Heredity 67 (1991) 205-213 Received 6 November 7991 OThe Genetical Society of Great Britain Genetic subdivision of the intertidal snail Bembiciurn vitta turn (Ga stro pod a: Littorinidae) varies with habitat in the Houtman Abroihos Islands, Western Australia M. S. JOHNSON & R. BLACK Departmentof Zoology, The University of Western Australia, Nedlands, Western Australia 6009, Australia Thelittorine gastropod Bembicium vittatum has benthic egg masses and is thought to have little or no period of planktomc development. A study of allozyme polymorphisms in populations from the Houtman Abroihos Islands, Western Australia, permitted examination of genetic subdivision within and among island groups, and among populations from contrasting habitats which occur through- out the Abroihos archipelago. Across all populations, the average FST for five polymorphic loci was 0.163, which is very large for a marine species but similar to values found for other gastropods that lack planktonic development. The level of genetic subdivision varied dramatically with habitat. Over similar distances, the average FST among lake populations was 0.277, compared with 0.138 among populations from protected lagoonal shores, and only 0.022 among populations from relatively exposed shores. This variation emphasizes the need to consider local conditions when attempting to determine the genetic structure of a species. Keywords:geneflow, genetic subdivision, FST, littorine snail. ledge of the extent of such connectedness is important Introduction both for the management of populations and for an Theamount of genetic subdivision within a species understanding of evolutionary processes in the marine depends upon the interactions among natural selection, environment. Direct measurements of gene flow, how- genetic drift, and gene flow. Gene flow among popula- ever, are especially difficult to obtain for marine tions decreases the potential for genetic divergence due species. The extent of genetic connectedness depends to either selection or drift. By retarding the accumula- upon the characteristics of the species and the environ- tion of genetic differences, gene flow disrupts the mental conditions. The duration of larval life has development of localized adaptation. Similarly, by received much attention as a predictor of gene flow, increasing effective population size, genetic connec- and hence retardation of geneticdivergence tions among populations reduce both random fluctua- (Scheltema, 1971, 1978; Crisp, 1978). Studies of tions in allelic frequencies and the rate of loss of allozyme polymorphisms have generally confirmed the genetic variation due to sampling within finite local expected inverse relationship between genetic differen- populations. Although it is fundamentally important, tiation and the probable length of larval life (e.g. gene flow is not easily studied in natural populations. Berger, 1973; Janson, 1987; Waples, 1987), although Measures of genetic subdivision are much easier to some species with apparently a great potential for gene obtain. In addition to their descriptive value, such flow have a surprisingly large genetic subdivision measures are being used increasingly to make infer- (Burton, 1983; Hedgecock, 1986). Littorinid snails ences about the level of gene flow (Slatkin, 1985, have provided some of the clearest examples of the 1987). greater localized genetic differentiation and adaptation Questions about gene flow are particularly impor- in species which lack a planktonic phase of develop- tant in the marine enviromnent, because the potential ment (Berger, 1973; Janson, 1987; Knight et al., 1987; for connectedness among populations is great. Know- Behrens Yamada, 1989). 206 M. S. JOHNSON & R. BLACK Although the relationship between planktonic Wallabi group dispersal and reduced genetic differentiation has received much attention, less work has examined environmental effects on gene flow. As measures of genetic subdivision, such as Wright's (1978) FST, are increasingly used to characterize species, it is especially important to take into account environmental effects on the potential for gene flow. For marine species, this includes the strength and direction of water currents, as well as potential barriers to movements of animals. The littorimd snail Bembiciurn vittatum (formerly Easter group :.::...: Bembicium melanostoma; see Reid, 1988) provides a good opportunity to examine the extent of genetic sub- division in the context of both life history and contrast- ing environmental conditions. This species lives on 7'5 sheltered intertidal rocky shores and mangrove roots in South Australia and southern Western Australia. It lays egg masses on the shore, and has little or no planktonic development (Reid, 1988). Because of this mode of development, there is the potential for substantial genetic subdivision over relatively short distances. At the northern end of its range in Western Pelsoert group Australia, B. vittatum is widespread in the Houtman 14 Abrlhos Islands. This archipelago includes a series of I separate island groups, within each of which B. 15 vittatum is found in contrasting areas of relatively exposed reef flats, calm lagoonal shores, and isolated lakes, which could be expected to affect the amount of 5 km gene flow. Because each of these habitats is found over the same distances, we have been able to use this -4— 29°20S 12 heterogeneity to examine the spatial scale of genetic 140 E subdivision in B. vittatum associated with different Fig.1Mapshowing collecting sites of Bembicium vittatum environmental constraints to gene flow. in the Houtman Abrothos Islands, Western Australia. (I) Eastern shore sites; (0)pairedlake sites; (0) western shore sites. Stippled areas are reef flats. Names of the numbered Materials and methods locations are given in Table 1. Samples the Easter and Pelsaert groups (Fig. 1). Although the Samples of B. vittatum were collected from rocky inter- reefs protect the island shores occupied by B. vittatum tidal shores in the Houtman Abrolhos Islands in May, from deep ocean waves, these western populations are 1987 (Fig. 1). These islands cap the southernmost exposed to local wind waves and substantial wind and coral reef structures in the Indian Ocean, and are wave-driven water currents from the western reef 70—100 km off the Western Australian coast. The margins. In contrast, the eastern edge of each platform Abroihos archipelago consists of three carbonate plat- is lined with long, narrow islands, which have calm forms supporting groups of many low islands, each lagoonal environments on their western shores. Sites platform being 20-30 km in extent, and separated by numbered 2, 7, and 14 to 17 (Fig. 1) were from the deep channels 10-20 km wide (for a summary of the more exposed western portions of the archipelago, characteristics of the Abrolhos Islands, see Hatcher, whereas the remaining shore sites were from the 1985). Between two and 10 samples from intertidal eastern lagoonal areas. shores were collected from each of the three major Another contrasting set of habitats is provided by island groups, thus allowing assessment of genetic lakes found on several of the islands, most of which divergence within and among groups. Within each have large populations of B. vittatum. A parallel sample island group, the southwestern portion is most was collected from a lake less than 100 m distant for 8 exposed, with broad reef flats being well developed in of the 17 shore sites, making a total of 25 samples. HABITAT AND GENETIC SUBDIVISION OF AN INTERTIDAL SNAIL 207 These lakes are actually sinkholes or dolines (Teichert, among populations (Wright, 1978). FST was calculated 1947), which differ in their exchange of water with the by the method of Weir & Cockerham(1984). The jack- sea. The lakes on Long Island (site 1), Suomi Island knife procedure was used to obtain a mean and (site 4), and Pelsaert Island (site 12) are shallow, with variance of F1 over the five loci, allowing comparison silty bottoms, and an impeded connection with the sea, of values obtained for different sets of samples as evidenced by their altered salinities and lag in tidal (Reynolds ci' a!., 1983). To test the significance of FST, phase. Similarly, the lake on Serventy Island (site 3), contingency chi-squared values were calculated, using although having a relatively solid bottom, has altered the method of Workman & Niswander (1970), in which salinities and tidal lags. In contrast, the lakes on Keru =2NFST(k —1),where N is the total sample size, and — Island (site 5) and Wooded Island (site 6) are separated k is the number of alleles; d.f. =(k—1)(m1),where m from the sea by only a narrow cobble berm, and appear is the number of samples. To ensure that the expected to experience fairly free subterranean tidal exchange, values were 4, rare alleles were pooled; if as a group as evidenced by their near-ambient salinities and tidal the pooled expected values were <4, the rare alleles oscillations. The exchange of water with the lakes on were excluded from the test. In order to determine the Post Office Island (site 8) and Arthur Island (site 9) is apportionment of geographical variation within and not as clear, but appears to be intermediate to the two among island groups, a hierarchical analysis of FST groups classified above. Thus, the lake samples as a (Wright, 1978) was conducted, using the rnosys-1 group provide a comparison of relatively isolated computer package. Only the shore samples were habitats, while differences among the lakes in their included in this hierarchical analysis. isolation allow a further test of subdivision. Pairwise comparisons between populations were made with Nei's (1978) unbiased genetic identity, calculated with niosys- 1. Because these genetic identi- Electrophoresis ties were calculated for this selected set of polymorphic Thesnails in each sample were frozen in the field in loci, they should not be compared directly with values liquid nitrogen, and were later transferred to —70°C from other studies. pending electrophoresis. Enzymes were extracted by Variation within populations was expressed as grinding 1 vol. hepatopancreas in 2 vol. of 10 per cent observed heterozygosity, H.