Fish and shellfish share the waters with many thousands of animal and plant species, some of which engage in nasty chemical warfare with each other. At least 60 species of one-celled algae called dinoflagellates produce defensive toxins that also poison the human digestive and nervous systems. Several of these toxins can kill.
We don’t consume dinoflagellates directly, but we do eat animals that eat them. Bivalve filter feeders—mussels, clams, scallops, oysters—concentrate algal toxins in their gills and/or digestive organs, and then transmit the poisons to other shellfish—usually crabs and whelks—or to humans. Accordingly, most dinoflagellate poisonings are called “shellfish poisonings.” Many countries now routinely monitor waters for the algae and shellfish for the toxins, so the greatest risk is from shellfish gathered privately.