Paralytic shellfish toxins in geoduck clams (Panope abrupta): Variability,anatomical distribution, and comparison of two toxin detection methods

The geoduck clam, Panope abrupta: is a valuable economic resource in Washin gton State. Prior to the mid 1970s, the levels of paralytic shellfish poiso ning (PSP) toxins in Washington State geoducks were not considered by the W ashington State Department of Health (WDOH) to be a risk to public health b ecause the viscera were presumed to be discarded. Recent monitoring informa tion indicates that geoducks accumulate high levels of toxins, primarily in the viscera. The purposes of this study were to determine: (1) the seasona l concentration of paralytic shellfish toxins in geoduck clams at two sites and at two depths within each site; (2) the variability of PSP toxin level s among individual clams within each site; (3) the anatomical distribution of toxins: and (4) the correlation between two methods for estimating PSP t oxins. From the summer of 1997 through the winter of 1998, 12-24 geoducks w ere collected biweekly from a shallow (7 m) and a deep (17 m) location in e ach of two tracts in Puget Sound, Washington: Quartermaster Harbor (QH) and Agate Pass (AP). Geoducks, dissected into siphon, mantle, and visceral por tions, were assayed separately using the mouse bioassay (MBA), while only t he visceral portions were assayed using the receptor-binding assay (RBA). R esults indicated that toxin variability between individual clams was high i n the shallow areas, with coefficients of variation (CVs) ranging from 20-9 8%, and lower in the deep areas (CV = 18-62%). In QH, only geoducks from th e shallow water had toxin levels greater than the regulatory level of 80 mu g saxitoxinequivalents (STX eq) . 100 g shellfish tissue(-1), while all ge oducks from AP contained toxin above the regulatory level, with clams from shallow water considerably more toxic than those from deep water. Anatomica lly, the highest concentrations of PSP toxins were localized in the viscera of geoducks. There was a significant positive correlation between toxin le vels measured by the MBA compared to values obtained using the RBA (r(2) = 0.83). The large differences in toxicity between geoducks sampled at differ ent depths and harvest tracts indicate that careful management plans must b e designed in order to ensure public health.