Shellfish Testing

Interns with the Sitka Tribe select frozen saxitoxin extracts for analysis. Photo: Bethany Goodrich

Phytoplankton monitoring is an important early warning for shellfish toxins, but the only way to be sure that shellfish are safe is to routinely test them as well. As with phytoplankton monitoring methods, shellfish testing in Alaska can also be split into public health techniques and research-centric techniques. For public health, shellfish testing needs to be fast, capable of capturing the full toxicity of a sample, cost-effective, and targeted toward the species that will be consumed. Since toxin results are extremely time sensitive for all three harvesting groups, the Mouse Bioassay or Receptor Binding Assay (RBA) are the preferred methods for paralytic shellfish toxin testing, since both are high-throughput, total toxicity tests. The mouse bioassay is currently approved by the FDA for all interstate, commercial shellfish testing, while the RBA is approved for non-commercial monitoring, precautionary commercial closures, and commercial blue mussels and is in review for commercial geoducks and oysters. In these methods, a sample of at least 100 grams of shellfish tissue (about the weight of a deck of cards) is blended into a slurry. The toxins are then extracted from the shellfish slurry and further analyzed for a total toxicity value, typically expressed in micrograms of saxitoxin equivalent per 100 grams of shellfish. The mouse bioassay and RBA do not produce a toxin profile (e.g. 10% saxitoxin, 90% neosaxitoxin), and instead produce an total toxicity value that can be directly compared to the FDA regulatory limit of 80 micrograms of saxitoxin equivalent per 100 grams of shellfish tissue.

For researchers, the toxin profile of a given shellfish sample can provide important information about which HAB species are producing the bloom or how potent the toxins are. For those measurements, high performance liquid chromatography (HPLC) or liquid chromatography – tandem mass spectrometry (LC-MS/MS) are the preferred methods. While the HPLC is an FDA-approved regulatory method for commercially harvested shellfish, the higher cost of HPLC testing and longer testing time mean that most states, including Alaska, use the mouse bioassay for commercial management. HPLC and LC-MS/MS tests separate the paralytic shellfish toxins into their constituent parts by running the toxins through specialized columns that force molecules with certain properties to travel faster or slower through the column, much like drawing water up through tissue paper will separate ink into its constituent colors. The individual compounds are then analyzed and identified. As with the mouse bioassay and RBA, results can then be communicated in micrograms of saxitoxin equivalents per 100 grams of shellfish tissue. but HPLC and LC-MS/MS data can also be presented as a full toxin profile.

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