The physiological optics of Dinopis subrufus L. Koch: A fish-lens in a spider

Abstract

The polymorphic brackish-water snail Clithon oualaniensis is widely spread in the Indo-Pacific Region. It has been studied in India and Ceylon, round the Malay Peninsula, and in Hong Kong; a smaller material from NE Queensland, Australia, has also been examined. The total is not far short of 100000 individuals. There are great geographical differences in gene frequencies. For instance, 'purple spirals' is polymorphic with an incidence of about 16% in NE Queensland (650 km of coastline), 5.5% round the Malay Peninsula (1500 km) and 2% on the east coasts of India and Ceylon (1400 km); but an abrupt drop to near-absence occurs in the Gulf of Mannar and persists as far as Goa on the Indian west coast (another 1400 km). The incidence thus falls from a polymorphic to a monomorphic level. Most variants are polymorphic in parts of the range and monomorphic in others. As large areas remain to be explored, the monomorphic phase of some polymorphic entities may yet be discovered. More gradual clines within the polymorphic phase have been observed round the coastline of Malaya. In general, gene frequencies tend to be remarkably uniform over long stretches of coastline which presumably indicates that, for the gene(s) in question, the conditions affecting survival value are similarly uniform. By the same token, changes in gene frequency presumably indicate changes in the environment. There is a strong prima facie case that this is so for the major discontinuity between western Clithon from India and Ceylon, and that from the eastern regions. In this instance, surface salinity appears to be one of the factors responsible, and the same parameter may be involved in the changes of incidence in purple spirals. It may be surmised that, in other instances, there are other environmental differentials whose nature remains to be discovered. The relation between selection and adaptation is also discussed, it being maintained that a change in gene frequency brought about by a change in the environment is not, in itself, evidence that the change is adaptive in nature. The incidence of most morphs in Clithon is stable over great distances. This 'linear stability' of gene frequencies is evidently the one-dimensional equivalent of the 'area effects' in land snails like Cepaea. Only a single entity, 'black' (and perhaps 'giant tongues') shows irregular fluctuations of gene frequency over short distances and thus seems to be influenced by local littoral conditions.