-Ophioplocus esmarki Lyman and Ophionereis annulata LeConte are common species of brittlestars inhabiting the coastal waters of the eastern Pacific Ocean. Ophioplocus esmarki internally broods its embryos, while Ophionereis annulata presumably produces a lecithotrophic, pelagic larva. Forty-five individuals of each species were collected from three sites near San Diego, California. Electrophoresis was employed to screen each species for genic variation at 13 genetic loci to determine if differences in larval developmental modes were reflected in levels of genic variability and the extent of genetic differentiation among sampling sites of each species. Ophioplocus esmarki was polymorphic for 46.2% of its loci, while the average individual was heterozygous at 11.1% of its loci. These estimates were 51.3% and 12.6% for Ophionereis annulata. Deviations in genotypic frequencies from those expected on the basis of the Hardy-Weinberg Law were found within sites for both species, but were somewhat more pronounced in Ophioplocus esmarki. Genetic heterogeneity among sites was assessed by chi-square and the computation of genetic distances. Similar levels of microgeographic variation were observed in both species despite their different modes of larval development. Both species displayed substantial differentiation on a microgeographic scale, which was tentatively attributed to population subdivision and genetic drift. Dispersal is defined as the movement of individuals over short distances away from the natal site (Endler, 1977). Since dispersal facilitates gene flow and may thus serve as a source of genetic variation within populations, it has been viewed as an important process in evolution (Dobzhansky et al., 1977; Endler, 1977; Hedrick et al., 1976). High levels of gene flow promote genetic homogeneity among local populations, while restriction in gene flow usually is seen as being a prerequisite for population differentiation. Marine invertebrates display a variety of larval developmental strategies, and several studies have attempted to assess how these strategies are related to patterns of genetic differentiation among populations. Gooch et al. (1972) reported genetic homogeneity for two isozyme loci among 11 populations of the mud snail Nassarius obsoletus. This species has a long-lived pelagic larva and gene flow among populations is presumably high. On the other hand, Snyder and Gooch (1973) observed significant heterogeneity at two isozyme loci for adjacent populations of the viviparous snail Littorina saxatilis. These results and those of other studies (Berger, 1972, 1973; Gaines et al., 1974; Marcus, 1977; Schopf and Gooch, 1971) suggest that larval dispersal strategy may play an important role in influencing the extent to which adult populations are genetically differentiated. Ophioplocus esmarki and Ophionereis annulata are common species of brittlestars which occur sympatrically in southern California. Ophioplocus esmarki is found in the shallow waters of the Pacific Ocean from central California to northern Baja California, while Ophionereis annulata has a distribution extending from Panama to San Pedro, California (Allen, 1967). These species display contrsting modes of larval development. In 0. esmarki, direct development is employed and embryos are brooded in internal bursae. A pelagic A GUST 20, 1982 THE SOUTHWESTERN NATUR LIST 27(3):255-262 This content downloaded from 157.55.39.92 on Tue, 21 Jun 2016 05:53:43 UTC All use subject to http://about.jstor.org/terms The Southwestern Naturalist larva is absent in 0. esmarki (Boolotian, 1966). On the other hand, 0. annulata has been found to have a short-lived, vitellaria larva which metamorphoses in 4 days (G. Hendler, pers. comm.). We report here the results of a study in which we employed starch gel electrophoresis to examine the genetic structure of Ophioplocus esmarki and Ophionereis annulata populations near San Diego, California. The objectives of this study were 1) to assess levels of genic variability in these species and 2) to determine if the interspecific differences in development are reflected in the extent to which their respective populations are genetically differentiated on a microgeographic scale. We predicted that 0. esmarki, by virtue of its reduced potential for larval dispersal, would display greater genetic heterogeneity among sampling sites than 0. annulata. MATERIALS AND METHODS. -A total of 90 brittlestars were collected from the intertidal region of False Point, La Jolla, California (32048'N). Fifteen individuals of each species were collected by hand from each of three sites. All sites were located approximately at the zero tide level (MLLW). Sites I and II were directly adjacent to one another while the center of site III was 20 m away from the center of site I. Each sampling was a circle 2 m in diameter. Individuals were collected during the nonreproductive season (November 1978 to January 1979) so that the genetic analysis of Ophioplocus esmarki would not be complicated by the presence of actively brooding females. Specimens of each species were obtained by overturning rocks at low tide, and only adult brittlestars having disc diameters greater than 10 mm were collected. The brittlestars then were transported to the laboratory where they were maintained without food for 48 h, thereby minimizing possible interference of stomach contents during electrophoresis. Following the holding period, each individual was given an identification number and its central disc was homogenized in a 2-ml Wheaton tissue grinder with an approximately equal volume of distilled water while submerged in an ice bath. The homogenates were centrifuged at 7719 X g for 25 min in a refrigerated centrifuge (0-4?C). Supernatants were transferred to labeled culture tubes and stored at -25?C for no longer than 2 weeks prior to electrophoresis. Horizontal starch-gel electrophoresis was performed following the methods described by Ayala et al. (1972). Small quantities of the thawed supernatants were absorbed on 4 by 10 mm wicks of Whatman chromatography paper. These wicks were then placed in gels of 11.5% Electrostarch (Otto Hiller, Madison, Wisconsin). Specific information concerning gel and tray buffers is presented elsewhere (Gaarde, 1980). Histochemical staining procedures (Ayala et al., 1972; Selander et al., 1971; Shaw and Prasad, 1970) were used to determine the allozyme phenotypes of each brittlestar at 13 structural gene loci. The most anodal form of each enzyme system within a species was designated one, with others number sequentially in order of decreasing anodal mobility. A locus was judged to be polymorphic if the frequency of the most common allele was < 0.99. Levels of genic variability within each sampling locality were measured for both species as the proportion of polymorphic loci and the mean heterozygosity per individual as determined by direct count. Sample site genotypic frequencies for each locus were compared to those expected from Hardy-Weinberg conditions by chi-square goodness-of-fit tests (Zar, 1974). Finally, inter-site differentiation within each species was examined by heterogeneity chi-square tests (Zar, 1974) on allelic frequencies for each locus and by computing genetic distances between sampling sites (Nei, 1972). RESULTS.-Considerable genic polymorphism was observed in both brittlestar species. Ophioplocus esmarki was polymorphic for 9 of the 13 loci surveyed. In Ophioplocus esmarki, two alleles were segregating for the following loci: alkaline phosphatase (ALK-1 and -2); leucine aminopeptidase (LAP-1, -2, and -3); indophenol oxidase (IPO); and esterase (EST-1, -2, and -3). Ophionereis annulata was polymorphic for ALK-1, ALK-2, LAP-I, LAP-2, IPO, EST-1, EST-2, EST-3. EST-1 had three alleles segregating, while the remaining polymorphic loci had two alleles. Heterozygotes at all polymorphic loci for both species displayed the double-banded zymogram pattern typical of enzymes having 256 vol. 27, no. 3 This content downloaded from 157.55.39.92 on Tue, 21 Jun 2016 05:53:43 UTC All use subject to http://about.jstor.org/terms Gaarde and McCleaghan-Genetics of Ophiuroids TABLE 1.-Proportion of polymorphic loci and observed mean heterozygosity per individual for Ophioplocus esmarki and Ophionereis annulata populations from San Diego, California. Each sampling site is represented by 15 individuals of each species. Proportion of Mean heterozygosity Species Site loci polymorphic per individual Ophioplocus esmarki I 0.385 0.087 II 0.462 0.128