Spatial and temporal evolution of PSP toxins along the Atlantic shore of Morocco.

A monitoring program for bivalve molluscs contaminated by algal toxins was established in 1992 at different stations along the Atlantic Moroccan shore. The presence of toxicity in bivalve molluscs commercially exploited was tested fortnightly using the mouse bioassay method. Results obtained from this surveillance indicate paralytic shellfish poisoning is responsible for bivalve molluscs contamination along the Atlantic coastline of Morocco. Toxin profile was established by automated pre-column HPLC/FLD in selected contaminated tissues. The study of individual toxins in mussel during a bloom in the northern Atlantic coastline in 1994 showed a very complex profile, typical to that obtained with cultures of the toxigenic dinoflagellate Gymnodinium catenatum isolated from the Iberian region. However, toxin composition of mussels and marine beans from later blooms in the southern Atlantic coastline in 1999 showed a strong resemblance with that of Alexandrium minutum, due to dominance of gonyautoxins 1/4. A minor contamination by G. catenatum due to the presence of decarbamoyl-saxitoxin is hypothesized.     

Identification of pectenotoxins in plankton, filter feeders, and isolated cells of a Dinophysis acuminata with an atypical toxin profile, from Chile.

A bloom of Dinophysis acuminata produced, in autumn of 2005, a closure of the scallop harvesting in Bahía Inglesa, in the Chilean III region. Isolated cells of this Dinophysis species were shown to contain 180 pg cell(-1) of pectenotoxin 2 but neither okadaic acid nor any of its analogs or derivatives (at least at a detectable level). Examination of plankton and filter-feeder samples covering an area of ca. 350 km, from the location where the toxicity was recorded to Bahía Tongoy, showed that the unique toxin profile found in the first bloom was widespread over that part of Chile and persisted for months. The analysis were carried out by HPLC-ESI-MS using positive ionization mode, with a detection limit below 2 ng of OA mL(-1) of methanolic extract. This is the first report of the presence of pectenotoxins in the plankton of the Pacific coast of America and in the studied filter feeders. This is also the first report of a Dinophysis species containing pectenotoxins and not any toxin of the okadaic acid group.     

Pectenotoxin-2 in single-cell isolates of Dinophysis caudata and Dinophysis acuta from the Galician Rías (NW Spain).

Dinophysis acuta and Dinophysis caudata are seasonal components of the dinoflagellate community in the Galician Rías Bajas (NW Spain). These species can be the main contributors to the occurrence of Lipophilic Shellfish Toxins (LST) in September-October, leading to prohibition of harvesting in an area of intensive mariculture (250 x 10(3)t of cultured mussels, 60 x 10(3)t of other shellfish in natural banks, per year). Previous analyses of okadaic acid (OA) and related toxins in these two species by HPLC revealed significant amounts of OA and DTX2 in D. acuta, but only trace amounts of OA in D. caudata cells, and led to the erroneous conclusion that the contribution of the latter species to autumn LST events was negligible. Recent analyses by LC-MS/MS of individually picked cells of D. acuta and D. caudata have shown that both species may have high levels of PTX2 (up to 30 pg and 130 pg cell(-1), respectively) and that this toxin can be the dominant toxin during toxic outbreaks associated with Dinophysis spp. Although the quick conversion of PTX2 to PTX-2SA in mussels may reduce the risks for human health, these results have important implications for monitoring programmes. The complex toxin profile of shellfish exposed to Dinophysis spp. populations should be taken into account when making decisions on the toxin control methods to be used as the basis of the programme. Conclusive toxicological studies are required to elucidate the public health relevance of the different PTXs derivatives and to provide the scientific basis for regulations.     

Toxicity assessment for the horseshoe crab Carcinoscorpius rotundicauda collected from Cambodia.

In this study, we assessed the toxicity of the horseshoe crab Carcinoscorpius rotundicauda collected from Cambodia within two successive months during rainy (April-May) and dry (December-January) seasons, respectively. Toxicity assessments of the collected specimens by standard mouse bioassay showed marked individual variation, and their toxin profiles by liquid chromatography/mass spectrometry (LC/MS) revealed tetrodotoxin (TTX) was the main toxin while no paralytic shellfish toxins (PSTs) were detected. All specimens were toxic and the highest toxicity values were 315, 113, 60, 47, 44 and 38 mouse units (MU)/g in the tissues of hepatic caecum, egg, viscera, muscle, intestine and testis, respectively. Although the current findings showed that the Cambodian C. rotundicauda was a moderately toxic species, they are not suitable for human consumption due to their toxicity. To the best of our knowledge, this is the first scientific study on toxic marine seafood ever investigated in Cambodian territorial waters.     

The loss of PSP toxin production in a formerly toxic Alexandrium lusitanicum clone.

Toxin production has always been considered a constitutive characteristic of dinoflagellates in the genus Alexandrium. Here we demonstrate that saxitoxin production can be lost by an Alexandrium species during routine culture maintenance. This is the first report of any marine saxitoxin-producing alga ever to have completely lost the ability to produce toxins. A clonal toxic isolate of Alexandrium lusitanicum from Portugal has been maintained in culture since 1962. In 1992, a subculture was established and sent to a different laboratory. Recent comparisons of the parental strain and the subculture revealed that the former had lost its toxicity, whereas the latter still produces saxitoxins. This loss of toxicity was confirmed by three independent toxin detection methods: mouse bioassay, mouse neuroblastoma assay and HPLC. Sequence analyses of different rRNA domains demonstrated that the toxic and non-toxic cultures are genetically identical for those markers. Morphological analysis showed that both cultures have the same plate tabulation and are A. lusitanicum. These results strongly argue that the loss of toxicity is not a result of a culturing artifact or mistake, such as mislabeling or contamination. The clonal cultures also show a significant difference in growth. Possible explanations for the change include genetic mutations or the effects of prolonged treatment of the non-toxic culture with antibiotics.     

Monitoring of brevetoxins in the Karenia brevis bloom-exposed Eastern oyster (Crassostrea virginica).

Brevetoxin uptake and elimination were examined in Eastern oyster (Crassostrea virginica) exposed to recurring blooms of the marine alga Karenia brevis in Sarasota Bay, FL, over a three-year period. Brevetoxins were monitored by in vitro assays (ELISA, cytotoxicity assay, and receptor binding assay) and LC-MS, with in vivo toxicity of shellfish extracts assessed by the traditional mouse bioassay. Measurements by all methods reflected well the progression and magnitude of the blooms. Highest levels recorded by mouse bioassay at bloom peak were 157 MU/100g. Oysters were toxic by mouse bioassay at levels >or=20 MU/100g for up to two weeks after bloom dissipation, whereas brevetoxins were measurable by in vitro assays and LC-MS for several months afterwards. For the structure-based methods, summed values for the principal brevetoxin metabolites of PbTx-2 (cysteine and cysteine sulfoxide conjugates), as determined by LC-MS, were highly correlated (r(2)=0.90) with composite toxin measurements by ELISA. ELISA and LC-MS values also correlated well (r(2)=0.74 and 0.73, respectively) with those of mouse bioassay. Pharmacology-based cytotoxicity and receptor binding assays did not correlate as well (r(2)=0.65), and were weakly correlated with mouse bioassay (r(2)=0.48 and 0.50, respectively). ELISA and LC-MS methods offer rapid screening and confirmation, respectively, of brevetoxin contamination in the oyster, and are excellent alternatives to mouse bioassay for assessing oyster toxicity following K. brevis blooms.     

Study of paralytic shellfish poisoning toxin profile in shellfish from the Mediterranean shore of Morocco

Since 1992, a monitoring program for bivalve molluscs contaminated by algal toxins was established at different stations along the Mediterranean Moroccan shores. The monitored stations were tested every 2 weeks. The presence of toxicity was determined using the mouse bioassay method. Toxin profile was carried out by HPLC/FD in selected contaminated tissues. According to the outcomes of this surveillance from 1994 to 1999, reliable information on toxicity of shellfish was obtained. They indicate that PSP is a recurrent toxicity in molluscs along the Mediterranean shore of Morocco. It has been noted a difference of PSP accumulation among individual shellfish. The cockle (Achanthocardia tuberculatum) presents toxicity throughout the year, while other specimens from the same area such as clam (Callista chione), warty venus (Venus gallina) and marine beans (Donax trunculus) accumulate it seasonally from January to April, after which they depurate the toxin. Moreover, the study of toxin profiles among individual shellfish was undertaken. It was found that shellfish presented a complex profile pointing to contamination by Gymnodinium catenatum.     

Ubiquitous 'benign' alga emerges as the cause of shellfish contamination responsible for the human toxic syndrome, azaspiracid poisoning.

A new human toxic syndrome, azaspiracid poisoning (AZP), was identified following illness from the consumption of contaminated mussels (Mytilus edulis). To discover the aetiology of AZP, sensitive analytical protocols involving liquid chromatography-mass spectrometry (LC-MS) were used to screen marine phytoplankton for azaspiracids. Collections of single species were prepared by manually separating phytoplankton for LC-MS analysis. A dinoflagellate species of the genus, Protoperidinium, has been identified as the progenitor of azaspiracids. Azaspiracid-1, and its analogues, AZA2 and AZA3, were identified in extracts of 200 cells using electrospray multiple tandem MS. This discovery has significant implications for both human health and the aquaculture industry since this phytoplankton genus was previously considered to be toxicologically benign. The average toxin content was 1.8 fmol of total AZA toxins per cell with AZA1 as the predominant toxin, accounting for 82% of the total.     

Lipophilic toxicity from the marine dinoflagellate Karenia brevisulcata: use of the brevetoxin neuroblastoma assay to assess toxin presence and concentration.

A method for assessing the presence and concentration of a major toxin produced by Karenia brevisulcata has been developed, based on the neuroblastoma assay for brevetoxins. This cytotoxicity assay, coupled with observations of the dose-response relationship and of changes in cellular morphology during the assay gave a characteristic toxin 'signature'. This signature was consistent whether the toxicity was assessed in extracts of shellfish, seawater samples, uni-algal cultures of K. brevisulcata, or in partially purified toxin samples. Using this method it was possible to confirm the presence of this toxicity during a mixed Karenia bloom, and infer that K. brevisulcata toxin (KBT) was the probable cause of the fish and shellfish mortality observed at that time. The neuroblastoma assay thus provides a solution to the problem of confirming the presence of KBT in suspected K. brevisulcata events, even in the absence of a known toxin structure.     

Studies of the effect of Psi-APONIN from Nannochloris sp. on the Florida red tide organism Karenia brevis.

Studies were conducted on the conditions under which the red tide organism, Karenia brevis (a.k.a., Gymnodinium breve), was treated with Nannochloris sp. The latter organism is known to produce cytolytic agents called Apparent Oceanic Naturally Occurring Cytolin (APONINs). Conventional wisdom might suggest that brevetoxins would be released upon destruction of the single-celled dinoflagellate K. brevis and that efforts to treat red tide outbreaks would lead to release of brevetoxins and enhanced toxicity toward marine species. Earlier studies described conditions by which K. brevis cells were converted to a non-motile form when cultures of K. brevis were treated with an isolate (Psi-APONIN) produced by Nannochloris sp. but when centrifuged only a small amount of the toxin was released. The present study confirms that the toxin is not released when the K. brevis is undisturbed, however, when the culture is stressed (stirred with a magnetic stirring bar for 24, 48, and 72h) toxin was released, and the toxicity could be measured using a Microtox analyzer. In the study, it was found that at as few as eighty cells of K. brevis produced a toxic effect of 20% as measured by the effect on Vibrio fischeri. Nannochloris sp. had no effect on the bacteria used in the Microtox analyzer, nor did interaction of Nannochloris sp. with K. brevis in the short term. This effect is presumed to be due to the production of Psi-APONIN and conversion of K. brevis to a non-motile or resting form.     

First Detection of Tetrodotoxin in Greek Shellfish by UPLC-MS/MS Potentially Linked to the Presence of the Dinoflagellate Prorocentrum minimum

During official shellfish control for the presence of marine biotoxins in Greece in year 2012, a series of unexplained positive mouse bioassays (MBA) for lipophilic toxins with nervous symptomatology prior to mice death was observed in mussels from Vistonikos Bay–Lagos, Rodopi. This atypical toxicity coincided with (a) absence or low levels of regulated and some non-regulated toxins in mussels and (b) the simultaneous presence of the potentially toxic microalgal species Prorocentrum minimum at levels up to 1.89 × 10³ cells/L in the area’s seawater. Further analyses by different MBA protocols indicated that the unknown toxin was hydrophilic, whereas UPLC-MS/MS analyses revealed the presence of tetrodotoxins (TTXs) at levels up to 222.9 μg/kg. Reviewing of official control data from previous years (2006–2012) identified a number of sample cases with atypical positive to asymptomatic negative MBAs for lipophilic toxins in different Greek production areas, coinciding with periods of P. minimum blooms. UPLC-MS/MS analysis of retained sub-samples from these cases revealed that TTXs were already present in Greek shellfish since 2006, in concentrations ranging between 61.0 and 194.7 μg/kg. To our knowledge, this is the earliest reported detection of TTXs in European bivalve shellfish, while it is also the first work to indicate a possible link between presence of the toxic dinoflagellate P. minimum in seawater and that of TTXs in bivalves. Confirmed presence of TTX, a very heat-stable toxin, in filter-feeding mollusks of the Mediterranean Sea, even at lower levels to those inducing symptomatology to humans, indicates that this emerging risk should be seriously taken into account by the EU to protect the health of shellfish consumers.     

Salinity effect on growth and toxin production of four tropical Alexandrium species (Dinophyceae).

Four tropical PSP toxins-producing dinoflagellates, Alexandrium minutum, Alexandrium tamiyavanichii, Alexandrium tamarense and Alexandrium peruvianum from Malaysian waters were studied to investigate the influences of salinity on growth and toxin production. Experiments were conducted on constant temperature 25 degrees C, 140 microE mol m(-2) s(-1) and under 14:10 light:dark photo-cycle with salinity ranged from 2 to 30 psu. The PSP-toxin congeners, GTX 1-6, STX, dcSTX, NEO and C1-C2 were analysed by high performance liquid chromatography. Salinity tolerance of the four species in decreasing order is A. minutum>A. peruvianum>A. tamarense>A. tamiyavanichii. Specific growth rates and maximum densities varied among these species with A. minutum recorded as the highest, 0.5 day(-1) and 6 x 10(4) cells L(-1). Toxin content decreased with elevated salinities in A. minutum, the highest toxin content was about 12 fmole cell(-1) at 5 psu. In A. tamiyavanichii, toxin content peaked at optimal growth salinity (20 and 25 psu). Toxin content of A. tamarense, somehow peaked at sub-optimal growth salinity (15 and 30 psu). Results of this study implied that salinity fluctuation not only influenced the growth physiology but also toxin production of these species.     

Brevetoxins, like ciguatoxins, are potent ichthyotoxic neurotoxins that accumulate in fish.

Brevetoxins and ciguatoxins are closely related potent marine neurotoxins. Although ciguatoxins accumulate in fish to levels that are dangerous for human consumption, live fish have not been considered as potential sources of brevetoxin exposure in humans. Here we show that, analogous to ciguatoxins, brevetoxins can accumulate in live fish by dietary transfer. We experimentally identify two pathways leading to brevetoxin-contaminated omnivorous and planktivorous fish. Fish fed with toxic shellfish and Karenia brevis cultures remained healthy and accumulated high brevetoxin levels in their tissues (up to 2675 ng g(-1) in viscera and 1540 ng g(-1) in muscle). Repeated collections of fish from St. Joseph Bay in the Florida panhandle reveal that accumulation of brevetoxins in healthy fish occurs in the wild. We observed that levels of brevetoxins in the muscle of fish at all trophic levels rise significantly, but not to dangerous levels, during a K. brevis bloom. Concentrations were highest in fish liver and stomach contents, and increased during and immediately following the bloom. The persistence of brevetoxins in the fish food web was followed for 1 year after the K. brevis bloom.     

Occurrence of paralytic shellfish toxins in Cambodian Mekong pufferfish Tetraodon turgidus: selective toxin accumulation in the skin.

The toxicity of two species of wild Cambodian freshwater pufferfish of the genus Tetraodon, T. turgidus and Tetraodon sp., was investigated. Tetraodon sp. was non-toxic. The toxicity of T. turgidus was localized mainly in the skin and ovary. Paralytic shellfish toxins (PSTs), comprising saxitoxin (STX) and decarbamoylsaxitoxin (dcSTX), account for approximately 85% of the total toxicity. Artificially reared specimens of the same species were non-toxic. When PST (dcSTX, 50 MU/individual) was administered intramuscularly into cultured specimens, toxins were transferred via the blood from the muscle into other body tissues, especially the skin. The majority (92.8%) of the toxin remaining in the body accumulated in the skin within 48h. When the same dosage of tetrodotoxin (TTX) was similarly administered, all specimens died within 3-4h, suggesting that this species is not resistant to TTX. Toxin analysis in the dead specimens revealed that more than half of the administered TTX remained in the muscle and a small amount was transferred into the skin. The presence of both toxic and non-toxic wild specimens in the same species indicates that PSTs of T. turgidus are derived from an exogenous origin, and are selectively transferred via the blood into the skin, where the toxins accumulate.     

Growth and toxin production in the ciguatera-causing dinoflagellate Gambierdiscus polynesiensis (Dinophyceae) in culture

The growth and toxin production in a clonal strain of Gambierdiscus polynesiensis, TB-92, was examined in batch culture conditions. The mean growth rate at exponential phase was (0.13 ± 0.03) division day⁻¹. Regardless of the age of cultures, all mice injected with dichloromethanolic and methanolic extracts showed symptoms specific to ciguatoxin (CTX) and maitotoxin (MTX) bioactivity, respectively. The highest total toxicity assessed in TB-92 cultures was 10.4 × 10⁻⁴ mouse unit cell⁻¹. The toxin production pattern reveals an enhanced cellular toxin content with the age of the culture. CTX- and MTX-like compounds each accounted for approx. 50% of the total toxicity of TB-92 cultures, except in aged cells where CTXs were dominant. The high ciguatoxic activity of TB-92 was further confirmed in dichloromethanolic extracts by means of the receptor-binding assay. The highest CTX level monitored at late stationary phase was (11.9 ± 0.4) pg P-CTX-3C equiv cell⁻¹. Further HPLC and LC-MS analysis revealed the presence of five CTXs congeners in lipid-soluble extracts, i.e. CTX-3C, -3B, -4A, -4B and M-seco-CTX-3C, and of new CTX congeners. Toxin composition comparison between two G. polynesiensis strains suggests that the toxin profile is a stable characteristic in this species. G. polynesiensis clones also proved inherently more toxic than other Gambierdiscus species isolated from other geographical areas.     

Comparative study on differential accumulation of PSP toxins between cockle (Acanthocardia tuberculatum) and sweet clam (Callista chione).

At the western Mediterranean coast of Morocco, the cockle (Acanthocardia tuberculatum) contained persistent high levels of paralytic shellfish toxins for several years, while other bivalve molluscs such as sweet clam (Callista chione) from the same vicinity were contaminated seasonally to a much lesser extent. In order to understand the causes of this prolonged contamination, a comparative study on PSP decontamination between sweet clam and cockle was conducted from November 2001 until June 2002. PSP toxicity was analysed by automated pre-column oxidation (Prechromatographic oxidation and LC-FD) in several organs of both species, namely digestive gland, foot, gill, mantle, muscle and siphon for sweet clams. The results showed that cockle sequester PSP toxins preferably in non-visceral organs (Foot, gill and mantle) contrary to sweet clam that sequester them in visceral tissues (digestive gland). The toxin profile of cockle organs indicated dominance of dcSTX, whereas sweet clam tissues contained especially C-toxins. Substantial differences in toxin profile between cockle and sweet clam, from the same area as well as from the composition of PSP toxin producer, Gymnodinium catenatum, confirm the bioconversion of PSP toxins in cockle.     

Accumulation and depuration profiles of PSP toxins in the short-necked clam Tapes japonica fed with the toxic dinoflagellate Alexandrium catenella.

A toxic dinoflagellate responsible for paralytic shellfish poisoning (PSP), Alexandrium catenella (Ac) was fed to the short-necked clam Tapes japonica, and the accumulation and depuration profiles of PSP toxins were investigated by means of high-performance liquid chromatography with postcolumn fluorescence derivatization (HPLC-FLD). The short-necked clams ingested more than 99% of the Ac cells (4 x 10(7)cells) supplied once at the beginning of experiment, and accumulated a maximal amount of toxin (185 nmol/10 clams) after 12h. The rate of toxin accumulation at that time was 23%, which rapidly decreased thereafter. Composition of the PSP toxin accumulated in the clams obviously different from that of Ac even 0.5h after the cell supply, the proportion of C1+2 being much higher than in Ac, although the reason remains to be elucidated. In contrast, a higher ratio of gonyautoxin (GTX)1+4 than in Ac was detected in the toxin profiles of clam excrements. The variation in toxin composition derived presumably from the transformation of toxin analogues in clams was observed from 0.5h, such as reversal of the ratio of C1 to C2, and appearance of carbamate (saxitoxin (STX), neoSTX and GTX2, 3) and decarbamoyl (dc) derivatives (dcSTX and dcGTX2, 3), which were undetectable in Ac cells. The total amount of toxin distributed over Ac cells, clams and their excrements gradually declined, and only 1% of supplied toxin was detected at the end of experiment.     

Non-selective retention of PSP toxins by the mussel Mytilus galloprovincialis fed with the toxic dinoflagellate Alexandrium tamarense

Mussels, Mytilus galloprovincialis, were contaminated by paralytic shellfish poisoning (PSP) toxins by being fed with the toxic dinoflagellate Alexandrium tamarense. Temporal variations in the toxin content and the profile of mussels during the feeding experiment were monitored by high-performance liquid chromatography (HPLC). The toxin profile of mussels was compared with that of A. tamarense to clarify the mechanism of uptake of toxins in mussels. The prominent toxins in mussels and A. tamarense were N-sulfocarbamoyl toxins (C1,2) and carbamate toxins, gonyautoxin-1,4 (GTX1,4). The toxin profiles of both mussels and A. tamarense were almost constant throughout the experimental period. There were no remarkable differences in the toxin proportion between mussel and A. tamarense. These results indicate that mussels do not selectively accumulate particular toxins.     

Toxic and lethal effects of ostreolysin, a cytolytic protein from edible oyster mushroom (Pleurotus ostreatus), in rodents.

Ostreolysin (Oly), an acidic, 15 kDa protein from the edible oyster mushroom (Pleurotus ostreatus), is a toxic, pore-forming cytolysin. In this paper, its toxic properties have been studied in rodents and the LD(50) in mice shown to be 1170 microg/kg. Electrocardiogram, arterial blood pressure and respiratory activity were recorded under general anaesthesia, in intact, pharmacologically vagotomised and artificially respirated rats injected with one mouse LD(50). A few seconds after intravenous Oly injection, a transient increase in arterial blood pressure was recorded, followed by a progressive fall to mid-circulatory pressure accompanied by bradicardia, myocardial ischaemia and ventricular extrasystoles. Similar changes produced by Oly were observed in vagotomised and artificially respirated animals, indicating that vagotomy and hypoxia play no primary role in toxicity. Oly induced lysis of rat erythrocytes in vitro, and probably also in vivo as indicated by the increase in serum potassium. Although direct action of the protein on the cardiomyocytes or heart circulation cannot be excluded, the hyperkalaemia resulting from the haemolytic activity probably plays an important role in its toxicity. The lethality and cardiorespiratory toxic action of Oly are thus shown to be candidates for the cause of the recorded adverse effects of oyster mushroom.     

Comparative effects of the toxic dinoflagellate Karenia brevis on clearance rates in juveniles of four bivalve molluscs from Florida, USA.

The effects of Karenia brevis (Gymnodiniales, Gymnodiniaceae) on the feeding activity of juveniles of four species of bivalve mollusc were examined in the laboratory to assess the potential impacts on these important shellfish populations from Florida. Clearance rates were determined under short-term (one hour) static and long-term (two days) flow-through conditions using both whole and lysed cultures of K. brevis. Under short-term conditions, the bay scallop, Argopecten irradians, was the most sensitive species, exhibiting a 79% reduction in clearance rate at 1000 cells ml(-1) of whole K. brevis culture compared to the control (no K. brevis). The eastern oyster, Crassostrea virginica, was the least responsive, showing a 38% reduction in clearance rate between the same treatments. The green mussel, Perna viridis, and the northern quahog, Mercenaria mercenaria, displayed intermediate responses. Similar results were also observed during long-term exposures to a continuous supply of K. brevis. Bay scallops showed a significant decline in clearance rate at 100 cells ml(-1) after 24h exposure; clearance rate of oysters was not affected by K. brevis at this concentration. No mortality was observed for any species during these brief exposures. The prospect for recovery of bay scallop populations in Florida estuaries where they were once abundant may be hampered by recurring blooms of K. brevis. Reduced clearance rates in M. mercenaria at high K. brevis densities could translate into poor growth of cultured Florida hard clams. On the other hand, P. viridis, which also showed reduced clearance rates at high K. brevis concentrations, might be negatively impacted by K. brevis blooms, thereby affecting their ability to spread into estuaries hampered by recurring toxic algal blooms.