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15 April 1995

Disgustinng sunblock keep snails off the menu

MOLECULES which protect tiny marine snails from being eaten by fish may also prevent them from getting scorched by the Sun, according to scientists who have unravelled the chemistry behind the bizarre behaviour of a shrimp-like Antarctic crustacean.

Five years ago, researchers working at the US Antarctic research station in McMurdo Sound found that the amphipod Hyperiella dilatata, which grows to a millimetre or so in length, regularly “kidnaps” small shell-less snails called Clione antarctica. These snails seem to be immune to attack from the plankton-feeding fish that are the amphipods’ main predators. So, to protect themselves, the amphipods often carry a snail on their backs, holding them firmly in place with two of their pairs of legs. The snails can grow to several times the size of H. dilatata, so the amphipods select only the smallest. Fish avoid amphipods carrying snails and will spit them out if they capture one by accident.

Ever since this remarkable behaviour was first observed, scientists have been working to find the chemical that makes the snails so unpalatable. Researchers led by Bill Baker of the Florida Institute of Technology extracted the chemicals produced by C. antarctica using two different solvents, which each extracted a different mixture of substances. They then added these chemical cocktails to pellets of fish food and compared the feeding patterns of fish given the doped food with those given untreated, control pellets. One of these mixtures stopped the fish feeding, indicating that it contained the defensive chemical.

The researchers purified each of the different chemicals in this mixture using a technique called high performance liquid chromatography, which separates chemicals on the basis of the differing strengths with which they bind to a silica gel. When these chemicals were tested individually only one made the fish stop feeding. This molecule, which the researchers call pteronenone, had a structure similar to that of chemicals which absorb the Sun’s damaging ultraviolet rays and are carried by other planktonic creatures (Journal of Organic Chemistry, vol 60, p 780). So it seems that the molecule which makes C. antarctica unpalatable to fish and causes the snails to get kidnapped may also stop them getting sunburnt.