Population studies on a polymorphic prosobranch snail (Clithon (Pictoneritina) oualaniensis Lesson).

Abstract

The prosobranch gastropodClithon (Pictonentina) oualaniensisLesson which is widely spread in the Indo-Pacific region, shows a high degree of variability as regards shell colour and pattern. The present report is based on some 72000 snails collected round the coasts of Ceylon, in Malaya and Singapore, and in Hong Kong; and on some preliminary breeding results (Nugaliyadde, appendix 4).Clithonhas an axial (transverse) pattern which most commonly consists of fine transverse lines (f.t.l.); less often, there are coarse transverse lines (c.t.l.) or more widely spaced patterns such as zebras or tigers (which are rare in Ceylon, but common in the eastern provinces). F.t.l. and, to a lesser extent, c.t.l. are often complicated by the presence of small triangles or tongues the density of which increases with age. Superimposed on the axial is often a spiral pattern which generally consists of three roughly equidistant spirals. The ratio of axial:spiral patterns is about the same over the whole area and about 62:38. The spiral patterns in Ceylon include spiral tongues, ladders and yellow spirals; these are fairly distinct in certain populations, but grade into each other to a varying extent in others. In the eastern provinces, ladders are completely absent, but there are other spiral patterns which may be their equivalents. Between them, these simple axial and spiral patterns account for the great majority of all animals. In addition, there are certain rarer types which are mostly sharply classifiable (dilution, purple-tipped tongues, purple spirals, black and a few others). Preliminary breeding experiments show that axial as compared with spiral patterns correspond to distinct genotypes, spirals being dominant over axial patterns. Within the axial patterns, there are numerous intergrades between f.t.l. and c.t.l.; the tongue pattern can vary from absence to a density which covers most of the shell; it is largely an age effect like greying of hair in man. In the same shell, these patterns can change into each other gradually or sometimes abruptly following a temporary cessation of growth (including attempted predation by hermit crabs). They are prob ably largely, if not entirely, non-genetic in origin; this is compatible with the few breeding data so far available. Zebras and tigers which are almost confined to the eastern provinces are probably genetically distinct separate entities. Both population studies and breeding experiments make it probable that the spiral group includes a significant element of genetic segregation. Purple spirals are probably due to a single (? recessive) gene, and a simple genetic basis is probable for most or all of the rarer variants. Within a given province, populations are remarkably uniform. The rarer variants are generally homogeneously distributed within a province (except purple spirals which are much commoner on the east than on the west coast of Ceylon). There is more variance between populations as regards the ratio of axials to spirals, and as regards the various spiral patterns. But the variance between populations, both in space and in time, is not significantly greater than the error variance, and it thus appears that what variance there is lacks permanence; i.e. there is oscillation round essentially stable mean values. There is thus no need to invoke permanent differences in gene frequencies or permanent differences in the environment as between populations. On the other hand, there are consistent and major differences between provinces, and these must clearly be genetic in nature. They may have arisen during the spread ofClithonfrom its original home which, presumably, has been coastwise and slow.Clithonhas probably no pelagic larva and, as localities where it can exist are separated by many miles of open coastline where it cannot, its spread must have been at the mercy of rare accidents: these probably convey only a few individuals at a time from one suitable locality to another and thus expose populations to genetic drift (founder effects). As, within provinces, there is little or no evidence for this, it seems most probable that gene frequencies are stabilized by selection, and as long as an allele is not lost altogether, the interplay of selective forces will tend to bring gene frequencies back to normality. Whereas it thus appears probable that the polymorphism ofClithonis kept in being by selection, there is no reason to suppose that any particular pattern is better adapted, over the life cycle as a whole, than any other. The whole array of variability may thus be adaptively neutral.