Layout 1 HOLOTHURIAN’S EXPLOITATION The worldwide consumption of fish food products from 1961 to 2017 increased at an average annual rate of 3.1%, with a consumption per capita of fish food raising from 9.0 kg to 20.5 kg in the same period (FAO, 2020). Although the wild caches have been followed by the development of fish farming, the state of the wild stocks has continued to decline with less than 66% of the stocks harvested in a sus- tainable way (FAO, 2020). In response to the over-exploita- tion of wild finfish stocks, the invertebrate fisheries rapidly increased, being a new available source of seafood proteins and socio-economic opportunities (Berkes et al., 2006, An- derson et al., 2008). Many of the new target species now belong to low trophic levels, as a response to the overall down effect of trophic webs caused by top predators (Pauly et al., 2002; Anderson et al., 2008). In many cases, the pres- sure on stocks within low trophic levels increased faster than their management policies (Anderson et al., 2011a, 2011b), causing the spread of unregulated fishery and rais- ing concerns for the possible consequences on ecosystem functioning and the sustainability of the fishery (Andrew et al., 2002; Leiva and Castilla 2002; Berkes et al., 2006; Anderson et al., 2008; FAO, 2008). Sea cucumbers, marine invertebrates belonging to the Echinodermata Phylum, include more than 1500 species (Horton et al., 2018) and, mainly being deposit feeders, represent a good example of low trophic level organisms. Their fisheries had rapidly grown and expanded since 1980 as a consequence of the increasing demand from in- ternational markets, aquaculture and biomedical research programs (Bordbar et al., 2011). Holothurians are present in almost all the marine biotopes, from the littoral to hadal depths (Purcell et al., 2012). Holothurians are part of the Chinese culinary tra- dition, are considered gourmet and luxury seafood and are generally sold as a dried product called bêche-de-mer or trepang (Wen et al., 2010; Yang and Bai, 2015). The mar- ket price of this product depends on the quality (grade low, medium, high) (Ram et al., 2014), with some partic- ularly valuable species as Apostichopus japonicus Se- lenka, which holds the highest price of 2950 US$ dried kg−1, followed by Holothuria scabra Jaeger, 1833 (115- 640 US$ dry kg−1), Holothuria lessoni Massin, Uthicke, Purcell, Rowe & Samyn, 2009 (240-790 US$ dry kg−1) (Purcell et al., 2012). The presence of high-value nutrients such as Vitamin A, Vitamin B1 (thiamine), Vitamin B2 (riboflavin), Vita- min B3 (niacin), and minerals (i.e., calcium, magnesium, iron and zinc) indicate that sea cucumbers are suitable tonic and restorative products, also rich in crude proteins (range 41-63%) (Wen et al., 2010, Bordbar et al., 2011). Moreover, sea cucumbers, containing a number of bi- ological and pharmacological bioactive compounds, have attracted attention for their potential medical value (Bor- dbar et al., 2011). Sea cucumbers contain numerous bioactive and anti-age substances that are already ex- ploited in the cosmetic and pharmaceutical industries (Fredalina et al., 1999; Saito et al., 2002; Zhao et al., 2007; Bordbar et al., 2011; Purcell, 2014). All these prop- erties and the high market price led to the overexploitation and decline of sea cucumbers Indo-Pacific populations and the expansion of the fishery to reach new virgin stocks in Galapagos Islands, Mexico, North America and the Mediterranean Sea (Conand, 2006; Purcell et al., 2012; Gonzàlez-Wangüemert et al., 2018). The estimated sea cucumbers harvest, from Asia and Pacific regions, ranges from 20.000 to 40.000 t per year of the dry product (FAO, 2012). Fisheries from African and Indian Ocean regions also contribute to the complex amount with the range of 2000-2500 t per year (FAO, 2012). REVIEW Biology, ecology and management perspectives of overexploited deposit-feeders sea cucumbers, with focus on Holothuria tubulosa (Gmelin, 1788) Viviana Pasquini,*1 Ambra Angelica Giglioli,1 Antonio Pusceddu,1 Pierantonio Addis1 1Department of Life and Environmental Sciences, University of Cagliari, Via T. Fiorelli 1, 09126 Cagliari, Italy ABSTRACT The increasing harvesting of low trophic level organisms is raising concern about the possible consequences on the ecosystem functioning. In particular, the continuous demand of sea cucumbers from the international market led to the overexploitation of either traditionally harvested or new target species, including the Mediterranean ones. Sea cucumbers are mostly deposit feeders able to consume sedimentary organic matter and, thus, are ideal candidate for the remediation of eutrophicated sediments, like those beneath aquaculture projects. Breeding and restocking of overexploited sea cucumbers populations are well-established prac- tices for Indo-Pacific species like Holothuria scabra and Apostichopus japonicus. Some attempts have also been made for the Mediterranean species Holothuria tubulosa, but, so far, the adaptation of protocols used for other species has presented several is- sues. We here summarize narratively the available information about sea cucumbers rearing protocols with the aim of identifying their major flaws and gaps of knowledge and fostering research about new triggers for spawning and feasible protocols to reduce the high mortality of post-settlers. No n- co mm er cia l u se on ly Biology, ecology and management of deposit-feeders sea cucumbers 37 Less information are available about sea cucumbers fisheries in the Mediterranean Sea, in particular for Holothuria tubulosa Gmelin, 1788, Holothuria mammata Grube, 1840, Holothuria sanctori Delle Chiaje, 1823, Holothuria forskali Delle Chiaje, 1823, Parastichopus re- galis Cuvier, 1817, and Holothuria arguinensis Koehler & Vaney, 1906 (Çakly et al., 2004; Antoniadou and Vafidis, 2011; Sicuro and Levine 2011; Gonzàlez-Wangüemert and Borrero-Perez, 2012; Mezali and Thandar 2014; Gonza- lez-Wangüemert et al., 2014a, 2015). Presently, more than half of global sea cucumber fisheries are considered de- pleted or overexploited to the extent that governments (in- cluding the Italian Government) have banned their harvesting (Anderson et al., 2011; González-Wangüemert et al., 2014, 2018). With the 38% of sea cucumber fisheries currently unregulated and an unknown level of illegal catches, this fishery is considered unsustainable and far from being adequately managed (Anderson et al., 2011; Choo, 2008; Toral-Granda, 2008). The unregulated exploitation of sea cucumbers is a ris- ing concern for their conservation, with 16 species world- wide now classified as “vulnerable” or “endangered”, according to the IUCN Red list (Conand et al., 2014, Ramírez-González et al., 2020). Concern also raises be- cause most of the harvested sea cucumbers are deposit- feeders, thus playing an ecological key role due to their feeding behaviour (Uthicke, 2001; Roberts et al., 2000), their decline could have severe consequences on sedimen- tary biogeochemistry and benthic ecosystem functioning. Here we reviewed the available information about the ecological role of sea cucumbers, with a focus on the Mediterranean H. tubulosa, their breeding, fishery man- agement issues, main gaps of knowledge and future per- spectives for their use as remediation of eutrophicated sediments. LIFE HISTORY AND POPULATION DYNAMICS OF SEA CUCUMBERS The increasing interest towards sea cucumbers and their use for food, medical and habitat remediation purposes, stimulated exploration about their reproductive cycle and population dynamics, both crucial aspects for the assess- ment of wild stocks and their eventual management. Almost all sea cucumbers are broadcast spawners with external fertilization that present an annual or bi-annual maturation season (Mercier and Hamel, 2009; Mohsen and Yang, 2021). With a few exceptions of hermaphrodite species, they are generally gonochoric that leak in sexual dimorphisms (Smiley et al., 1991; Mercier and Hamel, 2009). The life cycle of sea cucumbers is characterised by one or more planktonic larval stages starting with a feed- ing auricularia (early, mid and late), a non-feeding dolio- laria and then a feeding pentactula that settle on the substrate (Strathmann, 1975; Ito and Kitamura, 1997; Yanagisawa, 1998). Doliolaria actively explore the surrounding environ- ment to identify the best place to settle and made the last metamorphosis into the pentactula. If the conditions are not suitable for settlement, the larvae will keep swimming for several days (Mercier et al., 2000). The pentactula lose the ability to swim but can continue to explore the sur- rounding environment with the buccal podia, moving by small jumps (Mercier et al., 2000). Although rarely, Evans and Palmer (2003) reported the ability of the pentactula larvae of Parastichopus californicus Stimpson, 1857, to clone, forming a bud that, after separation, will normally develop into an auricularia larvae. The pentactula larvae will start to feed and grow, be- coming a juvenile in a variable time lag (Mercier et al., 2000; Agudo, 2006; Mercier and Hamel, 2009; Rakaj et al., 2018, 2019). Information about the mechanisms of settlement, physiology and cue that can stimulate the lar- vae to settle are poorly explored and understood, so far. Studies conducted in mesocosm investigated the success of the larval settlement, which can strongly depend on the larval nutrition state and the capacity to accumulate lipids (Peters-Didier and Sewell, 2019). In the late au- ricularia stage of H. scabra, the development of the hya- line spheres indicates an adequate feeding, and their size is a reliable indicator for subsequent performance (Duy et al., 2016). The settlement and the last metamorphosis, as for other echinoderms, represents a survivorship bot- tleneck that can lead to high mortality rates. The early juvenile stage (<5 mm length) is also vulnerable and a critical phase with substantial mortality rates (Agudo, 2006; Rakaj et al., 2018). The holothurians recruitment has been studied mainly on historically exploited species, and information about post-settlers and juveniles in the field is scarcely recorded in the literature and, even, referred to sporadic occasions. For instance, the recruitment of H. scabra has been found to occur on a monthly time scale on seagrasses, with adult specimens mainly observed in sandy sediments and juve- niles in organic matter (OM) enriched muddy sediments (Mercier et al., 2000). The lack of other information about holothurians recruitment can also be ascribed to the poten- tial misidentification of the species because they can have a considerably different morphology when compared with that of adults. Besides this, juveniles might occupy differ- ent habitats and can be obscured from the researchers’ view because of their cryptic behaviour (Shiell, 2004). H. scabra juveniles can also be affected by predation-medi- ated mortality by fish belonging to the Balistidae, Labri- dae, Lethrinidae and Nemipteridae families (Dance et al., 2003), sea stars, and crustaceans (Kinch et al., 2008). Holothurians’ recruitment can also be affected by geo- graphic distances, the duration of the larval period and to No n- co mm er cia l u se on ly V. Pasquini et al.38 the hydrodynamic retainment in coastal areas (Uthicke, et al., 1998, 1999, 2001; Uthicke and Purcell, 2004). Most studies about holothurians’ population dynamics explored species with a long history of exploitation, in- cluding A. japonicus, Cucumaria frondosa Gunnerus, 1767, and Isostichopus fuscus Ludwig, 1875, (Herrero- Pérezrul et al., 1999; Reyes-Bonilla and Herrero-Pérezrul, 2003; Hamel and Mercier, 2008; Anderson et al., 2011; Purcell et al., 2011; Yang et al., 2015; Glockner-Fagetti et al., 2016). Unfortunately, the absence of a rigid struc- ture in sea cucumbers and the high plasticity of the body wall make it difficult to investigate the growth rates of holothurians. Alternative methods proposed include mark- ing the calcareous (epi-pharyngeal) ring, chemical mark- ing of spicules, external and internal tagging (Kinch et al., 2008). However, all of these methods are affected by wide methodological biases but also by the bio-ecological traits of holothurians. In fact, the body size of holothurians can vary as a response to changing environmental conditions (Tolon et al., 2017b), the occurrence of asexual reproduc- tion through fission (Purwati and Dwiono, 2005; Uthicke and Conand, 2005; Laxminarayana, 2006; Purwati and Dwiono, 2007; Purcell et al., 2012; Dolmatov, 2014,2021) or the evisceration of their internal organs (intestine, go- nads and respiratory trees) through autotomy, in response to predation and other environmental stressors (Shukalyuk and Dolmatov, 2001; Wilkie, 2001; Spirina and Dolmatov, 2003; Zang et al., 2012). The evisceration is a typical be- havioural trait of holothurians that does not lead to the death of the organism, rather is followed by the re-growth of the internal organs (Dawbin, 1949; Murray and Gar- cía-Arrarás, 2004; García-Arrarás et al., 2006; Dolmatov and Ginanova, 2009). Interestingly, after evisceration, the respiratory function shifts to the body wall for the time necessary for the respiratory trees’ regrowth. During this period, sea cucumbers will consume endogenous sub- stances, which causes a significant body weight loss (Zang et al., 2012, Zhang et al., 2017). Because of the multiple factors regulating holothurians body size, small individuals are not necessarily the youngest ones (Kinch et al., 2008). BREEDING OF SEA CUCUMBERS The development of sea cucumbers ex situ breeding protocols derived from the need to reduce the pressure on wild overexploited stocks. Breeding sea cucumbers can be used for restocking activities (Purcell and Kirby, 2006) as already explored for other exploited echinoderms (Cou- vray et al., 2015; Giglioli et al., 2021). Moreover, produc- ing and releasing juveniles sea cucumbers reared in “conservation hatchery”, could be a useful tool for biore- mediation of eutrophicated sediments or in integrated multi-trophic aquaculture systems (see below) without burden on wild populations. The experimental reproduc- tion of sea cucumbers has been carried out for many species and the aquaculture is now established for largely exploited Indo-Pacific species like H. scabra (Agudo, 2006) and A. japonicus (Purcell et al., 2012; Shi et al., 2013, 2015; Pietrak et al., 2014). China, the largest consumer and producer country, is breeding annually about 10 000 t of dry weight A. japon- icus from aquaculture to supply the local demand, while in other countries this activity is still in a pilot scale or in early development stages (Choo, 2008). It has been esti- mated that once released in the field H. scabra can reach the commercial size of 700 g ind-1 in about 2-3 years, with a survivorship of 7-20% (Purcell and Simutoga, 2008). In the last decade, new attempts have been also made with the Mediterranean species H. tubulosa and Holothuria polii Delle Chiaje, 1823, (Rakaj et al., 2018, 2019); H. ar- guinensis (Domínguez-Godino et al., 2015); H. mammata (Domínguez-Godino and González-Wangüemert, 2018). FEEDING BEHAVIOR AND ECOLOGICAL ROLE OF SEA CUCUMBERS Deposit-feeders holothurians acquire food by swal- lowing large volumes of sediment (Ramon et al., 2019). They sift through the sediment with tentacles and feed on detritus, organic matter, sand and the relative grown-over biofilm, expelling sandy pellets after digestion (Hartati et al., 2020). The feeding starts with capturing the sedimentary food particles with tentacles and their release into the pharynx through the circum-oral tentacles. Once inside the mouth the particles are mixed with the digestive enzymes and compressed into a plug which moves throughout the gut following a plug-flow reactor model. The plug is then transported by peristalsis along the simple digestive sys- tem that ends in the posterior part of the animal (Zamora and Jeffs, 2011). Sea cucumbers predominantly feed on sedimentary or- ganic detritus associated with micro-organisms and small benthic organisms (Roberts et al., 2000). In the gut min- eral and organic particles are found along with fragments of shell, barnacles, seagrasses, echinoderms ossicles, fae- cal pellets, foraminifera shells, with a highly variable size (Roberts et al., 2000). Information about the potential selectivity of shallow- water holothurians is controversial. Some holothurians are able to choose OM enriched particles, whereas others ap- pear not to be (Moriarty 1982; Hammond, 1983; Uthicke and Karez, 1999; Battaglene et al., 1999; Slater et al., 2011; Navarro et al., 2013; Sun et al., 2015; Lee et al., 2018; Hartati et al., 2020). The selective ability can be re- lated to how sea cucumbers feed on the sediment, which is highly variable among species, depending on their ten- No n- co mm er cia l u se on ly Biology, ecology and management of deposit-feeders sea cucumbers 39 tacles dimension, the size and gut morphology (Roberts et al. 2001, Dar and Ahmad, 2006; Ramón et al., 2019). The selection of smaller organic-rich particles might be due to the greater ease of being caught and held by the tentacles, or to the potential chemo-selection ability of holothurians (Schneider et al 2013; Lee et al., 2018). The presence of a higher OM content in the gut compared to the one present in the sediment can be a consequence of a passive selection of the finest grain size of the particles which can be more easily ingested. This, in turn, can be explained because smaller grain size particles can have a higher OM content due to the wider surface available for the microbial colonization (Hargrave, 1972; Levinton, 1972; Dale, 1974; Yamamoto and Lopez, 1985; Manini and Luna, 2003). Considering their feeding behaviour, sea cucumbers are great seafloor bioturbators, able to rework large amounts of sediments via ingestion and excretion (9-82 kg ind−1 year−1) which can extensively blend and reform seafloor substrata (Coulon and Jangoux, 1993; Uthicke and Karez, 1999; Mangion et al., 2004). Bioturbation in- tensity can influence the sediment permeability, oxygen concentration, water content and chemical gradients in pore water, affecting the rate of remineralization and the inorganic nutrient flux and, finally, can redistribute food resources for the other benthos (Reise, 2002; Lohrer et al., 2004; Solan et al., 2004; Meysman, 2006a). Biotur- bation carried out by sea cucumbers can be circumscribed to the upper layer of the sediment or reach up to ten cen- timetres depth based on the habits of the species whether they are fossorial or not (Uthicke and Karez, 1999; Pur- cell, 2004a; Amaro et al., 2010). The role of holothurians in recycling the sedimentary OM is considered one of their main ecosystem functions (Purcell et al., 2016). The ability to reduce the OM con- tent in the sediment has been recently investigated (Dar and Ahmad, 2006; İşgören-Emiroğlu and Günay, 2007; Slater and Carton, 2009; Wolkenhour et al., 2010; Zamora and Jeffs, 2011; Tolon et al., 2017a; Neofitou et al., 2019; Hartati et al., 2020). The sea cucumber Australostichopus mollis Hutton, 1872, can significantly reduce total organic carbon (TOC), chlorophyll-a and phaeopigments contents of sediments impacted by green-lipped mussel biodeposits (faeces and pseudofaeces) (Slater and Carton, 2009). MacTavish et al. (2012) reported that A. mollis suppressed benthic microalgae and facilitated bacterial activity, caus- ing a shift in the balance of benthic production and de- composition processes. Juveniles of the same species decreased their ingestion rate with the increasing of the total sedimentary organic matter (TOM), showing the ability of this species to use different amounts of TOM, changing their feeding behaviour and digestive physiol- ogy (Zamora and Jeffs, 2011). H. tubulosa reduced the sedimentary OM and organic carbon (OC) by 31-59%, with an absorption rate of 43 and 55% respectively, both in manipulative laboratory and field experiments (Neofi- tou et al., 2019). The functioning of the digestive system of holothurians has been modelled and defined as a sort of ‘bioreactor’, where the ingested nutrients are quickly extracted and as- similated (Penry and Jumars, 1986, 1987; Jumars, 2000; Amaro et al., 2010). The grazing of holothurians could in- crease the exchange flux of nutrients across the sediment- water interface and promote nutrient regeneration (Zhou et al., 2006; Yuan et al., 2013; Slater and Carton, 2009; Slater et al., 2011; Zamora and Jeffs, 2011, 2012a, b). On the other hand, other species, like A. japonicus, could not affect TOC and total nitrogen (TN) sedimentary contents, but can cause OM particles redistribution and inhibit mi- crophytobenthos (Michio et al., 2003). THE MEDITERRANEAN SEA CUCUMBER HOLOTHURIA TUBULOSA A new target species candidate for sea cucumbers aquaculture is Holoturia tubulosa (Gmelin 1788), one of the most common and widespread holothurians in the coastal areas of the Mediterranean Sea and the Eastern At- lantic Ocean (Tortonese, 1965; Koukouras et al., 2007). In the last few years, H. tubulosa has been actively harvested in Turkey, Greece, Italy, Spain and the increasing of illegal and unregulated fishing is one of the main issues for its management (Rakaj et al., 2019). Overexploitation of this species led the Italian Ministry of Agriculture, Food and Forestry (MIPAAF) to ban sea cucumbers fishing along the entire national coastline (Ministerial decree 156/2018), as a precaution for the conservation of the species. H. tubulosa is a continuous deposit-feeder, generally encountered in organic matter enriched soft bottoms and seagrass meadows (Bulteel et al., 1992; Gustato et al., 1982). Coulon and Jangoux (1993) reported that large in- dividuals of H. tubulosa might ingest up to 17 kg of dry weight sediment ind-1 y-1. Using the data provided by Costa et al. (2014) it can be estimated that the quantity of seagrass detritus potentially ingested by H. tubulosa ranges between 12 and 28 g dry weight m−2 y−1 ind-1. The reproductive cycle of H. tubulosa was studied in specimens from the Adriatic Sea, Oran coast (Algeria) and Dardanelles Strait (Turkey). The development stages of male and female gonads showed a clear annual pattern and all authors agreed that the spawning period was set between June and October with minor local differences, and a resting period from October to January (Despala- tovic et al., 2004; Ocaña and Tocino, 2005; Dereli et al., 2015; Tahri et al., 2019). Rakaj et al. (2018) successfully bred and reared H. tubulosa in the laboratory, completing the larval development in 27 days, which, however, was followed by high mortality shortly after the settlement. A No n- co mm er cia l u se on ly V. Pasquini et al.40 recent study reported the use of H. tubulosa larvae as new model for embryo-larval bioassays to assess marine pol- lution (Rakaj et al., 2021), but, to date, rearing techniques of this species remain still not very efficient. SEA CUCUMBERS IN INTEGRATED MULTI-TROPHIC AQUACULTURE In the last two decades, to satisfy the demand for seafood product, aquaculture activities increased and the need to mitigate its impacts on the environment became an urgent need, especially in the presence of vulnerable habits like seagrass beds (Pusceddu et al., 2007; Holmer et al., 2008). Wastes coming from mariculture plants can affect sediments biochemistry, increasing the organic con- tents, ultimately exacerbating eutrophication (David et al., 2009; Keeley et al., 2014). In fact, wastes from maricul- ture can cause benthic hypoxia and anoxia, hydrogen sul- phite enrichment and, in extreme cases, also led to rising of methanogenentic bacteria populations, which, in turn, can significantly impact the abundance and biodiversity of benthic organisms (Karakassis et al., 2000; Angel et al., 2002; Mirto et al., 2002; Burford et al., 2003; La Rosa et al., 2004; Fodelianakis et al., 2015). The conceptual approach of integrated multi-trophic aquaculture (IMTA) is to use different trophic-levels or- ganisms in the same system: those belonging to the high- est trophic level (generally fish) are fed artificially and those belonging to the lowest trophic level (extractive species) feed on waste released by the specimens of the highest trophic level (Troell, 2009; Granada et al., 2015). The extractive species commonly used in IMTA include molluscs, seaweeds or detritivorous species (Zhou et al., 2006; Slater and Carton, 2007; Yuan et al., 2013; Slater et al., 2009; Zamora and Jeffs, 2011, 2012a, 2012 b; Lam- prianidou et al., 2015; Shpigel et al., 2018). Among de- tritivorous species, considering their feeding habits, sea cucumbers appear to be ideal candidates as extractive species for IMTA systems. Commercially valuable holothurians species most used in IMTA systems include A. japonicus (Zhou et al., 2006; Yuan et al., 2013; Kim et al., 2015), A. mollis (Slater and Carton, 2007; Slater et al., 2009; Zamora and Jeffs, 2011, 2012a, 2012b), and P. californicus, (Paltzat et al., 2008), mainly fed with scallops and mussels’ biode- posits alone, or mixed with powdered algae (Yuan et al., 2006). Other small-scale experiments used Actinopyga bannwarthi Panning, 1944 (Israel et al., 2019) and H. scabra (Mathieu-Resuge et al., 2020). The IMTA feasibility in the Mediterranean Sea is still in an experimental scale, whereas either pilot or commer- cial scale activities have been carried out in other regions (MacDonald et al., 2013; Marinho et al., 2013; Lampri- anidou et al., 2015). To our best knowledge, only two studies investigated the use of H. tubulosa in IMTA sys- tems in the Mediterranean Sea. Beneath fish cages, Tolon et al. (2017b) observed a biomass increase of holothurians ranging from 9 to 31 g ind-1 in just 90 days and suggested that these animals are ideal candidates to mitigate in IMTA the benthic eutroph- ication generated by fish farming. Neofitou et al. (2019) during an experiment carried in the field beneath farming cages of the sea bream S. aurata and the sea bass Dicen- trarchus labrax Linnaeus, 1758, reported that the maxi- mum extractive capacity of holothurians is reached at a density of ca. 10 individuals m-2. Such a density allowed abating OM and OC contents in sediments beneath the cages by 31 and 59%, respectively. These results, though spatially and temporally fragmented, corroborate the idea of using sea cucumbers beneath fish cages, in IMTA sys- tems, to mitigate the impacts of biodeposition on the sed- iment, at the same time providing a commercially important by-product, without any additional feed. With these assumptions, it can be envisaged that sea cucumbers in IMTA will increase the environmental sustainability of aquaculture and will also generate an important economic advantage, due to the high value of sea cucumbers. HOLOTHURIANS’ MANAGEMENT PERSPECTIVES The ecological consequences of holothurians overex- ploitation include a loss in bioturbation and a consequent reduction of benthic biomass, biodiversity, and ecosystem functioning (Lohrer et al., 2004; Solan et al., 2004; Meysman et al., 2006b). Therefore, sea cucumbers’ over- exploitation claims for urgent measures to preserve natu- ral populations and their ability to provide reproductive adults for either natural or artificial breeding. On the one hand, the peculiar biological and ecologi- cal traits of holothurians and the lack of reliable stock as- sessments make a scientific based management of this resource still far to be reached. Management and regula- tion of sea cucumbers fishery are currently being imple- mented in some countries, using different approaches. Among these, for example, a rotational zone strategy has been applied to the multispecies sea cucumber fishery in Australia’s Great Barrier Reef Marine Park, where this approach led to a substantial reduction of the risk of lo- calized depletion, higher long-term yields, and improved economic performance (Plagányi et al., 2015). To guarantee significant recruitment in an acceptable timeframe, future management policies of sea cucumbers should set a minimum population density threshold, below which exploitation should be banned (Battaglene and Bell, 2004), also to avoid the Allee effect, which oc- currence has been reported for overexploited populations of H. scabra in the Warrior Reef, Australia (Skewes et al., No n- co mm er cia l u se on ly Biology, ecology and management of deposit-feeders sea cucumbers 41 2000), I. fuscus in the Galapagos Marine Reserve, Ecuador (Toral-Granda and Martinez, 2007), and H. no- bilis (Selenka 1867) in the Great Barrier Reef, Australia (Uthicke and Benzie, 2000) and Holothuria mexicana Ludwig, 1875 (Rogers et al., 2018). Ultimately, we notice that adequate protocols of holothurians’ populations management still need large amount of quantitative information about their population dynamics, recruitment success, rates of growth and natural mortality (Romero-Gallardo et al., 2018), mechanisms al- lowing larval settlement. Concurrently, studies aiming at identifying new and more efficient ex situ rearing proto- cols, also to feed restocking actions and to preserve the natural genetic pools (Purcell, 2004b, Purcell and Kirby, 2006; González-Wangüemert et al., 2015) are also needed. ACKNOWLEDGMENTS This study has been carried out in the framework of the projects: “Marine habitats restoration in a climate change- impaired Mediterranean Sea [MAHRES]”, funded by the Ministero dell’Istruzione dell’Università e della Ricerca under the PRIN 2017 call (Protocol: 2017MHHWBN; CUP F74I19001320001); “Innovative species of commercial in- terest for Sardinian aquaculture: development of experimen- tal protocols for the breeding of sea cucumbers, (project n. 1/INA/2.47/2017)” founded by the European Maritime and Fisheries Fund (EMFF) programme 2014/2020, Measure: 2.47 – Innovation; and co-founded by the “lnEVal: lncreas- ing Echinoderm Value Chains” (grant n. ID 101 InEVal) founded by ERA-NET BlueBio programme. Corresponding author: v.pasquini@studenti.unica.it Key words: Sea cucumber; Holothuria tubulosa; biology; ecology; review. Contributions: All the authors have read and approved the final version of the manuscript and agreed to be accountable for all as- pects of the work. Conflict of interest: The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Availability of data and materials: All data generated or analyzed during this study are included in this published article. Received: 21 July 2021. Accepted: 12 Octobre 2021. This work is licensed under a Creative Commons Attribution Non- Commercial 4.0 License (CC BY-NC 4.0). ©Copyright: the Author(s), 2021 Licensee PAGEPress, Italy Advances in Oceanography and Limnology, 2021; 12:9995 DOI: 10.4081/aiol.2021.9995 REFERENCES Agudo N, 2006. Sandfish hatchery techniques. Australian Centre for International Agricultural Research, Secretariat of the Pacific Community and WorldFish Center. 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