Unravelling The Ecology Of Snail Shell Coiling

Ecologists are taking to the trees in a bid to unravel the ecology of shell coiling in snails. Speaking at the British Ecological Society’s Annual Meeting, being held at Manchester Metropolitan University on 9-11 September 2003, Dr Paul Craze of the University of Plymouth will explain how examining the proportion of right- and left-coiling individuals in a species of Bornean tree snail could help ecologists understand how new species arise.

The vast majority of snail species are almost exclusively dextral, or right-coilers, with just the occasional sinistral, or left-coiling, individual. However, in a small number of snail species there appears to be a stable balance between the number of right- and left-coilers. Coil direction in snails is inherited from the mother and is controlled by a single genetic locus or region, and coil direction is important because it is difficult for right-coiling snails to mate with left-coiling snails.

The fact that left- and right-coiling snails cannot align themselves properly during mating is, however, more than an irritation to the snails and an interesting puzzle for ecologists. Understanding how new species arise is a fundamental biological problem, and in the case of snails, some ecologists believe that the existence of left- and right-coiling individuals could be one mechanism for sympatric speciation (the development of new species by isolating them other than geographically). Since left-coiling and right-coiling snails find it hard to mate with each other they may, over time, develop into separate species.

Dr Craze and Dr Menno Schilthuizen of the Universiti Malaysia Sabah studied shell coiling in Amphidromus martensii, a snail that lives in the rainforests of Borneo, and one of the few species that is composed of roughly 50% right- and 50% left-coiling individuals, to find out what factors are responsible for maintaining this 50:50 split, and whether or not the right- and left-coilers show evidence of diverging towards becoming different species.

“There has been virtually no ecological study of shell-coiling polymorphism, so the first thing we were interested in was whether the 50:50 ratio that we know exists at the medium scale (ie between sites separated by tens of kilometres) also exists at a smaller scale (ie between sites separated by tens of metres), or whether the right coilers occupied different habitats from the left coilers,” Dr Craze says.

Because A. martensii lives in the forest canopy, Dr Craze chose to collect shells of dead snails from the ground. “We found equal numbers of left and right coilers no matter what scale we chose, which suggests that we are dealing with a truly balanced polymorphism and that these snails are not at the early stages of speciation. If that turns out to be the case, we need to identify what factors are responsible for keeping the polymorphism so balanced,” Dr Craze explains.

However, to make sure that the results they have found are not only confined to snail shells found on the ground, Dr Craze will be repeating the experiment by hanging from a rope in the rainforest looking at live snails in the forest canopy. “We also plan to use small snail-sized radio transmitters to track movements of snails to see how many snails of each coil-type an individual is likely to encounter and so work out the potential for gene flow between areas,” he says.

According to Dr Craze: “Our work may have some interesting implications. Studies of the ecological factors responsible for maintaining balanced polymorphism are still quite rare. On the other hand, if some degree of divergence between the coil-types is discovered, we have a model system for studying what may be an interesting mechanism of sympatric speciation.”

Dr Craze will present his full findings at 15:00 on Tuesday 9 September 2003.