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Advances in Oceanography and Limnology, 2016; 7(1): 1-15 REVIEW
DOI: 10.4081/aiol.2016.5591

INTRODUCTION

The genus Ostreopsis belongs to the family of Ostre-
opsidaceae (Gonyaulacales, Dinophyceae, Dinophyta). It
was first described by Schmidt (1901) after the finding of
O. siamensis Schmidt in plankton samples collected in the
Gulf of Siam in 1900. Since then, this species has rarely
been encountered in the phytoplankton, due to its predom-
inantly benthic habit. 

Ostreopsis species were reported since a long time in
tropical ciguatera endemic areas, associated with the ben-
thic toxic dinoflagellate Gambierdiscus toxicus (Ballan-
tine et al., 1985; Carlson and Tindall, 1985; Bomber and
Aikman, 1989) and, therefore, improperly considered in
association with ciguatera syndrome (Tosteson, 1995). In-
deed, some Ostreopsis species are toxic, but their toxins
(mostly belonging to the palytoxin group) are not those
implicated in ciguatera.

In the last decade, Ostreopsis blooms have become
common in temperate areas as well, and regularly occur

in the Mediterranean Sea during summer-autumn (Vila et
al., 2001; Turki, 2005; Aligizaki and Nikolaidis, 2006;
Mangialajo et al., 2008; Totti et al., 2010; Illoul et al.,
2012; Ismael and Halim, 2012; Pfannkuchen et al., 2012),
and in other temperate areas of the world (Chang et al.,
2000; Rhodes et al., 2000; Pearce et al., 2001; Taniyama
et al., 2003; Shears and Ross, 2009; Selina et al., 2014).
In these areas, Ostreopsis is well-known since its blooms
are often associated with noxious effects on health of both
humans (Gallitelli et al., 2005; Kermarec et al., 2008;
Tichadou et al., 2010; Del Favero et al., 2012) and benthic
marine organisms (Pagliara and Caroppo, 2012; Gorbi et
al., 2013; Carella et al., 2015). Additionally, Ostreopsis
often appeared in association with other toxic or poten-
tially toxic benthic dinoflagellates such as Prorocentrum
spp., Amphidinium spp. and Coolia monotis in both sev-
eral Mediterranean (Tognetto et al., 1995; Vila et al.,
2001; Aligizaki and Nikolaidis, 2006; Monti et al., 2007;
Mabrouk et al., 2011) and world areas (Okolodkov et al.,
2007; Parsons and Preskitt, 2007; Kim et al., 2011; Selina
and Levchenko, 2011).

The toxic benthic dinoflagellates of the genus Ostreopsis in temperate areas:
a review

Stefano Accoroni,* Cecilia Totti

Dipartimento di Scienze della Vita e dell'Ambiente, Università Politecnica delle Marche, via Brecce Bianche, 60131 Ancona, Italy
*Corresponding author: s.accoroni@univpm.it

ABSTRACT
The genus Ostreopsis includes species largely distributed from tropical to temperate marine areas worldwide. Among the nine

species of the genus, O. siamensis, O. mascarenensis, O. lenticularis and O. cf. ovata can produce toxins of the palytoxin group. In the
last decade Ostreopsis cf. ovata and O. cf. siamensis originated intense blooms in all the rocky Mediterranean Sea coastal areas, typically
during summer-late summer. The correct identification of Ostreopsis species in field samples is often problematic as Ostreopsis species
are morphologically plastic and hardly discriminable under light microscopy and, therefore, molecular analyses are required. Ostreopsis
blooms are often associated with noxious effects on health of both humans and benthic marine organisms mainly carried by aerosol and
direct contact with seawater. Environmental factors have been shown to affect toxin content of Ostreopsis which generally produces
more toxins per cell when growing under suboptimal conditions. O. cf. ovata is able to produce both temporary and resting cysts. In
particular, the resting cysts are able to germinate in laboratory conditions for as long as 5 months after their formation at 25°C, but not
at 21°C; the presence of a temperature threshold affecting cyst germination in the laboratory suggests that temperature represents a key
factor for Ostreopsis cf. ovata bloom onset in natural environments as well. Several studies conducted to assess the role of abiotic
factors (mainly hydrodynamics, water temperature and nutrients) on the bloom dynamics, revealed that the synergic effects of hydro-
dynamics, temperature and N:P ratios would lead the Ostreopsis blooms in temperate areas. Ostreopsis abundances showed a significant
decrease with depth, likely related to light availability, although there are conflicting data about the relationship between light intensity
and Ostreopsis growth in experimental conditions. The relationship between Ostreopsis blooms and salinity is not completely clear,
complicated by the influence of high nutrient levels often associated to low salinity waters. Finally, Ostreopsis colonize a variety of
substrata, although living substrata seems to allow lower concentration of epibionts than any other substrate, probably due to the pro-
duction of some allelopathic compounds.

Key words: Ostreopsis; palytoxin; harmful algae; benthic dinoflagellates; Mediterranean Sea.

Received: October 2015. Accepted: February 2016.

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2 S. Accoroni and C. Totti

Given their negative implications on both marine
ecosystem functioning and human health and activities,
Ostreopsis blooms attracted the attention of researchers
in the last decade. This paper is meant to be a narrative
review of the existing information about taxonomy, geo-
graphical distribution, toxin production, life cycle and
ecology of Ostreopsis in temperate areas.

TAXONOMY

There is a considerable confusion regarding the de-
scriptions of the Ostreopsis species, since its first descrip-
tion carried out by Schmidt (1901): in the original
drawings of Ostreopsis siamensis, both the epitheca and
the hypotheca in anteroposterior view were presented but
with marked different shapes (one appeared rounded and
the other appeared elongated) insomuch hardly they can
belong to the same cell. Much later, Fukuyo (1981) re-de-
scribed O. siamensis and described two new species O.
lenticularis and O. ovata. The rounded shape reported by
Schmidt for O. siamensis was one of the main differences
between O. lenticularis and O. siamensis, so probably in
the original description Schmidt confused these two
species. O. ovata was distinguished from the other two
species by having a more ovoid shape and a smaller size.
In the following years, several other species have been
described by other authors: O. heptagona (Norris et al.,
1985), O. mascarenensis (Quod, 1994), O. labens (Faust
and Morton, 1995), O. marinus, O. belizeanus and O.
caribbeanus (Faust, 1999) (recently, the latter three have
been renamed as O. marina, O. belizeana, O. caribbeana,
Hoppenrath et al., 2014). So, at present, nine species of
Ostreopsis have been described.

In the Mediterranean Sea, only two species have been
recorded until now, O. cf. ovata and O. cf. siamensis (Vila
et al., 2001; Penna et al., 2005, 2010, 2012; Battocchi et
al., 2010; Totti et al., 2010; Mangialajo et al., 2011; Perini
et al., 2011; Mabrouk et al., 2012). The correct identifi-
cation of these Ostreopsis species in field samples is often
highly problematic. The most suitable taxonomical char-
acter used to discriminate O. cf. ovata and O. cf. siamen-
sis, is the dorsoventral/anteroposterior diameter ratio
(DV/AP) which is <2 and >4, respectively (Penna et al.,
2005; Aligizaki and Nikolaidis, 2006; Selina and Orlova,
2010). However, O. cf. ovata cells in the Adriatic Sea
(Mediterranean Sea) have a DV/AP ratio slightly higher
than 2 (i.e., 2.3-2.4) (Monti et al., 2007; Guerrini et al.,
2010; Accoroni et al., 2012b). Since the molecular analy-
ses clearly confirmed the presence of only O. ovata
species in the N Adriatic samples (Perini et al., 2011), it
can be hypothesized that the Adriatic population has a dif-
ferent cell morphology, with cells more flattened than in
other areas. These morphological problems and the lack
of genetic data for the holotype specimens from which the

original species descriptions of both O. siamensis and O.
ovata were made, lead to the conclusion that O. siamensis
and O. ovata examined nowadays should be referred to
O. cf. siamensis and O. cf. ovata respectively, until more
accurate morphological data and genetic sequences will
be gathered to clearly define each species (Penna et al.,
2005, 2010).

Given these ambiguities in defining morphological
characteristics, many researchers have been induced to re-
vise the description of Ostreopsis species by sequencing
the ITS and 5.8S rDNA regions, using these data in com-
bination with morphometric ones. Leaw et al. (2001) iso-
lated several Ostreopsis strains from Malaysian coastal
waters and showed that O. cf. ovata isolates were sepa-
rated into two genetically distinct geographic groups, a
Malacca Strait group and a South China Sea group, while
there were minor morphological differences among the
strains. The 5.8S and ITS sequences of these Malaysian
strains differed from Mediterranean strains ones (Penna
et al., 2005), suggesting a genetic variability in relation
to the geographic distribution within the species. Nowa-
days, the ITS-5.8S and LSU rDNA allowed to distinguish
various clades among the Ostreopsis species (Penna et al.,
2014): the species complex Ostreopsis cf. ovata includes
the Atlantic/Mediterranean/Pacific clade (i.e., isolates
from Japan Sea, and Mediterranean Sea), the Atlantic/In-
dian/Pacific clade (i.e., isolates from Belize) and the Pa-
cific clade (i.e., isolates from Vietnam); Ostreopsis cf.
siamensis forms an Atlantic/Mediterranean clade. The Os-
treopsis cf. lenticularis/O. cf. labens contains isolates
from Hawaii and Pacific Asia.

DISTRIBUTION IN TEMPERATE AREAS

The first record of Ostreopsis in the Mediterranean Sea
dates back to 1972 in Villefranche-sur-Mer (France) by
Taylor (1979). Later, O. cf. ovata was detected in 1994
along both the Italian coasts of the Tyrrhenian Sea
(Tognetto et al., 1995) and the Catalan coast in Spain in
1997-1998 (Vila et al., 2001). In the last decade, Ostreopsis
spp. blooms have been more intense, frequent, and widely
distributed in many Mediterranean areas, including Spain,
France, Greece, Italy, Algeria, Tunisia, Turkey (Turki,
2005; Aligizaki and Nikolaidis, 2006; Ciminiello et al.,
2006, 2008; Riobó et al., 2006; Turki et al., 2006; Monti et
al., 2007; Riobó et al., 2008; Guerrini et al., 2010; Mabrouk
et al., 2011, 2012; Mangialajo et al., 2011; Illoul et al.,
2012). As reported above, genetic analyses indicate that two
genotypes corresponding to the morphotypes O. cf. ovata
and O. cf. siamensis are present in the Mediterranean Sea
(Penna et al., 2010, 2012). Along the Mediterranean rocky
coasts, the genotype ovata is the most abundant and widely
distributed (Battocchi et al., 2010; Perini et al., 2011). The
genotype siamensis was detected along the Catalan coast,

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in the eastern Atlantic coast of Morocco, Portugal, northern
Spain and southern Italy (Vila et al., 2001; Amorim et al.,
2010; Bennouna et al., 2010; Laza-Martinez et al., 2011;
Ciminiello et al., 2013) and its morphotype has also been
reported along the northern African coast (Turki, 2005;
Turki et al., 2006; Mabrouk et al., 2011, 2012). Moreover,
Penna et al. (2012) found a new genotype, probably corre-
sponding to a new species of Ostreopsis, in both the At-
lantic coast (Canary Islands) and Mediterranean Sea
(Greece and Cyprus).

Ostreopsis spp. have been recorded in other temperate
areas as well. In Japan, toxic strains of both O. siamensis
and O. ovata have been recorded not only in sub-tropical
southern Okinawan waters in late 1970s (Fukuyo, 1981;
Nakajima et al., 1981; Yasumoto et al., 1987) but also in
the more temperate northern waters of western Kyushu,
eastern Miyazaki, Kochi and Shikoku (Taniyama et al.,
2003; Adachi et al., 2008; Sagara, 2008). Ostreopsis cf.
siamensis cells were found in temperate New South Wales
and Tasmanian waters (Murray, 2010) and, in the latter,
in the gut contents of wild mussels (Pearce et al., 2001).
Moreover, O. cf. siamensis has become a major bloom
former in New Zealand, causing extensive mats covering
seaweeds in the eastern Northland waters, and has been
reported as far south as temperate Wellington waters
(Rhodes et al., 2000, 2010; Shears and Ross, 2009); in the
northern New Zealand waters also O. lenticularis and O.
cf. ovata have been recorded (Chang et al., 2000). Finally,
Ostreopsis cf. ovata and O. cf. siamensis were a constant
component of the epiphytic communities during the sum-
mer–fall period in Peter the Great Bay, Sea of Japan since
2006 (Selina and Orlova, 2010; Selina et al., 2014).

TOXIN PROFILE AND FACTORS AFFECTING
TOXIN PRODUCTION

Ostreopsis species produce different toxins, mostly
belonging to the palytoxin group. Among the nine species
of the genus Ostreopsis, toxicity has been demonstrated
in O. siamensis, O. mascarenensis, O. lenticularis and O.
cf. ovata (Nakajima et al., 1981; Yasumoto et al., 1987;
Holmes et al., 1988; Mercado et al., 1994; Meunier et al.,
1997; Lenoir et al., 2004; Ciminiello et al., 2006; Scalco
et al., 2012; Uchida et al., 2013; Brissard et al., 2015;
García-Altares et al., 2015). Moreover, O. heptagona was
determined to be toxic as methanol extracts of culture of
this species isolated from Knight Key (Florida) were
weakly toxic to mice (Babinchak, according to Norris et
al., 1985).

Palytoxin (PlTX) has a molecular formula of
C129H221N3O54 and a molecular weight of 2680 Da (Moore
and Bartolini, 1981). It has been primarily isolated from
the marine zoanthid Palythoa toxica (Moore and Scheuer,
1971), from which the name comes. PlTX is a very com-

plex molecule with both lipophilic and hydrophilic groups
and is slightly less toxic than maitotoxin in total potency.
The PlTX analogues produced by Ostreopsis species have
a similar chemical structure as the parent PlTX, as well
as a similar mode and site of action. Ostreocin-D was the
first PlTX analogue isolated from cultures of O. siamensis
(Usami et al., 1995; Ukena et al., 2001). This compound
has the chemical formula C127H220N3O53 and a molecular
weight of 2634 Da, a little lower than PlTX. Another
PlTX analogue, mascarenotoxin (McTX), was isolated
from O. mascarenensis (Lenoir et al., 2004) and O. cf.
ovata (Rossi et al., 2010; Scalco et al., 2012). The molec-
ular weight of the three identified mascarenotoxin con-
geners ranges from 2500 to 2628 Da (Rossi et al., 2010).
In addition, a third PlTX analogue was isolated from O.
cf. ovata, the ovatoxin (OvTx) (Ciminiello et al., 2008,
2010, 2012a; Rossi et al., 2010). Mediterranean cultures
of O. cf. ovata were found to produce isobaric palytoxin,
ovatoxin-a, b, c, d, e, f, g and h and mascarenotoxin-a and
c (Scalco et al., 2012; García-Altares et al., 2014; Brissard
et al., 2015). On the contrary, the Mediterranean O. cf.
siamensis strain seems to be devoid of any appreciable
toxicity (Ciminiello et al., 2013). Finally, ostreotoxins
(produced by O. lenticularis) do not display the same
mode and site of action as PlTX-analogues and the clas-
sification of these compounds as PlTX analogues is still
unclear (Mercado et al., 1994; Meunier et al., 1997).

As far as the studies on the action mechanism are con-
cerned, almost all studies refer to the commercial PlTX
standard (Tubaro et al., 2014). PlTX targets membrane
sodium-potassium pumps (Na+/K+-ATPase) responsible
for maintaining ionic gradients (Artigas and Gadsby,
2003). Characteristic aspects of PlTX include delayed
haemolysis with a loss of potassium, converting Na/K
pump into a non-specific ionic channel leading to the dis-
ruption of ion homeostasis exerted on excitable tissues
(Habermann et al., 1981). This results in nausea, vomit-
ing, hyper-salivation, abdominal cramps, diarrhoea,
numbness of extremities, severe muscular spasms and res-
piratory distress (Yasumoto et al., 1986; Alcala et al.,
1988; Kodama et al., 1989). On the contrary, the possible
effects recorded in non-excitable cells are less clear: in
this case, the toxin can affect different sets of proteins and
signalling pathways, stressing the complexity of the mode
of action of PlTX (Bellocci et al., 2011; Rossini and Bi-
giani, 2011; Wattenberg, 2011). For example, protein ki-
nases involved in the control of cell proliferation can be
activated by PlTX (Wattenberg, 2011), providing a pos-
sible role of this toxin in the tumour-promoting activity
(Fujiki et al., 1986; Fujiki and Suganuma, 2009). How-
ever, PlTX has been also shown to possess a potent cyto-
toxic activity (Bellocci et al., 2011).

Environmental factors (e.g. temperature, salinity, light,
nutrients) and the characteristic of the strains (e.g., isolation

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4 S. Accoroni and C. Totti

site, growth phase and the age of the strain) have all been
shown to affect toxin content in Ostreopsis cultures (Guer-
rini et al., 2010; Ciminiello et al., 2012a,b; Pezzolesi et al.,
2012; Scalco et al., 2012; Vanucci et al., 2012b). Several
phytoplankton species produce more toxins per cell when
growing under suboptimal conditions (Johansson and
Granéli, 1999a,b; Etheridge and Roesler, 2005). In the same
way, some authors observed that O. cf. ovata isolated from
both the Tyrrhenian and Adriatic Seas produces higher
toxin contents per cell when growing under suboptimal
temperature and salinity conditions (Granéli et al., 2011;
Pezzolesi et al., 2012; Vidyarathna and Granéli, 2013),
which differ among strains from different geographical
areas (see Paragraph 7). On the contrary, optimal nutrient
conditions seem to be required for toxin production and
both P- and N-depleted media decreased O. cf. ovata toxi-
city (Vanucci et al., 2012b). The growth phase affects Os-
treopsis toxicity as well. Although toxin production rate has
been found to increase during the exponential phase (Pez-
zolesi et al., 2014), toxins concentration on a per cell basis
increased from the exponential to the senescent phase, in-
dependently of the growth conditions (Guerrini et al., 2010;
Pistocchi et al., 2011; Vanucci et al., 2012a, 2012b). This
behaviour was recently explained (Pinna et al., 2015) as
due to the strong influence of the internal nutrient status
(i.e., carbon to nutrient ratio) on toxin synthesis. On the
contrary, the toxin profile of O. cf. ovata was relatively sta-
ble during the growth stages and independent of culture
conditions (Pistocchi et al., 2011; Scalco et al., 2012; Pez-
zolesi et al., 2014). 

VECTORS OF EXPOSITION AND EFFECTS
ON ORGANISMS

The main vectors for Ostreopsis intoxication of hu-
mans include marine aerosol (Casabianca et al., 2013,
2014; Ciminiello et al., 2014), direct contact (Tichadou et
al., 2010) and the per os ingestion (the latter mainly asso-
ciated to clupeotoxism syndrome, see below). Blooms of
O. cf. ovata caused serious problems on human health,
mainly due to inhalation of sea water droplets containing
Ostreopsis cells or fragments and/or aerosolized toxins
(Gallitelli et al., 2005; Kermarec et al., 2008; Tichadou et
al., 2010; Honsell et al., 2011; Del Favero et al., 2012).
One of the most intense episode occurred in summer 2005,
when about 200 people exposed to marine aerosols along
the Ligurian coasts required medical first aid due to similar
symptoms of respiratory intoxications, and 20 persons
were subjected to extended hospitalization (Brescianini et
al., 2006; Durando et al., 2007). The typical intoxication
symptoms of Ostreopsis aerosol and direct contact expo-
sure (fever, dyspnoea, broncho-constriction, conjunctivitis
and skin irritations) resolve within a few days.

Regarding the oral ingestion, although Ostreopsis has

not been confirmed as the source of toxin in clupeotoxism
yet, it was strongly suspected of that intoxication in sev-
eral events (e.g., Onuma et al., 1999; Randall, 2005). Clu-
peotoxism is one of human intoxications due to
consumption of contaminated sardines and herrings (Clu-
peidae) or anchovies (Engraulidae). Symptomology of
clupeotoxism is similar to that of ciguatera (Yasumoto et
al., 1986), though the former has a much higher mortality
rate (Onuma et al., 1999). Several outbreaks were re-
ported in tropical insular areas of the Pacific and the
Caribbean during the last 30 years (Yasumoto et al., 1986;
Fukui et al., 1987; Gleibs et al., 1995). Recently, clu-
peotoxism occurred in the southwestern Indian Ocean,
mainly in Madagascar where palytoxin analogues were
involved in fatalities occurred after consumption of Sar-
dinella fish (Yasumoto, 1998; Hansen et al., 2001).

Ostreopsis toxins may contaminate seafood: ostre-
ocin-D produced by O. cf. siamensis were accumulated
in wild mussels (Mytilus edulis planulatus) from Tasman-
ian coasts (Pearce et al., 2001). Rhodes et al. (2002), feed-
ing New Zealand mussels (Perna canaliculus), Pacific
oysters, and scallops (Pecten novaezealandiae) with O.
cf. siamensis cells detected trace amounts of palytoxin-
like compounds in some of the fed animals. In the
Mediterranean Sea, Aligizaki et al. (2008) analysing field
samples of shellfish (Mytilus galloprovincialis, Venus ver-
rucosa, Modiolus barbatus) reported that shellfish toxicity
coincided with seasonal peaks in Ostreopsis abundance,
providing the most compelling evidence to date that Os-
treopsis-borne palytoxin analogues likely accumulate in
shellfish. Although it has been shown that the oral toxicity
of palytoxin and 45-hydroxy palytoxins is about 1000-
fold less than that observed by intraperitoneal injection
(Sosa et al., 2009; Munday, 2011; Tubaro et al., 2011), a
regulatory threshold of 30 µg kg–1 has been proposed for
shellfish flesh (EFSA, 2009). However, the effects of the
ingestion of products contaminated by O. cf. ovata toxin
are still unknown.

Ostreopsis blooms are often accompanied by mortality
of benthic marine organisms, such as sea urchins, limpets,
mussels, crustaceans, holothurians, sponges and even
macroalgae (Di Turi et al., 2003; Shears and Ross, 2009,
2010; Accoroni et al., 2011). In fact, several recent studies
have shown that Ostreopsis toxicity affects also various
marine organisms, both invertebrates and fish (Gorbi et
al., 2012, 2013; Simonini et al., 2011; Faimali et al., 2012;
Pezzolesi et al., 2012; Privitera et al., 2012; Carella et al.,
2015), interfering with embryonic development as well
(Pagliara and Caroppo, 2012). 

A study carried out on natural banks of Mytilus gallo-
provincialis sampled during various phases of O. cf. ovata
bloom in the north-western Adriatic Sea (Mediterranean
Sea) demonstrated a significant accumulation of algal tox-
ins in mussels exposed, which exhibited a marked inhibi-

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Ostreopsis in temperate areas

tion of the Na+/K+-ATPase activity and alterations of im-
munological, lysosomal and neurotoxic responses (Gorbi
et al., 2012).

LIFE CYCLE AND CYST FORMATION

As the largest part of marine dinoflagellates, Ostreop-
sis has a haplontic life cycle with a dominant motile hap-
loid biflagellate stage (Pfiester and Anderson, 1987;
Litaker et al., 2002).

The asexual and sexual reproduction of Ostreopsis has
been studied mainly in Ostreopsis cf. ovata, both in the
field and in cultures isolated from the Mediterranean Sea
(Bravo et al., 2012; Accoroni et al., 2014). The asexual
reproduction occurs with the division of haploid vegeta-
tive cells that takes place in the motile stage: cells divide
by desmoschisis in the sagittal plane, with each daughter
cell inheriting part of the parental thecal plates (Bravo et
al., 2012).

In general, sexual reproduction of dinoflagellates usu-
ally begins with the production of gametes that can fuse
with each other forming a diploid zygote. The actual de-
tails of gamete formation vary, but in any case, two hap-
loid cells (both typically motile) fuse to yield a diploid
cell. The sexual reproduction in O. cf. ovata has been ob-
served in both natural and cultured populations (Bravo et
al., 2012). Gamete pairs are observed in either inter-
crosses or intracrosses of different strains (Bravo et al.,
2012) and nutrient limitation seems to stimulate sexual
reproduction, which however occurs also in normal cul-
ture conditions (Accoroni et al., 2014). In O. cf. ovata,
two types of mating gametes were identified: i) gametes
joined by epitheca, with the point of attachment posi-
tioned almost centrally (Bravo et al., 2012), and ii) ga-
metes joined laterally with the two cingula perpendicular
to each other, with melting of the two thecae (Accoroni
et al., 2014), as observed in Coolia monotis by Faust
(1992). In dinoflagellates, the newly formed motile
diploid cell produced when karyogamy and plasmogamy
are complete, is known as a planozygote (Pfiester, 1989).
The planozygote of some species undergoes meiosis in
the plankton, while that of most other dinoflagellates
swims for a variable amount of time (hours to weeks),
sheds its flagella, rounds up, and settles to form a non-
motile hypnozygote. Hypnozygotes often differentiate
into long-term resting stages (hypnocysts) that accumulate
in sediments and may remain dormant for years before
germinating (Wall, 1975; Anderson et al., 1987). Different
types of cysts were identified for O. cf. ovata; some of
them are non-dormant as germinate within 3 days (tem-
porary cysts), while some others are resting cysts able to
germinate for as long as 5 months after their formation. A
study on the life cycle of O. cf. ovata conducted with
northern Adriatic strains highlighted that resting cysts ger-

minated in laboratory conditions at 25°C, but not at 21°C
(Accoroni et al., 2014). The presence of a temperature
threshold affecting cyst germination in laboratory condi-
tions would highlight that temperature could represent a
key factor for Ostreopsis cf. ovata bloom onset also in
natural environments.

ROLE OF ENVIRONMENTAL PARAMETERS
ON BLOOMS

Hydrodynamic conditions

Several studies considered hydrodynamic condition as
the main factor affecting Ostreopsis bloom trends, high-
lighting that higher abundances are observed in sheltered
sites compared with exposed ones (Barone, 2007; Shears
and Ross, 2009; Totti et al., 2010; Mabrouk et al., 2011).
The abundances of benthic dinoflagellates are highly af-
fected by wave action, since they are only loosely attached
to the substrata and can be easily removed and re-sus-
pended in the water column, although in literature we
often found controversial data. Chang et al. (2000) sug-
gested that O. cf. siamensis was more abundant on the
northern New Zealand’s eastern coast because it is a less
energetic, more stable environment than the western one.
This suggestion has been confirmed later by Shears and
Ross (2009) that observed higher Ostreopsis siamensis
abundances at sheltered vs exposed locations. Vila et al.
(2001), on the other hand, concluded that Ostreopsis sp.
prefers ‘moderately shaken’ waters in the NW Mediter-
ranean and Parsons and Preskitt (2007) observed higher
abundance of O. cf. ovata on the windward coast of the
island of Hawaii, whereas Ostreopsis sp.1 was more
prevalent on the leeward coast.

Observations in the northern Adriatic Sea highlighted
that 1) significantly higher abundances were observed in
the sheltered sites compared with the exposed ones; 2) hy-
drodynamics may have an important effect on the tempo-
ral variability of bloom, because stormy events can result
in a sudden decrease of cell abundances on the benthic
substrata, with cell proliferation being re-established at
high densities after some days of calm sea conditions
(Totti et al., 2010; Accoroni et al., 2012a). Moreover, it
has been highlighted that turbulence can affect O. cf.
ovata growth rate and consequently its cell size (Accoroni
et al., 2012b).

The effect of hydrodynamics has been separately
tested for each bloom phase, i.e. initial (no more than 102
cells cm–2 recorded over all substrata), proliferation (pe-
riod of intense cell division, when O. cf. ovata rapidly in-
creased abundances until reaching maximum peak) and
decline phase (decrease in cell abundances and bloom de-
cline), and it has been shown that cell abundances in shel-
tered sites were significantly higher than those in the

5

No
n-

co
mm

er
cia

l u
se

 on
ly



6 S. Accoroni and C. Totti

exposed ones during the proliferation phase (Accoroni et
al., 2012b). This result suggested that hydrodynamics af-
fect O. cf. ovata abundances mainly during phases when
the highest abundances are reached. In fact, a well-devel-
oped benthic mat (i.e., brownish pellicle loosely attached
to benthic substrata) is produced only during the most in-
tense proliferation period (Totti et al., 2010) and this
structure is easily removed by effect of the hydrodynamic
conditions. This result may explain why such effects
linked to hydrodynamic conditions were not observed in
those areas where high abundances were not reached and
a mat did not develop, as observed in the Tyrrhenian Sea
(Zingone, personal communication), and in Johnston
Atoll (Pacific Ocean) (Richlen and Lobel, 2011).

Water temperature

Many authors suggested that Ostreopsis spp. need rel-
atively high temperatures to proliferate, proposing that the
global warming might have influenced Ostreopsis expan-
sion in temperate areas such as the Mediterranean Sea
(Hallegraeff, 2010; Granéli et al., 2011), but a more care-
ful analysis of literature data shows that temperature role
is not the same in all coastal areas around the world (Tab.
1). Ostreopsis blooms are summer events in temperate
areas, although comparing the bloom trend in several
Mediterranean areas it can be observed that peaks can
occur from spring to autumn, with a certain inter-annual
variability. In the northern Adriatic Sea, the peaks of the
blooms occur generally in September-October (Monti et
al., 2007; Totti et al., 2010; Accoroni et al., 2015a). On
the contrary, in the Ligurian Sea highest cell abundances
were mostly recorded in mid-summer (end of July) (Man-
gialajo et al., 2008). Vila et al. (2001) observed abundance
peaks for Ostreopsis sp. in the north-western Mediter-
ranean even early, in springtime. Also in the Aegean Sea,
Spatharis et al. (2009) found O. cf. ovata being most
abundant in May, contrarily to what observed in the same
area by Aligizaki and Nikolaidis (2006) that reported peak
abundances from midsummer to late fall. 

Experimental studies about the response of Ostreopsis
growth to temperature provide a possible explanation for
the above different temporal trends observed in situ: Guer-
rini et al. (2010) and Pezzolesi et al. (2012) highlighted
that O. cf. ovata strains from different Italian coasts (in
the Tyrrhenian and Adriatic Seas) displayed different
growth temperature optima that parallel with the in situ
temperature values typical of the blooming period of the
single strain. Scalco et al. (2012) observed that Mediter-
ranean O. cf. ovata strains grew within a window of 18-
30°C with the best performances recorded between 22 and
26°C, suggesting that O. cf. ovata is adapted to interme-
diate temperatures and day length conditions, such are
those recorded in the natural environment at the beginning
of summer and/or of autumn (Mangialajo et al., 2011). In Ta

b.
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4

No
n-

co
mm

er
cia

l u
se

 on
ly



Ostreopsis in temperate areas

the same way, Tawong et al. (2015) observed that optimal
and tolerable temperature conditions differ among Ostre-
opsis cf. ovata subclades: strains of O. cf. ovata Thailand
subclade and O. cf. ovata South China Sea subclade
showed the semi-optimal temperature ranges of 22.7-
27.4°C and 27.9-30.8°C, with optimal temperature of
25°C and 30°C, respectively.

Although the reaching of the highest abundances of Os-
treopsis is not in concomitance with the highest water tem-
perature values in all areas, a temperature threshold would
seem to be important to let the bloom start: a study con-
ducted along the Conero Riviera (northern Adriatic Sea)
showed that, although the bloom peak occurred in late sum-
mer (when temperatures ranged between 18.8 and 24°C,
decreasing from the seasonal maximum), the bloom onset
was always observed at higher temperature (25-28.6°C),
suggesting that Ostreopsis needs to reach a fairly well fixed
temperature threshold to start its bloom, probably in rela-
tion to the cyst germination that generally occurs at around
25°C (Accoroni et al., 2014). As in this area the bloom can
persist until temperature values are much lower (14.4-
17.5°C) than that threshold, one would guess that once Os-
treopsis cysts are germinated, its vegetative forms seem to
actively proliferate even if temperature values decrease.
This discrepancy between optimal temperature range for
the cysts germination and the algal growth has been re-
ported for the dinoflagellate Scrippsiella trochoidea as well
by Binder and Anderson (1987) in experimental conditions.
However, in the northern Adriatic Sea the bloom onset is
often observed about 30 days after the reaching of the 25
°C-temperature threshold, suggesting that other environ-
mental factors, besides temperature, may affect the devel-
opment of O. cf. ovata blooms. In this regard, O. cf. ovata
blooms appear to be triggered by a combination of optimal
temperature and available nutrients, where the temperature
threshold plays a key role on the germination of O. cf. ovata
cysts and an N:P ratio around the Redfield value is a nec-
essary condition to allow cell proliferation.

Role of salinity and nutrients

Contrasting results are reported in the literature about
the effect of salinity on the development of Ostreopsis
blooms. Rhodes et al. (2000) observed that O. cf. siamen-
sis had a preferred salinity range of 28-34 in cultures iso-
lated from Northland (New Zealand). Salinity measured
in the north-western Mediterranean Sea during the Ostre-
opsis spp. blooms showed values around 37-38 (Vila et
al., 2001; Mangialajo et al., 2008) and similar salinity lev-
els were measured in the Gulf of Trieste (northern Adriatic
Sea) by Monti et al. (2007) and in the southern Adriatic
Sea by Ungaro et al. (2005); in the Conero Riviera (north-
ern Adriatic Sea) during the Ostreopsis blooms occurred
from 2007 to 2012, salinity ranged in a much wider range,
from 31.3 to 39.3 (Accoroni et al., 2015a). 

Several authors suggested that benthic dinoflagellates
proliferation is favoured by low salinity waters. In the Vir-
gin Islands, abundance maxima of Ostreopsis were cor-
related with the period of maximal rainfall (Carlson and
Tindall, 1985), and the same negative correlation with
salinity has been found for O. ovata along the Hawaiian
coasts (Parsons and Preskitt, 2007).

On the contrary, in the north-western coast of Cuba,
in Catalonian and in French Mediterranean coasts, Del-
gado et al. (2006), Blanfuné et al., (2015) and Carnicer et
al., (2015) suggested that low salinity values possibly hin-
der Ostreopsis spp. as they found conspicuously lower
Ostreopsis abundances in sites more affected by river
plumes (i.e., Jaimanitas River, Rhone River and Ebro
Delta) than in the rest of the studied areas. No significant
correlation between cell abundances and salinity values
were found in the Gulf of Mexico for O. heptagona
(Okolodkov et al., 2007) and in the Conero Riviera for O.
cf. ovata (Accoroni et al., 2015a)

When the effect of salinity has been investigated in
experimental conditions, it has been shown that such ef-
fect may be strain-specific: Tawong et al. (2015) showed
that optimal and tolerable salinity conditions differed
among Ostreopsis cf. ovata subclades as the optimal salin-
ities for the O. cf. ovata Thailand and South China Sea
subclades were 30 and 25, respectively. Pezzolesi et al.
(2012) demonstrated that an Adriatic O. cf. ovata strain
cultured at salinity ranging from 26 to 40 showed that al-
though the lowest growth rates were observed at the low-
est salinity, growth rates did not significantly differ in
different salinity conditions. 

Indeed, the relationships between algal blooms in the
field and salinity are more complicated, and other factors,
such as nutrient levels (which are typically associated to
low salinity waters) have to be considered. Recent studies
have provided increasing evidence of a link between the
nutrient enrichment of coastal waters (anthropogenic eu-
trophication) and harmful algal events (Glibert and Burk-
holder, 2006; Glibert et al., 2010). However, there is very
limited information on the relationships between nutrient
concentrations and the occurrence of Ostreopsis blooms
(Tab. 1). Vila et al. (2001) and Cohu et al. (2011) in north-
western Mediterranean Sea and Shears and Ross (2009)
in north-eastern New Zealand did not find any relation be-
tween epiphytic O. cf. ovata abundance and nutrients,
while Parsons and Preskitt (2007) found that Ostreopsis
sp.1 abundance was positively correlated with nutrient
concentrations in the waters surrounding Hawaii. In the
Conero Riviera (northern Adriatic Sea), although no clear
relationship was found between nutrient concentrations
and O. cf. ovata abundances, it was observed that in the
bloom onset period, PO4 concentrations were significantly
higher than in the rest of the study period. Interestingly,
the following bloom development is maintained in a con-

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8 S. Accoroni and C. Totti

dition of elevated N:P ratios, suggesting that such blooms
may be initiated at low N:P levels (possibly stimulated by
a ‘flush’ of nutrients or organic materials) that may allow
the newly germinating cells to increase growth rate while
other adaptive mechanisms (e.g., metabolic dissipatory
strategies, allelopathic and mixotrophic interactions),
would enable the maintenance of blooms at less than max-
imal growth rates and at not-optimal N:P ratios (Accoroni
et al., 2015a). A decrease in N:P ratio values has previ-
ously been associated with the onset of a number of plank-
tonic dinoflagellate blooms as well (Hodgkiss and Ho,
1997; Zhang and Hu, 2011; Glibert et al., 2012). In this
regard, experimental studies conducted on different Adri-
atic O. cf. ovata strains showed that the depletion of P was
proportionately more rapid than that of N, highlighting
the strong P demand of this dinoflagellate (Vanucci et al.,
2012b; Pezzolesi et al., 2014).

Moreover, further studies are needed to clarify the
trophic behaviour of Ostreopsis spp., considering that for
these species also mixotrophy was hypothesized (Barone,
2007; Burkholder et al., 2008), which may play an impor-
tant role in Ostreopsis development, as already observed
in other potentially toxic microalgae (Cucchiari et al.,
2008; Heisler et al., 2008).

Depth

The role of depth on Ostreopsis abundances along the
Conero Riviera (northern Adriatic Sea) was investigated
in 2007 (Totti et al., 2010) in target sites where samples
were collected at depths comprised between 0.5 and 10
m. O. cf. ovata abundances showed a significant decrease
with depth, in agreement with what observed by Richlen
and Lobel (2011) and Cohu and Lemée (2012), suggesting
a potential effect of light intensity. This may explain why
Ostreopsis blooms mainly develop in shallow waters.
However, such effect has not been observed in shallow
sites affected by high hydrodynamics, such as on the
fringing reefs of the higher infralittoral shelf, where O. cf.
ovata abundances were lower than those recorded imme-
diately deeper, due to the hydrodynamic effect of wave
actions (Totti et al., 2010).

As previously suggested, the role of depth may be re-
lated to light availability, although there are conflicting
data about the relationship between light intensity and Os-
treopsis growth in experimental conditions. Morton et al.
(1992) reported that O. cf. siamensis and O. heptagona
isolated from the Florida Keys displayed maximal growth
at approximately 200 µmol photons m–2 s–1 and did not
grow rapidly at over 240 µmol photons m–2 s–1 (i.e., >10%
full sun light). Scalco et al. (2012), analysing the growth
performance of some Italian O. cf ovata strains, observed
that this species grew better at relatively low photon flux
density (50 instead of 200 µmol photons m–2 s–1). Yam-
aguchi et al. (2014) observed that Ostreopsis sp. from

Japan grew proportionally when light intensity was in-
creased from 49.5 to 199 µmol photons m–2 s–1, but its
growth appeared to be inhibited slightly at >263 µmol
photons m–2 s–1. Heil et al. (1993) observed that O. cf. sia-
mensis cultured in spinner flasks would stay planktonic
when light intensities are maintained low (25 µmol pho-
tons m–2 s–1), but would produce mucus and settle at the
bottom at higher light intensities (75 µmol photons m–2 s–
1). This has been interpreted as a protective measure to
shade the cells. Therefore, in experimental conditions Os-
treopsis sp. seems to suffer too high light intensities,
therefore being exposed to potentially detrimental conse-
quences of photodamaging.

In field conditions, there are only few data about the
relationship between Ostreopsis abundances and light in-
tensity, which, moreover, seem to disagree with the ex-
perimental evidences described above. A study carried out
along several Italian coastal areas affected by Ostreopsis
blooms showed that during the blooms, the values of light
intensity at depth of Ostreopsis sampling were quite high,
up to 1800 µmol photons m–2 s–1 (ISPRA, 2012). How-
ever, it is known that light availability amidst macroalgal
vegetation is generally low (Raniello et al., 2004), and
Ballantine et al. (1988) suggested that Ostreopsis cells can
migrate to shaded areas of the algal host thallus to escape
high light levels.

Anyway, further studies are required to clarify the real
role of light intensity in the bloom dynamics of Ostreopsis.

Substratum

Ostreopsis has often been indicated to be preferen-
tially epiphytic on macroalgae (Bomber et al., 1989; Vila
et al., 2001), although it has been recorded on a variety
of other substrata (Table 1), including marine angiosperms
(Turki, 2005; Aligizaki and Nikolaidis, 2006; Turki et al.,
2006; Battocchi et al., 2010; Mabrouk et al., 2012), rocks
(Bottalico et al., 2002; Totti et al., 2010; Accoroni et al.,
2011), coral rubble (Norris et al., 1985), soft sediments
(Vila et al., 2001; Aligizaki and Nikolaidis, 2006), and in-
vertebrates (Bianco et al., 2007; Totti et al., 2010). They
also can be found as free-living in the plankton (Faust and
Morton, 1995; Tognetto et al., 1995; Chang et al., 2000;
Totti et al., 2010; Accoroni et al., 2011).

The possibility of Ostreopsis to colonize a variety of
substrata, living either as epiphytic, epilithic, or epizoic,
indicates that this species is not an obligate epiphyte.

A number of studies underlined the importance of host
thallus architecture (Lobel et al., 1988; Bomber et al.,
1989). Vila et al. (2001) observed that three-dimensional
flexible thalli are more suitable for the growth of Ostre-
opsis spp. It has been suggested that the higher abun-
dances found in branched than in flattened thalli, might
be explained by a different response of such thallus mor-
photypes to the wave action (Totti et al., 2010).

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Ostreopsis in temperate areas

Indeed, the relationships between Ostreopsis and
macrophytes are more complicated. In studies carried out
on natural populations of O. cf. ovata, significantly higher
abundances were reported on pebbles than on macroalgae
(Totti et al., 2010; Accoroni et al., 2011), suggesting that
living substrata allow lower concentration of epibionts
than any other substrate, probably due to the production
of some hypothetical allelopathic compounds (Jin and
Dong, 2003). In this regard, a study conducted to assess
any possible allelopathic interactions between Ostreopsis
cf. ovata and macroalgae showed that all the investigated
seaweeds [Dictyota dichotoma (brown alga), Rhodymenia
pseudopalmata (red alga) and Ulva rigida (green alga)]
exerted negative effects toward the benthic dinoflagellate,
with the highest inhibitory effect observed in D. di-
chotoma and the lowest in R. pseudopalmata (Accoroni
et al., 2015b).

UNRESOLVED ISSUES

Despite the number of studies on Ostreopsis biology,
ecology and toxin production and actions, several aspects
about the environmental concerns associated with this
genus remain still unclear. Regarding the action mecha-
nism of the implicated toxins, almost all studies refer to
the commercial PlTX standard, while a more accurate
analysis should be addressed on both effects and action
mechanism of all toxins produced by Ostreopsis species.
Moreover, among the vectors of Ostreopsis intoxication
i.e., marine aerosol, direct contact and per os ingestion,
the latter needs certainly further studies given its possible
implications on human health almost unknown in temper-
ate areas nowadays.

Numerous studies have highlighted the influence of
environmental factors on bloom dynamics of Ostreopsis
and the complexity of conditions leading to blooms of this
dinoflagellate is becoming clearer but not totally under-
stood. Although the mechanisms for bloom onset seems
clarified, those driving both bloom development and de-
cline are still far from being understood, and other both
abiotic and biotic factors, such as the interactions with
other organisms and the ability to use organic forms of
nutrients, should be investigated. Actually, several HAB
genera have been shown to use organic forms of nutrients
for their nutritional demands (Cucchiari et al., 2008;
Heisler et al., 2008) and a mixotrophic behavior has been
hypothesized in Ostreopsis (Barone, 2007; Burkholder et
al., 2008; Pinna et al., 2015). Effects of biotic interactions
on Ostreopsis should be considered as well and only few
studies have been carried out on bacteria (Vanucci et al.,
2012a), diatoms (Pichierri et al 2015) and macroalgae
(Accoroni et al., 2015b). In this regard, the role of viruses,
bacteria and parasites in both cysts formation and bloom
termination have been recognized in several microalgae

(Nagasaki et al., 1994; Nagasaki et al., 2000; Tarutani et
al., 2001; Mizumoto et al., 2008; Garcés et al., 2013) and
should be studied in Ostreopsis bloom dynamics as well.

ACKNOWLEDGMENTS

This study was partially supported by ISPRA-Italian
Ministry of the Environment, MURST (PRIN 2007), and
ENPI CBCMED M3-HABs project.

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