Shellfish toxins are naturally occuring compounds produced by microscopic planktonic algae (phytoplankton). Under normal conditions the concentrations of these compounds are very low and do not cause any problem. However, filter feeders such as clams, cockles, mussels, oysters and crustaceans such as crabs and lobsters can consume large quantities of these algae when environmental conditions result in algae blooms. Such blooms may discolour the surface of the sea from red (the so-called ‘red tides’) to yellow, green, brown or blue.
High concentrations of shellfish toxins can cause illness amongst people who consume contaminated shellfish or crustaceans. The most common illnesses that result from shellfish toxins are Paralytic Shellfish Poisoning (PSP), Diarrheic Shellfish Poisoning (DSP) and Amnesic Shellfish Poisoning (ASP). PSP is caused by a wide number of derivatives of saxitoxin (STX). DSP is primary caused by a group of polyether toxins including okadaic acid (OA). ASP is caused by domoic acid (DA), an excitatory amino acid containing the structure of glutamic acid and resembling kainic acid.