This toxin is produced by marine algae. It is a complex polyether, whish is highly lipophillic. The structure is shown below.
This complex organic molecule acts as a potent inhibitor of protein phosphatases. Recent research points towards effects upon testosterone levels in mammals.
Shellfish poisoning is not all down to okadaic acid. Neurotoxins such as brevetoxins and amnesiacs such as domoic acid are also responsible for the illness which can follow shellfish consumption.