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: ) varies with habitat in the Houtman Abroihos Islands,

M. S. JOHNSON & R. BLACK Departmentof Zoology, The University of Western Australia, Nedlands, Western Australia 6009, Australia

Thelittorine gastropod 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 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 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 . The littorimd snail Bembiciurn vittatum (formerly :.::...: 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 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 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 (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. Departures of hetero- (w/v) sucrose containing 0.1 per cent (v/v) mercapto- zygosity from Hardy—Weinberg ,expectations were ethanol and 0.1 per cent (w/v) bromophenol blue. measured as D='(HHe)IHe, where He is the Electrophoresis was carried out in horizontal starch expected number of heterozygotes in the sample. Over- gels. The buffer systems used were discontinuous tris- all values for D were obtained by summing observed citrate/NaOH (Poulik) and continuous tris-EDTA- and summing expected numbers over samples. Statisti- borate (TEB), which are buffers 3 and 6, respectively, cal significance of the departures from Hardy—Weinberg from Selander eta!.(1971).From each sample, 45—48 expectations was tested with goodness-of-fit x2. individuals were examined for each of four poly- morphic enzymes, representing five polymorphic gene loci: leucine aminopeptidase (E.C.3.4.—.—; Lap locus; Results TEB buffer); leucyl-proline peptidase (E.C.3.4.1.3; Lpp locus; TEB buffer); 6-phosphogluconate dehydro- Allelicfrequencies for all loci in each sample are genase (E.C.1.1.1.43; Pgd locus; TEB buffer); and shown in Table 1. The most variable locus is Vlp2. valyl-leucine peptidase (E.C.3.4.1.3; Vipi and V1p2 Although this locus was polymorphic in all samples loci; Poulik buffer). At each locus, alleles were labelled except Serventy lake (site 3L), the range of frequencies numerically according to the electrophoretic mobility was very great: 0—0.98 for the V1p2'°° allele (Fig. 2). of their corresponding allozymes, relative to that of the There is some regional pattern to this variation, with most common allele, which was given a value of 100. the islands in the having the highest The allozymes of the Yip] locus migrated faster than frequencies of Vlp2'. There is also much localized those of V1p2. variation, with changes in allelic frequency over short distances in each of the three island groups. These differences, however, are among the lagoonal and lake Analyses samples of the eastern areas; in contrast, the western Variationin allelic frequencies among samples for each samples are relatively consistent in allelic frequencies, locus was quantified using FST, the standardized regardless of island group (Fig. 2; Table 2). variance in allelic frequencies, which measures the At each of the other four loci, one allele predomi- proportion of allelic variation that is due to differences nates throughout the Abrolhos Islands, but each locus 208 M. S. JOHNSON & R. BLACK

Table 1 Allelic frequencies for five polymorphic enzymes in samples of Bembicium viuatum from the Abroihos Islands. Location numbers refer to Fig. 1; western shore samples denoted by W, eastern shore samples by E, and lake samples by L

Lap Lpp Pgd Vipi V1p2

Sample 112 100 94 88 100 89 100 80 114 100 94 87 100 76 70

Wallabi group lELongis. 0.001.000.000.001.000.001.000.000.001.000.000.000.460.540.00 1L Long Is. 0.001.000.000.001.000.001.000.000.001.000.000.000.230.770.00 2WLagoonls. 0.000.990.010.001.000.001.000.000.001.000.000.000.620.380.00

Easter group 3EServentyls. 0.001.000.000.001.000.001.000.000.100.900.000.000.180.820.00 3LServentyls. 0.060.940.000.001.000.001.000.000.150.850.000.000.001.000.00 4ESuomils. 0.001.000.000.001.000.001.000.000.080.920.000.000.170.830.00 4LSuomils. 0.310.690.000.000.920.081.000.000.280.710.000.010.350.650.00 SEKeruIs. 0.001.000.000.001.000.001.000.000.060.940.000.000.420.580.00 5LKeruIs. 0.220.780.000.001.000.001.000.000.040.960.000.000.310.690.00 6E Wooded Is. 0.001.000.000.001.000.001.000.000.001.000.000.000.700.300.00 6L Wooded Is. 0.001.000.000.001.000.001.000.000.020.980.000.000.710.270.02 7WRatIs. 0.020.980.000.001.000.000.960.040.190.740.020.050.670.330.00 Pelsaert group 8E Post Office Is. 0.020.980.000.000.990.010.990.010.080.920.000.000.940.060.00 8LPostOfficels. 0.020.980.000.000.890.110.990.010.050.950.000.000.980.020.00 9E Arthur Is. 0.001.000.000.000.980.021.000.000.001.000.000.000.790.210.00 9LArthurls. 0.020.980.000.001.000.001.000.000.001.000.000.000.890.110.00 l0EPelsaertls.,north0.010.990.000.000.850.150.980.020.010.990.000.000.900.100.00 llEFishhookBay 0.020.980.000.000.990.010.870.130.001.000.000.000.570.430.00 12E Pelsaert Is., south0.001.000.000.000.780.220.910.090.001.000.000.000.540.460.00 12L Pelsaert Is., soith0.001.000.000.000.920.080.970.030.010.990.000.000.710.290.00 l3EJonJimIs. 0.001.000.000.000.720.281.000.000.001.000.000.000.640.360.00 l4WGunIs. 0.050.950.000.001.000.001.000.000.001.000.000.000.730.270.00 lSWSevenIs. 0.001.000.000.001.000.001.000.000.001.000.000.000.820.180.00 l6WSweetIs. 0.001.000.000.001.000.001.000.000.010.990.000.000.600.400.00 17W Middle Is. 0.010.960.000.031.000.001.000.000.020.980.000.000.640.360.00

Table 2 Comparison of FSTamongdifferent groups of samples of Bern bicium vittatum

Setofsites Lap Lpp Pgd Vipi V1p2 Mean±S.E.

Ailsamples 0.031" 0.1100.022"0.116***0.258***0.163±0.011 Western shore0.009" 0.000" 0.032"0.167***0.025* 0.030±0.002 Easternshore0.002" 0.145***0.025"0.048" 0.258***0.138±0.012 Lakes 0.077***0.040" 0.010"0.140***0.418***0.277±0.017

'l'*'K "= P<0.05; P<0.001; n =validx2couldnot be calculated.

shows some regional and localized variation. For Vipi locus is also greatest in the Easter Group (Fig. 2). example, Lap'°° is fixed in nearly all samples, but the In contrast, at the Lpp locus allele100isfixed in all but Lap'12 allele occurs at frequencies of 0.06—0.31 in the one sample from the Wallabi and Easter groups, but in lakes on Serventy, Suomi, and Keru islands, in the the Pelsaert Group, Lpp89 was found in several of the Easter Group (Fig. 2; Table 1). The variation at the eastern populations, at frequencies as high as 0.28 HABITAT AND GENETIC SUBDIVISION OF AN INTERTIDAL SNAIL 209

l.0 • restricted to the Easter Group (Table 1). Only the 0.9 Lap'°° Wallabi Group lacked such genetic distinctness. 0.8 Overriding these regional differences, however, are substantial local variations within island groups. These 0.7 f include many differences between lake and shore 0 •o C samples, but we will first consider the differences 1.0 0 among shore samples. For all of the loci examined o Lpp except V1p2, the variation in allelic frequencies among shore samples within one of the island groups encom- passes that among all shore samples (Fig. 2). Even at the V1p2 locus, differences among sites within groups are substantial, with the frequency of l'7p2' ranging Pgd° from 0.17 to 0.70 in the Easter group and 0.54 to 0.94 0.9 in the Pelsaert group (Fig. 2). To quantify this compari- 0.8 son of variation within and among island groups, we U 1.0 0. C 0' performed a hierarchial analysis of FST: only 20 per V/Pt cent of the geographical variation among shore popula- tions is due to differences among island groups, with U i: 80 per cent being among populations within groups. .5 The most interesting aspect of this variation among .. . p. shore populations is that it is largely confined to the : eastern islands; the populations from the more exposed V/p20 0.9 western islands are relatively similar, regardless of island group. At the most variable locus, for example, :1: f the frequency of the V1p2'°° allele varies within the f relatively narrow range of 0.60—0.82 among the western populations, compared with 0.17-0.94 among the shore populations from the eastern islands (Fig. 2; Table 1). The average FST (±S.E.)is only 0.030 0.002 among the western samples for all five loci examined, compared with 0.138±0.012 among the eastern shore samples (Table 2). H In addition to this geographical variation among shore sites, there is an even greater variation associated 25 30 35 40 45 50 55 60 with the lake populations. Differences between lake Latitude(south of °S) samples and the adjacent shore sites were widespread Fig.2 Variation of alleic frequencies among samples from and often substantial, and were found at all loci except the Houtman Abrolhos Islands. ()Westernshore samples, Pgd (Table 3). Among the most dramatic differences (i)eastern shore samples, (0) lake samples. Vertical lines were the frequencies of 0.06, 0.31, and 0.22 for Lap"2 indicate approximate standard errors. in the lake populations from Serventy, Suomi, and Kern islands, respectively, despite the absence of that allele from the adjacent shore samples. There is a (Table 1). The Pgd locus shows a similar, but less tendency among the lake populations for those four extreme, pattern to that of Lpp: Pgd'°° is fixed in most lakes which are more isolated from the ocean (1, 3, 4, samples, including all but one from the Wallabi and 12) to be less similar to their adjacent shore popula- Easter groups, but there are localized low frequencies tions (average genetic identity S.E. =0.985 0.007) of Pgd'°° in the eastern Pelsaert Group (Fig. 2). than the populations from the four less isolated lakes The overall picture is clear despite the uniqueness of (0.997 0.003), although the differences was not quite patterns of geographical variation for each locus. There significant (t=1.659; d.f.=6; 1-tailed P=0.074). The is some regional variation, with particular alleles being substantial differences between lake and adjacent associated with individual island groups: Lpp" and shore samples are not consistent among sites. The lake Pgd8° were found almost exclusively in the Pelsaert samples do not represent a group of similar popula- Group, which also had the highest frequencies of tions but show a substantial amount of variation V1p2Iü0, whereas Lap"2 and Vip]"4 were nearly amongst themselves, with an average FST of 210 M. S. JOHNSON & R. BLACK

Table3 Values of x2fromcomparisons of allelic frequencies between paired samples from lakes and adjacent shore populations of Bembicium vittatum. Each individual comparison has 1 d.f. n.t. indicates that a valid x2couldnot be calculated

Site island Lap Lpp Pgd Vip] T'1p2 1 Long n.t. n.t. n.t. nt. 11.17*** 3 Serventy n.t. n.t. n.t. 0.76 18.65*** 4 Suomi 3555*** 8.35** n.t. 13.68*** 8.76** 5 Keru 21.49*** n.t. n.t. 0.42 1.60 6 Wooded n.t. n.t. n.t. n.t. 0.00 8 Post Office n.t. 8.89** n.t. 0.69 2.09 9 Arthur nt. n.t. n.t. n.t. 3.17 12 Pelsaert nt. 6.75** 3.20 n.t. 5.29* Summed x2 57.04*** 23.99*** 3.20 15.55** 50.73*** Summedd.f. 2 3 1 4 8

* = P<0.05; **= P<0.01; "°'= P<0.001.

Table 4 Heterozygosities (H) and departures from Hardy—Weinberg expectations (D) for five polymorphic enzymes in samples of Bern bicium vittatum from the Abroihos Islands. Location numbers refer to Fig. 1. Western shore samples denoted by W, eastern shore samples by E, and lake samples by L

Lap Lpp Pgd Vipi V1p2

Sample H D H D H D H D H D

Wallabi group lELongis. 0.000 — 0.000 — 0.000 — 0.000 — 0.333 iLLongIs. 0.000 — 0.000 — 0.000 — 0.000 — 0.383 0.057 2WLagoonls. 0.021 0.000 0.000 — 0.000 — 0.000 — 0.396 —0.086 Easter group 3EServentyls. 0.000 — 0.000 — 0.000 — 0.208 0.1050.271 —0.080 3L Serventyls. 0.068 —0.369 0.000 — 0.000 — 0.292 0.1590.000 — 4ESuomils. 0.000 — 0.000 — 0.000 — 0.109 —0.235 0.298 0.043 4LSuomils. 0.209 0.167 0.0800.000 — 0.417 —0.016 0.333 5EKeruIs. 0.000 — 0.000 — 0.000 — 0.083 —0.296 0.458 —0.067 SLKeruIs. 0.159 0.000 — 0.000 — 0.083 0.0330.292 6E Wooded Is. 0.000 — 0.000 — 0.000 — 0.000 — 0.479 0.125 6LWoodedls. 0.000 — 0,000 — 0.000 — 0.044 0.0110.375 —0.126 7WRatIs. 0.000 —1.000 0.000 — 0.083 0.0330.396 —0.055 0.500 0.113 Pelsaert group 8EPostOfficels. 0.000 —1.000 0.022 0.0000.022 0.0000.167 0.0800.083 —0.296 8LPostOfficels. 0.042 0.011 0.085 —0.557 0.021 0.0000.104 0.0440.042 0.011 9EArthurls. 0.000 — 0.043 0.0110.000 — 0.000 — 0.333 0.000 9LArthurls. 0.044 0.011 0.000 — 0.000 — 0.000 — 0.227 0.103 l0EPelsaertls.,north0.021 0.000 0.292 0.1590.042 0.0110.021 0.0000.208 0.105 llEFishhookBay 0.042 0.011 0.250 0.1340.083 0.0330.000 — 0.479 —0.031 l2EPelsaertls., south0.000 — 0.396 0.1460.146 —0.151 0.000 — 0.417 —0.170 l2LPelsaertls.,south0.000 — 0.167 0.0800.062 0.0220.021 0.0000.417 —0.002 l3EJonJimIs. 0.000 — 0.438 0.0710.000 — 0.000 — 0.583 0.131 l4WGunIs. 0.109 0.046 0.000 — 0.000 — 0.000 — 0.375 —0.060 l5WSevenIs. 0.000 — 0.000 — 0.000 — 0.000 — 0.271 0.080 l6WSweetIs. 0.000 — 0.000 — 0.000 — 0.021 0.0000.468 —0.039 l7WMiddlels. 0.089 0.026 0.000 — 0.000 — 0.042 0.0110.383 —0.179 *=P<0.05. HABITAT AND GENETIC SUBDIVISION OF AN INTERTIDAL SNAIL 211

0.277±0.017 (Table 2), compared with 0.152± studies ranged from approximately 70 km to more than 0.0 16 among their counterpart shore populations. 1800 km. In contrast, studies throughout Britain The observed heterozygosities and deviations from (distances on the order of 1000 km) of three species Hardy—Weinberg expectations for each locus in each which lack planktonic larvae have reported values of sample are shown in Table 4. The average level of FST ranging from 0.131 to 0.188. For Littorina saxa- heterozygosity for the five polymorphic enzymes was tilis, the most extensively studied of the species which as high in the lake samples as in the shore samples, lack planktonic larvae, values of FST ranged from 0.101 0.043 (S.E.) and 0.097 0.061, respectively. At 0.070 on 1 km of shore, to 0.287 over distances of the Lap locus, heterozygotes were found in five of the more than 10,000 km. Thus, the overall value of 0.163 eight lake populations, but not in the eight adjacent for the populations of B. vittatum from the Abrolhos shore populations. This trend was not consistent across Islands lies well within the range previously observed loci, however, and no patterns of different levels of for species that lack planktonic larvae, but is especially heterozygosity were found between lake and shore high, given that the maximum distance between samples. Similarly, there were no detectable differences samples is only about 70 km. in heterozygosity between the western and eastern Although it is useful in gross comparisons among shore populations. species, the overall average FST in Bembicium vittatum Most genotypic frequencies were close to expecta- masks substantial variation in the degree of genetic tion under Hardy—Weinberg equilibrium; of 43 tests in subdivision among different sets of samples. Particu- which the expected values were at least 4, there were larly striking is the effect of the lakes on genetic sub- five significant (P<0.05) departures from expecta- division, with nearly a twofold increase in FST tions. The significant departures were all deficits of compared with the shore populations. Indeed, varia- heterozygotes, and they were confined to the Lap (two tion among the lake populations of B. vittatum is cases) and l'7p2 (three cases) loci (Table 4). Further- similar to that of land snails, which are renowned for more, the lake populations from Suomi and Keru their limited gene flow (e.g. Johnson, 1988; Johnson et islands accounted for all but one of the departures, as al., 1988). The tendency for the apparently more both Lap and J'7p2 showed large deficits of hetero- isolated of the lake populations to have lower genetic zygotes in these samples (Table 4). A possible explana- similarities amongst themselves could reflect a grada- tion for this concordance would be the coexistence of tion in the effectiveness of barriers to gene flow two genetically distinct groups within each of these between lake and shore populations. Isolated popula- lakes. To test that possibility, we looked for linkage dis- tions, such as those in the lakes, would be expected to equilibrium between Lap and V1p2 in the two lake show greater genetic divergence due to both selective samples with deficits of heterozygotes but there was no and random causes. One expectation from random sign of the anticipated association between loci. genetic drift, however, is that isolated populations should have lower levels of polymorphism. This was not found in B. vittatum. Genetic drift is an especially Discussion unlikely explanation for the finding of the uncommon Thegeneral picture that emerges for Bembicium Lap'2 allele at moderate frequencies in three of the vittatum is one of a high degree of genetic subdivision lake populations in the Easter Group. With this but with striking variation in the level of that sub- localized exception, however, differences between lake division. This combination reflects both the charac- and shore samples are inconsistent, making them diffi- teristics of the species and the importance of the cult to interpret as a result of selection. The possibility environment in determining the amount of genetic of differences occurring in the localized selection is differentiation. The level of genetic subdivision in B. particularly strong in the lakes because they are subject vittatum is best appreciated by comparisons with other to greater variations in temperature and salinity, and species. In contrast with the large overall FST of 0.16 3 experimental tests of possible selection would be found for B. vittatum, values for other marine inverte- worthwhile. brates studied over much larger distances on the The contrast in the levels of subdivision among lake Western Australian coast range from 0.004 to 0.013 and shore populations is a clear example of the import- (Johnson & Black, 1984, 1990; Watts et al., 1990). ance of barriers to gene flow. Note, however, the Ward (1990) summarized the studies of genetic extremely low level of genetic divergence found among diversity in species of Littorina, providing comparisons the shore samples from the western islands. The rela- with other littorine gastropods, and finding an average tive uniformity of these populations both within and GST (equivalent to FST) of 0.047 for seven species with between island groups is shown by their average value planktonic larvae. The geographical scale of these of FST of only 0.030, nearly an order of magnitude 212 M. S. JOHNSON & R. BLACK smaller than that for the lake populations, and only disruptions to gene flow. For example, the milkfish one-fifth as large as that for the shore populations from Chanos chanos shows very little genetic differentiation the eastern islands. This low level of genetic differ- across much of the Pacific Ocean, but is genetically entiation must be explained by some combinations of subdivided within the Hawaiian Islands, probably high gene flow or strong uniform selection. Because of because of localized effects on the flow of currents their position facing the open reef, these western (Winans, 1980). An even clearer example of the populations are subject to a greater flow of water, importance of particular types of environment on gen- which should both increase dispersal and decrease at etic subdivision is the pattern of variation in snapper, least some environmental differences between sites. In Chysophrys auratus, in Australia. Although there is the absence of details of the extent and direction of very little genetic divergence among populations over flow, and the consistency of patterns of flow over time, thousands of kiometres along the outer coast, there are we cannot infer the actual potentials for gene flow genetically divergent populations in the embayments of between particular populations. Consequently, we Shark Bay in Western Australia, Spencer Gulf in South cannot separate the relative contributions of gene flow Australia, and Port Phillip Bay in Victoria (Johnson et and selection. al., 1986; Richardson et al., 1986). The findings for Nevertheless, the extremely low level of genetic Bembicium vittatum show that environmental differ- divergence among the western samples has implica- ences can affect genetic subdivision on a much smaller tions for the probable mode of development in B. geographic scale. Taken together, these examples of vittatum. In a review of types of larval development in localized disruptions to gene flow indicate that evolu- littorines, Mileikovsky (1975) lists B. vittatum (as tionary changes in marine species are more likely to Bembicium melanostotna) as having direct develop- result from peculiar local conditions that favour isola- ment. This is based on the observation of development tion, than from the more typical, highly connected past the veliger stage within benthic egg masses populations. (Anderson, 1958). These egg masses did not hatch, A great advantage of the Abrolhos Islands for however, so we cannot be certain that development genetic studies is that different habitats can be sampled was normal. As Reid (1988) points out, the relatively over the same spatial scale, both within and between large protoconch of B. vittatum is characteristic of island groups. Thus, the contrasting levels of genetic species with little or no planktotrophic development, subdivision in Bembicium vittatum reflect different but the conclusion that development is direct is ques- types of environment, rather than biogeographic tionable. The high degree of genetic uniformity among histories or geographically defined areas of subdivi- the western populations in the Abroihos Islands sion. Although a similar effect of environmental condi- strongly suggests that there is a short planktonic stage. tions has been found in arthropods inhabiting caves Otherwise, it is difficult to reconcile the high degree of (Caccone, 1985), such variation in the levels of gene uniformity of the western populations with the high flow has received little attention. It is clear that esti- degree of genetic subdivision of the eastern popula- mates of the average levels of subdivision or gene flow tions. may be misleading, as they mask biologically significant Regardless of the actual length of the planktonic variation in the degree of connectedness of popula- stage, if there is one, the contrast in levels of genetic tions. As genetic studies are used increasingly to make subdivision from different habitats emphasizes the inferences about levels of gene flow, it is important to importance of environmental context. Such environ- take account of this heterogeneity, and to recognize mental effects have important implications both for the that gene flow depends upon environmental condi- measurement of gene flow or genetic subdivision and tions, even in the marine environment. for the understanding of evolutionary processes in the sea. On a broader geographical scale, it is well recog- nized that the magnitude of genetic differentiation Acknowledgements varies within a species, often as a result of major barri- Wewould like to thank Dr Bruce Hatcher for introduc- ers. A good example is the concordant pattern of gen- ing us to the Houtman Abrolhos Islands, for his stimu- etic divergence found in many species between lating help in the field, and for perceptive comments on populations on the coasts of the Atlantic and the Gulf the manuscript. Ms Robyn Watts provided assistance in of Mexico, in the United States, representing a major the laboratory and helpful discussion of the manu- biogeographic split (e.g. Johnson, 1975; Reeb & Avise, script. Financial support was provided by the Austral- 1990). Of greater interest in terms of ongoing influ- ian Research Council. ences on genetic subdivision are examples of localized HABITAT AND GENETIC SUBDIVISION OF AN INTERTIDAL SNAIL 213

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