I was attracted to a colour photo on a recent Guardian obituary showing a cluster of Pacific banded snails that inspired Bryan Clarke to study their genetic variation.
These tree snails were first recorded by Captain Cook in 1774 and resembled banded snails (Cernuella virgata) found on the sand dunes of Northumberland which have been studied by zoology students for several decades. Their white to rust coloured shell usually has a dark brown spiral band which may be thin, broken or missing altogether. The internal lip may be white or pinkish and the spire comprises 5-7 whorls.
Bryan Clarke began his research on a slightly different snail species, Cepaea while he taught at Edinburgh University during the 60’s but then his interest moved to a Polynesian snail Partula. He made crosses between endemic species on different islands and identified some of the mechanisms that controlled the movement of genes across the species barrier. It seems that the inherited banding on shells may be crucial for survival as the banding provides camouflage from predators. Apart from birds snails are eaten by shrews, hedgehogs and various rodents.
Bryan Clarke and his wife were regular travellers to Polynesia but were horrified to find their unique snails were being eaten by an alien mollusc, introduced by the French in a misguided attempt to control the giant African snail. This was introduced as a delicacy in 1967 but became a serious pest within a few years. The predatory snail tracks its prey chemically, and sits on the quarry’s shell to suck out the contents. The native thin-shelled Partula were easier prey than the hefty African species. Partula species soon became extinct on the islands. The only survivors of the original Polynesian snails are now carefully nurtured in a few zoos across the world.
This unfortunate experience led Clarke to help create the Frozen Ark in 1996 which now involves the Natural History Museum in London and several other science institutions. Its objective is to save samples of frozen cells containing DNA from endangered animals before they go extinct.
Only very small samples are required and they are taken from the animal concerned without pain. Mouth swabs, hairs or feathers or routine blood samples, even faeces provide DNA. Once frozen cells can be stored safely at very low temperatures in a small space for potentially hundreds of years. Hopefully many cells can be revived and regrown by cloning. Currently the Frozen Ark holds 48,000 samples from more than half a million endangered and non-endangered animal species. These include the Scimitar Horned Oryx from Africa, the Socorro dove from the Revillagegido Islands off Mexico, the Seychelles Fregate Beetle and of course the Polynesian tree snails. Habitat decline due to climate change and desertification, overhunting and human activities including introduction of inappropriate alien species all contribute to extinctions.
Professor Bryan Clarke won the 2010 Darwin Medal awarded from Royal Society for his outstanding contribution to understanding the genetic basis of evolution. He died in February 2014 aged 81.