"HE WAS a brave man", said Jonathan Swift, "that first ate an oyster." Understandable as that may be in retrospect, oysters and other shellfish have become delicacies that most of us enjoy whenever we have the opportunity, often taking the advice of a well known expert on the topic:
"A loaf of bread", the Walrus said,
Is what we chiefly need,
Pepper and vinegar besides
Are very good indeed".
But such pleasures sometimes have their price if a "red tide" blooms in their vicinity.
Red tide is a species of plankton. These tiny, often microscopic, organisms float on the surface of the sea and are at the mercy of every wind and current that may come their way. Those that are animals in nature are called zooplankton, those that are plants are phytoplankton.
Now at certain times of the year, phytoplankton can become very abundant for short periods - an event called a bloom Like land plants, phytoplankton require light and nutrients to live and thrive. When the sun is bright and the concentrations of nitrates and phosphates in the water are coincidentally high, a bloom occurs and the numbers of phytoplankton may increase many thousandfold in a very short time.
When blooms occur in the open sea, they can be a positive benefit to fish life, by providing a much greater abundance of food than usual. But there are snags. During the process of evolution, some of these organisms learned to fight back in a rather clever way against been seen by the denizens of the deep as a free floating lunch.
A few species produce chemicals that discourage fish from eating them, and in some cases these substances are also toxic to humans. Although we rarely absorb enough plankton ourselves to become ill, the toxin may be concentrated in significant amounts of shellfish and other consumers of the plankton, and the poison may then be ingested by an unsuspecting gourmet.
While some of these less pleasant species of phytoplankton are transparent or neutral in colour, others have a distinct red or brownish tinge that colours the water when they bloom. It is the latter, known as dinoflagellates, that are responsible for the red tides that occur from time to time off the southeast coast of Ireland, the most common offenders being Dinophis acuta and Dinophis acuminata, both of which produce a toxin known as okadic acid.