Laboratory Methodology for Shellfish Toxins

Abstract

Paralytic shellfish poisoning (PSP) occurs following the consumption of shellfish that have been exposed to toxic marine dinoflagellates and contain a potent source of neurotoxins, called paralytic shellfish toxins (PSTs) (1). PSTs consist of at least 18 different toxins that can be divided into subclasses containing hydrogen atom (e.g., saxitoxin) or hydroxyl (neosaxitoxin-related toxins) groups on the R1 position. Gonyautoxins toxins (GTXs), are characterized by 11-hydroxy-saxitoxin sulfate derivatives (Fig. 1). Dinoflagellates are important ocean plankton that contribute to the foundation of the marine food chain as a primary source of carbohydrates, proteins, and lipids. The organisms are microscopic, single-cell photosynthetic algae, which during periodic maxima produce local discolorations of coastal waters. Shellfish poisonings are difficult to control because of the unpredictable and sporadic occurrence of the dinoflagellates which produce PSTs. The initial 184relationship between PSTs and the presence of plankton from the genus Gonyaulax was reported by Sommer et al. (2). In fact, toxic dinoflagellate blooms occur sporadically in large numbers from as many as 20 species of dinoflagellates throughout the world, with the greatest frequencies occurring in the north and south temperate zones (3). Outbreaks of PSP occur in both protected and unprotected waters in which conditions are prone to an upwelling of nutrient-rich water often associated with organic loading of seawater from run off or sewage disposal (4), or unique changes in salinity, temperature, light, and water turbulence. Seasonal outbreaks of red tide vary in coastal regions, as evidenced by the 185reported outbreaks between mid-May and late October along the Pacific Coast, in contrast to the period of mid-July to late September in Atlantic Canada (5). Dinoflagellates represent a rich source of food for bivalve shellfish and changes in water conditions or the presence of predators, such as diatoms, will change the dinoflagellate population. The principle group of shellfish associated with PSP outbreaks has been the bivalve mollusks, such as clams, mussels, scallops, and oysters. These species accumulate PSTs from poisonous dinoflagellates in the hepatopancreas and are then metabolized, excreted, or released within several weeks. One exception is the Alaskan butter clam (Saxidomus giganteus), which mobilizes PSTs from the hepatopancreas to the siphon, where they can remain for several years (6). Shellfish become toxic with PSTs only after a few days of becoming exposed to a red tide bloom or extremely high concentrations of poisonous dinoflagellates in cohabitat waters. Other intertidal organisms such as gastropods, periwinkles, whelks, and certain crab species have also been found to contain PSP (7–9). The blooms of Gonyaulax species are often referred to as red tide blooms due to the color produced from a xanthophyll derived from light-harvesting antenna (10) and the fact that the blooms are 90–95% monospecific to this species of dinoflagellate that reach as many as 20,000–30,000 organisms/ml of seawater. The distribution of PSP in shellfish is variable and depends on the species of bivalve and the specific tissue involved. In the case with red tide blooms, the correct environmental conditions produce a generation of algae that produce domoic acid poisoning, also referred to as amnesic shellfish poisoning (ASP). Subsequently, shellfish and finfish which feed by filtering seawater concentrate the algae and accumulate domoic acid toxin, principally in hepatopancreatic tissue. As is the case with other sources of seafood toxins, the principle agent is confined to the viscera.