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Shellfish play no role in Pfiesteria piscicida-related pathology. This toxin-producing unicellular alga has an improbably complex life cycle and many (24) morphological forms. These vary greatly in size from 5 µm to 750 µm. It is able to change quickly from one shape to another, which makes detection difficult. The algae lie for a long time inactive on the bottom of the sea or estuary and then suddenly burgeon massively and release toxins if there is a local increase in the number of fish in the neighbourhood. In recent years it has caused massive fish death in rivers and estuaries in North Carolina and parts of Mexico. There are two kinds of toxins: water-soluble and fat-soluble. The fat-soluble one causes skin lesions in fish. The water-soluble toxin affects the central nervous system. It stuns the fish so that they do not swim away, and then the fat-soluble toxin can do its work. The fish skin is damaged, ulcers result and the internal salt balance is disturbed. The fish dies, decomposes and the breakdown substances which contain lots of nitrogen and phosphorous, are used by the algae for their growth. This alga has not been known to date outside North American waters, but account must be taken of the possibility that in future it may appear on other coasts, for example via ballast water from freighters. However, the North Sea is probably too salty and too cold, although there could be a real danger at river mouths.
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This alga can also have important effects on humans. Humans may have problems if they breathe in the toxins via aerosol or if they get it on their skin. A range of symptoms has been associated with Pfiesteria piscicida. Only a few of these are well documented.
