which by forming aggregates of various inorganic and organic par- ticles caught in the water column, greatly increases its sedimentation speed (Wild et al., 2004; Huettel et al., 2006). It is easy to hypothesize that such a mucus-induced and accumulated material cycle could support the recy- cling of essential nutrients within marine ecosystems, con- tributing to its functioning. A study done in the Red Sea documented that particle entrapment by coral mucus oc- curs while the mucus is still attached to the surface, due to the weaker velocities of tidal currents causing the mucus to remain longer at the surface of the coral (Mayer and Wild, 2010) than reported for Great Barrier Reef, in which the mucus is rapidly detached (Wild et al., 2004; Huettel et al., 2006). This could result in a faster cycling of matter within the reef. Thus, organic compounds can be rapidly recycled (Wild et al., 2004; 2006), so that re- generated nutrients are released (Wild et al., 2005) and feed new primary production by autotrophic coral reef or- ganisms, including zooxanthellae. Coral mucus can also trap picoplankton particles (Davy and Patten, 2007; Futch et al., 2010) from the water column and transform them into larger mucus aggregates (Naumann et al., 2009) al- lowing benthic filter of using picoplankton as a food source and once again facilitating the coupling between the water column and the benthic environment. Even the mucus secreted by jellyfish can have impor- tant ecological implications, although less explored than corals. Jellyfish can provide small but significant amounts of nutrients that support primary production (Pitt et al., 2009). This is particularly important given the fact that jellyfish populations are characterized by large and rapid fluctuations in abundance. It is conceivable that the adap- tive characteristics of jellyfish will allow them to thrive in anticipation of future catastrophic events directed by climate change, namely warming, acidification, oxygen loss and increasing human exploitation of ocean services; therefore, understanding the response of marine ecosys- tems to this natural and/or anthropic perturbation is of fun- damental importance (Richardson et al., 2009; Purcell, 2012; Steinberg and Landry, 2017). Regardless of the divergent scientific opinions on the causes of these fluctuations in jellyfish populations, the combined effect of natural oscillations and anthropogenic factors can lead to an increase in their populations, with consequent ecological and socio-economic impacts (Richardson et al., 2009; Purcell, 2012; Condon et al. 2012, 2013; Sanz-Martín et al., 2016). Precisely because of their demographic dynamics of boom and bust, jellyfish are likely to influence the cycles of matter in the ecosystems they inhabit. Jellyfish blooms represent a significant and largely overlooked source of organic matter, especially at local and regional scales. Re- cently, jellyfish have been recognized as key carbon ex- port agents to the interior ocean, demonstrating the need to include jellyfish in oceanic biogeochemical models as a key component of the biological soft tissue pump (Le- brato et al., 2012; Steinberg and Landry, 2017) so far not considered. Jellyfish acquire C, N, and P by assimilating organic compounds from ingested prey, by absorbing small amounts of dissolved organic material, and some species instead actively assume dissolved inorganic forms. A portion of the ingested elements is incorporated into their biomass and the undigested material is elimi- nated in the faeces or released via “sloppy food”. The or- ganic forms of C and N are recycled into the environment as mucus together with both organic and inorganic meta- bolic products. During bloom formation, populations in- crease in size acting as a net sink for C, N and P. During decomposition processes the elements bound to their bio- mass are “recycled” into the water column as dissolved inorganic and organic compounds, finally available for pelagic and benthic microbial communities. Regenerated organic C by mucus production and decomposition sup- ports microbial production, while regenerated inorganic N and P support algae production. Additionally, few species, such as Linuche unguiculata Schwartz and many Rhizostome species (genera Cassiopea, Mastigias and Phyllorhiza) form a symbiosis with zooxanthellae. In zooxanthellate jellyfish, translocation of photosynthetic products from zooxanthellae is likely to be the major source of C for the host (Cates, 1975). Furthermore, inor- ganic excretory products can be translocated from the host to the zooxanthellae instead of being released into the ex- ternal environment. Once released into the environment, Non -co mmerc ial us e o nly Mucus secretions in cnidarian, an ecological, adaptive and evolutive tool 83 jellyfish mucus is metabolized by bacteria, creating im- portant variations in microbial assemblages and diverting carbon to bacterial respiration (Condon et al., 2011). Mucus release therefore not only initiate biogeochem- ical cycles, but also control a range of processes including the microbial activity. So, again microbes are the main actor. As stated above, jellyfish host and interact with tax- onomically and metabolically diverse microorganisms throughout their entire lives. The limited research avail- able highlights the importance of the environmental mi- crobial community for the recruitment of members of the jellyfish microbiome and a certain degree of microbiome specialization with some preferences for specific jellyfish taxa, life stages and body parts (Basso et al., 2019; Kra- mar et al., 2019; Tinta et al., 2019). The role of the mi- crobiota associated with jellyfish (or their mucus) is generally related to digestion, defense mechanisms against pathogenic microorganisms and possible predators (Basso et al. 2019 and references therein), and reproduc- tion (Weiland-Bräuer et al., 2020) Collectively, the review of the current state of knowl- edge on the jellyfish microbiome reveals that this topic is grossly underestimated and should be studied in more de- tail in the future. Mucociliary transport not only aids feeding by serving as a trap for nutrient particles, but also acts to remove non- nutritive particles from the surface of cnidarians. This is actually a common mechanism in several invertebrates, wherein the secreted mucus traps particulate debris which is then directed through the ciliated surface to a disposal site (Sleigh, 1989). Duerden (1906) was among the first to demonstrate the movement of mucus-bound non-nutri- tive particles up the edge of the disc of fungiid corals to the underlying substrate. It has been suggested that mucus have structural prop- erties to efficiently accumulate, absorb or maybe bind pol- lutants, such as aromatic hydrocarbons (Neff and Anderson, 1981) or heavy metals (Brown and Howard, 1984; 1985), thus giving some protection to the underlying coral tissues by physically protecting them and acting as a pathway for pollutant release (Neff and Anderson, 1981). It seems likely that one of the primary detoxification mechanisms em- ployed is increased mucus production. Increased mucus secretion has been described in re- sponse to exposure to a wide range of pollutants: crude oil (Mitchell and Chet, 1975; Neff & Anderson, 1981), drilling mud (Thompson et al., 1980), mercury (Bastidas and Gar- cia, 2004), copper sulphate (Mitchell and Chet, 1975), peat (Dallmeyer et al., 1982). In histological studies, an increase in the number and size of mucosal secretory cells has been observed after exposure of Manicina areolata (Linnaeus, 1758) coral to chronic oil pollution (Peters et al., 1981). In experiments using drilling mud, colonies of Acropora cer- vicornis (Lamarck, 1816) produced mucous filaments after 30 minutes of exposure, while mucus production by other species (Porites astreoides Lamarck, 1816, Porites divari- cata Le Sueur, 1820, Porites furcata Lamarck, 1816 and M. annularis) was observed only after 24 hours (Thompson et al., 1980). An ecotoxicological study conducted by Howe et al. (2012), tested copper toxicity on Exaiptasia diaphana (Rapp, 1829), a tropical symbiotic anemone; specifically, acute tests were conducted on juveniles (1-2 mm). The or- ganisms responded to copper with severe (and often com- plete) retraction of tentacles, the collapse of the column, overproduction of mucus, the expulsion of grouped and in- dividual zooxanthellae, and necrosis within 96 hours. These effects were rapid, with obvious changes in morphology occurring at concentrations ≥27 µg/L within the first hour. However, most anemones that survived acute copper ex- posure also survived continuous exposure, likely because they have successfully reduced their copper exposure and uptake through significant tissue retraction and employed detoxification mechanisms by increased mucus production, which allows the expulsion of metal-rich zooxanthellae. Mucus produced by cnidarians can act as a trap for par- ticles because of its adhesive properties. This can justify the presence of microplastic fibres found in Cnidaria and Ctenophora, as reported by a recent study conducted in Orkney, Shetland, and the North Sea (Devereux et al., 2021). This aspect deserves the attention of the scientific community, as the entry of microplastics through the cnidarians along the trophic chain can represent a potential risk both for ecosystems functioning and implications for human food safety and health due to the trophic transfer (Barboza et al., 2018; de Oliveira Soares et al., 2020; Al- bano et al., 2021; Devereux et al., 2021; Bruno et al., 2022). Microplastic effects on freshwater cnidarians, par- ticularly Hydra thomseni (Cordero, 1941) belonging to the class Hydrozoa and on jellyfish of the genus Aurelia were evaluated. These studies show that these organisms are ca- pable of ingesting microplastics with associated impacts on feeding, causing significant morphological changes in H. thomseni. In Aurelia sp., however, short-term exposure to microplastics compromises behavior as well as survival (Murphy and Quinn, 2018; Costa et al. 2020). Riegl and Branch (1995) attempted to measure energy expenditure during coral cleaning processes, concluding that mucus se- cretion in sediment cleansing was energetically very costly. In fact, under stress, the carbon requirement for mucus se- cretion doubled. Similar data were obtained by Edmunds and Davies (1989), who suggested that mucus secretion by Porites porites stressed by the presence of non-food parti- cles constituted an important pathway for energy loss. Moreover, mucus overproduction has been also seen in cases of increased water temperature (Neudecker, 1983) and decreased salinity (Coffroth, 1985), associated with high energetic costs. Climate change and anthropogenic stressors are threatening the long-term survival of many Non -co mmerc ial us e o nly S. Savoca et al.84 marine species, and coral reefs are among the most threat- ened ecosystems (Hughes et al., 2017). Long-lived organ- isms, such as scleractinian corals, are particularly vulnerable to environmental stresses and climate change, the rate of which occurs exceeds the time it takes for a pop- ulation to adapt through natural selection (van Oppen et al., 2017). One of the first signs of anthozoic suffering is rep- resented precisely by the increased production of mucus (Brown and Bythell, 2005), which in turn requires huge en- ergy investments, which can lead to the depletion of meta- bolic reserves, compromising the body’s immune system (Riegl and Branch, 1995; Sheridan et al., 2014;). In Acro- pora acuminata, energy expenditure through mucus exuda- tion following environmental stress can account for up to 40% of the net carbon fixed by photosynthesis (Crossland et al., 1980). An energy investment of this magnitude could therefore increase the risk of secondary adverse effects de- riving from subsequent exposure to environmental stressors (Anthony et al., 2009). Subsequently, environmental stress could further deplete energy resources by inducing immune upregulation, known to be very expensive in terms of en- ergy (Armitage et al., 2003). Unfavourable environmental conditions are compromising the health of many anthozoan species; global warming and therefore warmer and eu- trophic waters, seem to promote the seizure of nutrient re- sources by symbiotic algae at the expense of their hosts (Wooldridge, 2017; Baker et al. 2018) and this shift from symbiosis to parasitism may presage the well-known ex- pulsion of symbiotic coral partners and a change in the na- ture of their microbial communities (e.g., Bourne et al., 2009; Sokolow, 2009; Work and Meteyer, 2014; Hughes et al. 2017). Symbiotic relationships are highly sensitive to environmental change. Regardless of their narrow temper- ature range, zooxanthellae become photosynthetically im- paired, undergoing an expulsion process and varying degrees of bleaching. Hosts in these conditions vary in their ability to obtain heterotrophic nutrition (Grottoli et al., 2006; Rodrigues and Grottoli, 2007; Palardy et al., 2008; Ferrier-Pagès et al. 2011), and if bleaching is severe, het- erotrophy it must supply 100% of the metabolic needs of the coral (impossible for some species). Thus, disruption of this mutualism results in a state of starvation and energy deficit, leaving the host vulnerable to infection, (e.g., Siva- Jothy and Thompson, 2002). Similarly, infection is ener- getically very costly as resources are concentrated and invested in resisting or tolerating any disease (Mayack and Naug, 2009). According to ecological immunology theory, variations between and within constituent immunity and immune re- sponses are due to energetic trade-offs between costly func- tional traits, such as reproduction, growth, and maintenance/immunity (Sheldon and Verhulst, 1996; Sadd and Schmid-Hempel, 2009). Therefore, studies of molecu- lar ecology and immunity could offer a better understand- ing of the drivers that underlie the health status of both corals and other marine organisms in order to more effec- tively conserve and restore ecosystems of high ecological and socio-economic value such as the coral reefs. Evolution of the mucus layer The development of the mucus layer is a key event in the evolutionary history of the kingdom Animalia (Bak- shani et al., 2018). Secretion of a functional mucus layer first appeared in the Phyla Cnidaria and Ctenophora (Lang et al., 2016). In a study published in 1984, the luxury car- bon hypothesis was introduced, where the only function of mucus was to remove excess photosynthetic carbon from the coral-zooxanthellae symbiosis (Davies, 1984). This hy- pothesis originated because mucus contains very high per- centages of carbon. However, mucus did not evolve exclusively for this purpose, because this hypothesis is only applicable to Cnidaria (Bakshani et al., 2018). Later, other important functions to the evolution of mucus were attrib- uted, including capturing particles for feeding, preventing suffocation due to sediment, and providing a physical bar- rier that can keep pathogens out (Edmunds and Davies, 1989; Riegl & Branch, 1995; Wild et al., 2004). Sponges, belonging to the Phylum Porifera (which evolutionarily predates Cnidaria), have genes that could be precursors of mucins (Iwai et al., 2002; Lang et al., 2007; 2016), so some sponges are even capable of pro- ducing mucus (Biggerstaff et al., 2017; McGrath et al., 2017). Sponges are constantly in contact with environ- mental bacteria (found in abundance) (Kennedy et al., 2008), and as Cnidaria, have a distinct microbiota from the environmental one (Cooney et al., 2002; Frias-Lopez et al., 2002; Brown and Bythell, 2005; Savoca et al., 2019), with numerically fewer bacterial populations than Porifera (Ainsworth et al., 2010). The occurrence of microbes in mucus samples was confirmed by Scanning Electron Microscopy (SEM) And Transmission Electron Microscopy (TEM) microscopy analysis; however, they have not been found on the ep- ithelial surfaces of polyps or on the exumbrella of jellyfish (the external part of the umbrella) (Turk et al., 2021). Cnidaria is often exploited as model organisms in de- velopmental biology, yet the evolution of a functional mucus layer from invertebrates is rarely examined (Bak- shani et al., 2018), even though similarities are evident by comparing the cnidarians and vertebrates (including human) mucus composition (Lai et al., 2009). Bakshani et al. (2018), established the evolutionary conservation of the antimicrobial function of mucus by fo- cusing on mucin genes and structure of mucus secreting cells among invertebrates and vertebrate. In the review, the authors resumed how secreted mucins evolved in early metazoans as Cnidaria and the mechanisms lying at its bases is functional conserved from the first metazoans to Non -co mmerc ial us e o nly Mucus secretions in cnidarian, an ecological, adaptive and evolutive tool 85 mammals. CONCLUSIONS AND FUTURE RESEARCH NEEDS The importance of mucus in invertebrates, especially in Cnidarians, is evident, as it is a fundamental secretion for the biology of these organisms and the marine ecosystems they inhabit. Scleractinian corals have long been studied for their importance as ecosystem engineers and often used as model organisms for the study of symbiosis and recip- rocal nutrient exchange. However, in examining the current state of knowledge on cnidarian mucus secretion, we found several gaps to be filled in the future. i) Further efforts are required to determine and deepen the role played by the microbiome, its composition in species and the ecological implications of its species- specific variability, dependent on the life stage, or due to environmental stresses. Similarly, it is not clear whether there is a substantial difference between organ- isms adhered to the surface and those present in the mucus and whether they act differently in the recycling of matter and/or in the flow of energy in coral ecosys- tems. ii) Mucus production by scleractinian corals is prolific, but the amount of dissolved or particulate forms of this se- cretory material and their fate in the environment are still unresolved. iii) Although the involvement of mucus in the innate im- mune system of cnidarians is clear, there are still dis- crepancies that prevent a complete evolutionary understanding of mucus. In this, and for many other as- pects relating to the biological and ecological sphere of the cnidarians, studies on the hologenome could be par- ticularly useful, involving all members of the holobiont and accepting the fact that this acts as a single unit of evolution, where the microorganisms, by adapting more rapidly, provide the plasticity necessary for the host to adapt to the rapidly changing, and probably for the worse, environment. iv) Mucus secretion by jellyfish is a poorly explored field, although it has important ecological functions in antimi- crobial defense, chemical defense and protection from environmental stress. The few existing studies on the jellyfish-microorganism association demonstrate the importance of these associations in various fundamental aspects for the life cycle, well-being and the ecological role played by these organisms. However, further stud- ies on the interannual and seasonal dynamics of the as- sociated jellyfish microbial community and possible variations under the pressure exerted by anthropogenic factors are needed. Furthermore, new investigations are needed to produce potential common models of impacts of jellyfish biomass and their mucus production on both marine microbial communities and ecosystems world- wide. In particular, the link between jellyfish, mucus and microorganisms as final recipients and recyclers of the oceanic dissolved organic matter should be ad- dressed more thoroughly. Overall, an increased focus on this topic will contribute to understanding the dy- namics of marine biodiversity and related ecological processes, as well as addressing the implications arising from climate change. v) It is somewhat demonstrated that the increase in mu- cous secretion in cnidarians has a high energy cost, which very often translates into an increased vulnera- bility of the organism. However, although described for a few coral species, it is largely misunderstood for cnidarians what the energy trade-off between costly functional traits (growth and reproduction) and main- tenance/immunity is and how much energy investment is required for increasing mucus secretion in the An- thropocene era. In conclusion, it is strongly suggested for these future studies that the combined use of ECO-EVO-DEVO ap- proaches and ecological genomics would allow the indi- vidual disciplines and fields of investigation to enrich each other and would allow to improve not only the un- derstanding of this topic in an evolutionary key, but also of the ecological, biological, and genetic mechanisms un- derlying the responses of organisms to their natural envi- ronments. Corresponding author: Gioele Capillo, Department of Veterinary Sci- ences, University of Messina, Polo Universitario dell'Annunziata, 98168, Messina (Italy). E-mail: gcapillo@unime.it Authors’ contributions: All the authors made a substantive intellec- tual contribution. All the authors have read and approved the final version of the manuscript and agreed to be accountable for all aspects of the work. Conflict of interest: The authors declare no potential conflict of in- terest. Funding: None. Availability of data and materials: All data generated or analyzed during this study are included in this published article. Key words: Cnidarian, mucus, ecological function, evolution, jelly- fish, polyps. Received: 30 November 2022. Accepted: 16 December 2022. Publisher’s note: All claims expressed in this article are solely those of the authors and do not necessarily represent those of their affiliated organizations, or those of the publisher, the editors and the reviewers. Any product that may be evaluated in this article or claim that may be made by its manufacturer is not guaranteed or endorsed by the publisher. ©Copyright: the Author(s), 2022 Licensee PAGEPress, Italy Advances in Oceanography and Limnology, 2022; 13:11054 DOI: 10.4081/aiol.2022.11054 This work is licensed under a Creative Commons Attribution- NonCommercial 4.0 International License (CC BY-NC 4.0). Non -co mmerc ial us e o nly S. Savoca et al.86 REFERENCES Aglieri, G., Papetti, C., Zane, L., Milisenda, G., Boero, F., Piraino, S., 2014. First evidence of inbreeding, relatedness and chaotic genetic patchiness in the holoplanktonic jellyfish Pelagia noc- tiluca (Scyphozoa, Cnidaria). PLoS One. 9:e99647. Ainsworth, T.D., Thurber, R.V., Gates, R.D., 2010. The future of coral reefs: a microbial perspective. Trends Ecol. Evol. 25:233–40. Albano, M., Panarello, G., Di Paola, D., D’Angelo, G., Granata, A., Savoca, S., Capillo, G., 2021. 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