The rock scallop Crassadoma gigantea can achieve market size in 2-4 years and shows promise for commercial culture along its native range, from Baja California to southeastern Alaska. One serious unresolved issue, however, is the lack of information on accumulation and retention of algal toxins that can cause paralytic shellfish poisoning (PSP) in humans. This is critical because toxins associated PSP, e.g. saxitoxin (STX) and its derivatives, are currently the most widely reported toxins in bivalves along the west coast of North America. To address this issue, hatchery-reared rock scallops were deployed at three locations prone to PSP closures in Washington State, and sampled when a closure occurred and weekly thereafter until the closure was lifted. Induced toxicity under controlled conditions of algal cell density, temperature and salinity were also conducted at the NOAA Northwest Fisheries Science Laboratory in Manchester, WA. This work represents the first attempt to induce toxicity in C. gigantea.
The overarching goal of this research was to address the needs of public health agencies and shellfish producers by investigating where biotoxins accumulate in scallops, and how long the toxins remain. Temporal field exposure trials and lab experiments examined bioaccumulation and subsequent detoxification of STX in the adductor muscle and viscera of individual scallops. Both field and lab studies demonstrated that rock scallops can attain very high toxin loads for long periods of time, but toxicity was generally confined to the viscera (gut). Mean toxicity levels in rock scallop adductor muscles (the part that is consumed) were below the regulatory limit for STX (80µg/100g tissue). Induced toxicity experiments revealed a pattern of toxification and detoxification similar to other studies of bivalves fed toxic dinoflagellates, with rock scallops increasing toxin loads in digestive gland tissues at significantly higher rates than for other tissues (Bricelj and Shumway 1998). It is not clear from any of our datasets, field exposure or laboratory, how long rock scallops take to completely depurate STX from all tissues. This aspect of research will need to be further explored with future monitoring efforts to identify complete depuration rates. This data gap aside, it is clear that very high, persistent levels of STX in scallop viscera will likely preclude this species from safe, whole product consumption. The potential for a shucked, adductor only market will also require careful scrutiny due to persistent toxicity and variability of toxicity among individual rock scallops. This work was supported by NOAA-OAR-SG-2016-2004807.