|Biology of the gastropod family Littorinidae; I. Evolutionary aspects|
McQuaid, C.D. (1996). Biology of the gastropod family Littorinidae; I. Evolutionary aspects. Oceanogr. Mar. Biol. Ann. Rev. 34: 233-262
In: Oceanography and Marine Biology: An Annual Review. Aberdeen University Press/Allen & Unwin: London. ISSN 0078-3218, more
Evolution; Littorinidae Children, 1834 [WoRMS]; Marine
Gastropods of the family Littorinidae are abundant and ecologically important animals in shallow marine systems throughout the world. Ecological studies on this family have focused on geographical areas where the taxonomy is clear at species level. In some areas, notably the North Atlantic, this happy state does not exist and there has been extensive taxonomic confusion. As a result, considerable effort has gone into the study of the evolutionary biology of the littorinids over the last 20 years. Many of these studies have been aimed at understanding the taxonomic status of species. I have not addressed these. Others have used the group as a tool to consider evolutionary questions. These concern the maintenance of polymorphism in shell colour or shape, and especially the application of life history theory to mode of larval development and the influence of development type on dispersal and population heterozygosity. Overall the littorinids conform poorly or not at all to theoretical predictions.Colour polymorphisms seem to be maintained by selective forces based on predation (although the ecological literature suggests predation is not important at the population level) and heat uptake. The control of variation in shell shape is more difficult to explain and is influenced by the type of development. Direct developing species lack a planktonic larva, which is assumed to result in low gene flow. In examples of these species, shape appears to be largely genetically determined. For species with highly dispersive larvae, phenotypic responses to local factors, including growth rates, are important. But this generalization is not clear-cut, the evidence is conflicting and important counter-examples exist. The most direct evidence comes from breeding experiments. These are rare and, in at least two cases, provide examples that do not conform to this pattern.Dispersal and gene flow are expected to correlate with mode of larval development. In fact, we have little direct information on events between spawning and settlement and again this link is not clear. Direct developing species have been shown to raft or drift over long distances as adults. Species with long-lived planktonic larvae are expected to be highly dispersive, but correlations between settlement and adult density can be interpreted as indicating limited dispersal. A number of other observations on actual dispersal and geographic range of littorinids with different larval types do not fit the theory.Direct developing species can show striking examples of founder effects. Despite this, there are only imperfect correlations between larval type and both within- and among-population genetic heterozygosity, implying that gene flow is not tightly bound to larval type. This may be partly explained by the lack of correlation between larval type and dispersal. We may also be misled by assuming that juvenile recruitment is not genotype dependent. But most important is the fact that, while development mode is fixed for any species, heterozygosity is influenced by many factors. The absence of a clear correlation suggests that larval type and/or dispersal is only one of many factors that influence heterozygosity and does not have an overriding effect.There is a poor fit between predicted and actual reproductive strategies used by littorinids. One of the most useful ideas to emerge recently is the need to recognize phylogenetic constraints on mode of reproduction (and many other facets of biology for that matter). Evolutionary explanations can supersede a welter of imperfect and superfluous ecological explanations for the use of different strategies by different species. We require adaptive explanations only when a species shows a derived condition different from the primitive condition for the taxon to which it belongs. Thus ecological explanations are appropriate only in comparisons between species of similar ancestry or between populations of the same species exhibiting morphological or genetic differences.