Poisons in Food Chains: The case of ciguatera Poisoning (Heinemann 1 pp 53-54)
Because organisms eat one another, their products (such as poisons) move along food chains. A
poison found in a producer organism can end up affecting top carnivores far removed from the first
organism of the food chain.
Ciguatera, for example, is a term used to describe a type of fish poisoning in humans. It has been
long known in the tropical Pacific. Captain Cook and his crew were poisoned by fish in the New
Hebrides in 1774, and in the Pacific today about one person in every 1000 is affected each year. In
Australia, ciguatera poisoning has only been reported in Queensland.
Ciguatera poisoning is not well understood. It seems to involve a nerve poison which affects the
transmission of impulses from nerves to muscles in mammals. The poison causes vomiting,
diarrhoea, numbness, muscular weakness, reduced vision and confusion of the senses, such that an
affected person cannot distinguish hot from cold. In severe cases, shock, convulsions and muscular
paralysis occur, followed by death.
Ciguatera poisoning can occur after eating any one of a large number of tropical marine fishes,
including barracudas, groupers, sea basses, surgeonfishes, parrotfishes, jacks, wrasses, toadfishes,
herrings and eels. Over 400 species of fishes have been blamed for cases of poisoning. Since almost
all of the fishes are normally edible, and many are valuable food fishes in the world, ciguatera
poisoning is the most common fish poisoning affecting humans. So far as is known, the fishes are
unaffected by the poison.
The majority of these poisonous fishes are coral-reef or shore fishes between latitudes of 35°N and
34°5. They are usually bottom dwellers. They are herbivores and carnivores, feeding on algae or
other fishes.
Detrital feeders like mullet are seldom poisonous. Among herbivores, the liver and the gut are most
toxic. Among the carnivores, the flesh can be more toxic and the amount of fish in the diet is
proportional to the level of toxicity. Big fish also tend to be more toxic than small fish of the same
species because poison accumulates in the muscle tissue with age.
It seems certain that ciguatera poisoning is associated with the food chains of the fish (� 3:17). The
poison is formed in marine cyanobacteria. The product of the cyanobacteria is transferred to
herbivorous fishes, which in turn are eaten by humans. Herbivorous fishes are also eaten by
carnivorous fishes which are eaten by humans. Since some invertebrates, echinoderms, molluscs
and arthropods are poisonous, the poison probably passes from algae to these animals, which are
then eaten by fishes.
What triggers an outbreak of ciguatera is not known. Since the poison is transferred through food
chains, ciguatera is difficult to predict and impossible to control.
QUESTIONS
1.
What part of the sea food chain produces the poison ciguatera?
2.
How does the flesh of carnivorous fish become toxic?
3.
Why are detrital feeders seldom poisonous?
4.
Starfish (echinoderms) may be poisonous. Why?
5.
The red snapper is one of the most consistently toxic fish species causing poison in the
Pacific. What does this tell you about its position in the food chain?