POPULATION GENETICS OF TIGRIOPUS CALIFORNICUS. II. DIFFERENTIATION AMONG NEIGHBORING POPULATIONS.

Many species of intertidal marine invertebrates are restricted in their geographic distribution to discrete patches of habitat which occur irregularly along the shoreline. The rocky intertidal fauna of the Pacific coast of North America exhibits such restriction. Though locally abundant, rocky habitat is irregularly interrupted by stretches of sandy beach which vary in length from a few meters to tens of kilometers. Since many of the species inhabiting the rocky intertidal are relatively sedentary as adults and will not leave rock substrate, sandy beaches result in geographic subdivision of adult populations. Dispersal, primarily via planktonic larval stages, is expected to be important in the ecology and evolution of these species not only because it permits recolonization of habitat patches following local extinction, but also because it can maintain the genetic continuity of geographically separated populations. Despite its general importance in the biology of marine invertebrates, we currently have few data addressing the extent to which larval (or adult) dispersal effectively homogenizes gene pools of isolated coastal populations within a species. In the past decade, the discovery of extensive protein polymorphism in a variety of marine invertebrate species has contributed to our understanding of the genetic structure of natural populations of these organisms. Littorinid gastropods, for example, have been the focus of several studies. Using enzyme polymorphisms as genetic markers, Berger (1973, 1977) showed that the extent of population differentiation in three species of littorinids is correlated with differences in larval dispersal patterns among the species. Littorina littorea, a mid-intertidal species with planktonic larval development, showed little differentiation among geographically separated populations, while L. saxatilis (high intertidal) and L. obtusata (low intertidal), both without planktonic larvae, were significantly more differentiated. Snyder and Gooch (1973) obtained similar results in comparing population differentiation in L. saxatilis to Nassarius obsoletus, a species with long-lived planktonic larvae. Gooch (1975) and Crisp (1978) have reviewed some additional data concerning the relationship between dispersal ability (as measured by length of planktonic larval life) and genetic differentiation and have found it to be generally consistent with the inverse correlation observed by Berger: species with longlived planktonic larvae tend to be genetically homogeneous over broad geographic ranges, while species with short-lived larvae or direct development show some degree of genetic differentiation among neighboring populations. Exceptions to the above relationship have been well documented (e.g., Struhsaker, 1968; Koehn et al., 1980; others). Two factors can account for such apparent exceptions: (1) Strong differences in natural selective forces among populations arising from habitat heterogeneity can result in genetic differentiation even where there is significant inter-population dispersal. (2) Species which appear to have high dispersal ability may not always experience high levels of gene flow. Scheltema (1975) has pointed out that knowledge of the length of larval life is insufficient for understanding the dispersal capabilities of invertebrate species. He suggests that in order to estimate the ex-