NEOPLASIA AND BIOTOXINS IN BIVALVES - IS THERE A CONNECTION

In the past 25 years, there has been an increase in the frequency of t wo major types of cancer in bivalves: disseminated neoplasia and germi nomas, which cause debilitation and mortality in shellfish stocks. Dis seminated neoplasia is common in softshell clams, Mya arenaria; the co ckle, Cerastoderma edule; and blue mussels, Mytilus trossulus; and les s common in edible oysters, Ostrea edulis; macomas, Macoma balthica; b lue mussels, Mytilus edulis; and Olympia oysters, Ostrea conchaphila. Germinomas occur more frequently in northern quahogs, Mercenaria merce naria, and softshell clams, Mya arenaria. Certain geographical locatio ns, especially along the northwest Pacific and northeast Atlantic Coas ts of North America and the Atlantic Coast of Europe, are ''hot spots' ' for neoplasia. A genetic susceptibility of bivalves to tumor formati on has been suggested, and the etiologies proposed include chemical ca rcinogens, viruses, and other transmissible agents. However, no clear cause-and-effect relationship has yet been conclusively demonstrated, nor has the potential role of biotoxins as etiological agents been exa mined. In the past 25 years, there has also been an increase in the fr equency with which humans have been poisoned by consuming toxic bivalv es. Filter-feeding bivalves accumulate biotoxins produced by toxic mic roalgal blooms. This study traces the worldwide distribution of paraly tic shellfish poisoning (PSP), diarrheic shellfish poisoning, neurotox ic shellfish poisoning, amnesic shellfish poisoning, and venerupin she llfish poisoning and of the microalgae and bivalve species associated with the poisonings and then compares these distributions with the dis tribution of neoplasia in bivalves. The incidence of disseminated neop lasia in some affected bivalve species appears to parallel, both spati ally and temporally, outbreaks of PSP that are associated with the tox igenic dinoflagellates Alexandrium tamarense, A. minutum, A. fundyense , and A. catenella. Shellfish that have accumulated potent saxitoxin a nd its derivatives (neosaxitoxin and gonyautoxins) produced by these d inoflagellates are highly toxic to humans. The presence of disseminate d neoplasia parallels the presence of certain toxin derivatives in bot h the bivalve and the Alexandrium spp. to which the bivalves are expos ed. Disseminated neoplasia is common in softshell clams, M. arenaria, that have apparently been exposed to and have accumulated gonyautoxins , (GTX), and in particular GTX1 and GTX4, that are produced by A. tama rense or A. fundyense. M. mercenaria is apparently not affected by dis seminated neoplasia and does not usually accumulate toxins associated with A. tamarense or A. fundyense. Bivalves that accumulate high conce ntrations of saxitoxin or neosaxitoxin, such as butter clams, Saxidomu s giganteus; surf clams, Spisula solidissima; sea scallops, Placopecte n magellanicus; and California mussels, Mytilus californianus, are app arently not affected by disseminated neoplasia or germinomas. In M. ar enaria, the incidence of germinomas appears to be related to the distr ibution of Alexandrium spp. blooms. In M. mercenaria, however, the dis tribution of germinomas is not related to those Alexandrium spp. that are commonly associated with PSP. The incidence of disseminated neopla sia and germinomas is nor correlated with PSP outbreaks associated wit h Pyrodinium bahamense var. compressum or Gymnodinium catenatum. Altho ugh the epizootiological evidence presented here for a correlation bet ween dinoflagellate toxin profiles, the deposition of toxins in bivalv e tissues, and the presence of neoplasia in such bivalves is circumsta ntial, it should be investigated in field and laboratory experiments.