The microscopic plant, an alga known as a dinoflagellate, usually lurks in the waters of the Palmico and Neuse estuaries on the Atlantic coast of the south-eastern United States. But the presence of live fish can provoke the algae to secrete a powerful poison which kills everything around it.
In one episode, about a million menhaden - a fish related to the herring - died from the effects of the nerve toxin produced by an algal bloom of this dinoflagellate. The fish twitch, appear disoriented then suffocate. At least eight other incidents of mass poisoning have been documented.
Toxic algal blooms - sometimes known as 'red tides' - have become notorious in recent years. The poisons secreted by the algae can accumulate in shellfish, apparently without harm until eaten by humans. One incident in Guatemala resulted in 26 deaths of people who had eaten shellfish containing the nerve poison saxitoxin. Last year, in Monterey Bay, California, many pelicans died as a result of feeding on anchovy which had fed on the algae.
But, according to four biologists from North Carolina State university, writing in today's issue of Nature, their local dinoflagellate are different. These microscopic plants eat the fish that they poison. They grow an appendage, called a 'peduncle', swim towards the dying fish to reach flecks of sloughed flesh using the peduncle to attach to, and digest, its food.
According to the biologists, 'this dinoflagellate completes its sexual cycle while killing fish', then becomes dormant. Within two hours the algae have changed into cysts which sink to the sediments to await a new predatory foray. Thus, when investigators arrive, the killers of the fish have already vanished.
The cysts are almost indestructible. The biologists write: 'After treatment with concentrated sulphuric acid or ammonium hydroxide, 35 days of dessication, or nearly two years of dormancy, small percentages of the cysts have yielded viable toxic cells . . .'