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Bivalve molluscs, the primary vectors of paralytic shellfish poisoning (PSP) in humans, show marked inter-species variation in their capacity to accumulate PSP toxins (PSTs) which has a neural basis. PSTs cause human fatalities by blocking sodium conductance in nerve fibres. Here we identify a molecular basis for inter-population variation in PSP resistance within a species, consistent with genetic adaptation to PSTs. Softshell clams (Mya arenaria) from areas exposed to 'red tides' are more resistant to PSTs, as demonstrated by whole-nerve assays, and accumulate toxins at greater rates than sensitive clams from unexposed areas. PSTs lead to selective mortality of sensitive clams. Resistance is caused by natural mutation of a single amino acid residue, which causes a 1,000-fold decrease in affinity at the saxitoxin-binding site in the sodium channel pore of resistant, but not sensitive, clams. Thus PSTs might act as potent natural selection agents, leading to greater toxin resistance in clam populations and increased risk of PSP in humans. Furthermore, global expansion of PSP to previously unaffected coastal areas might result in long-term changes to communities and ecosystems.

Aureococcus anophagefferens is a picoplankionic alga that since 1985 has bloomed in coastal embayments of the western mid‐Atlantic, ranging from Narragansett Bay, Rhode Island, to Barnegat Bay, New Jersey, with greatest incidence of recurrence in Long Island bays, New York. Blooms of this small alga, referred to as “brown tide,” can persist for several months during late spring and summer at densities in excess of 1.0×106 cells ml−1. They are not associated with anomalous chlorophyll a, dissolved oxygen, or inorganic macronutrient (N, P) levels. Meterologically induced reduced flushing rates, elevated salinities, and delivery of micronutrients (e.g. iron) from the watershed have been implicated in bloom initiation. Brown tides have had severe detrimental effects on the benthos, especially eelgrass (Zostera marina) and suspension‐feeding bivalves, including bay scallops (Argopecten irradians) and blue mussels (Mytilus edulis). Adult bivalves experience sublethal effects (e.g. inhibition of clearance rates) at Aureococcus concentrations as low as ∼2×105 cells ml−1 and mortalities at ∼106 cells ml−1, attributed to toxicity of this microalga. Impacts of brown tide on zooplankton are less clear, but reduced egg production rates of copepods and reduced population growth rates of ciliates are documented at higher brown tide concentrations (≥1.0×106 cells ml−1). We summarize the state of knowledge about the physical, chemical, and biological factors that may contribute to brown tide initiation, maintenance, and decline and assess its ecological effects.

A laboratory study ~n v e s t~g a t e d cell ingestion, absorption of organlc matter, and paralytic shellfish poisoning (PSP) toxin ~ncorporation by iclytilus eduljs exposed to a high-tox~city isolate of the red tide dinoflagellate Alexandrium fundyense (strain GtCA29, toxicity = 66 pg saxltoxin equivalents (STXeq) cell-'). Maximum ingestion rate was achieved at 150 to 250 cells ml-' Clearance rates on A. fundyense were about 48'1.;) lower than those for a non-toxic control diet of the diatom Thalassioslra weissflogii. Mussels with no prior h~story of exposure to PSP maintained a constant ingest~on rate over 17 d of exposure to a A. fundyense (at 256 cells ml-l), and absorbed ca 6 2 % of the organic matter ingested. They experienced no mortality or sublethal adverse effects during intoxication. Maximum (saturation) toxin levels of 4.5 X 10"ig STXeq 100g-' were attained after 12 to 13 d. a value comparable to maximum toxicities reported during major toxic bloom events. Mussels could exceed the quarantine toxin level (80 pg STXeq 100g-l) in < 1 h of exposure to high densities of this isolate. At saturation, they incorporated 79 O/o of the toxin ingested, primarily in the viscera. This provides the first estimate of toxin incorporation efficiency in a bivalve under steady state conditions. Dinoflagellate toxins, determined by HPLC, were dominated by carbamate derivatives. The muscle, mantle/gill and foot of M. edulis showed significant enrichment in STX and reduction in the gonyautoxins GTX2+3 and neoSTX relative to ingested cells. The toxin composition of the viscera more closely resembled that of ingested cells, reflecting the presence of numerous intact cells in gut contents. Through its potential use of A. fundyense as a sole food source, M eduljs is thus capable of remarkably efficient toxln accumulation at environmentally realistic dinoflagellate cell densities.

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