Viviparous, branching corals such as Seriatopora hystrix are expected to generate most recruits through asexual reproduction (fission or fragmentation) but are expected to use sexual reproduction to produce widely dispersed colonists. In this study, allozyme electrophoresis was used to test for variation in the relative contributions of sexual and asexual reproduction to recruitment and to assess the apparent scale of larval dispersal (gene flow) in the central Great Barrier Reef. Fifty-seven collections (within ::s 25 m-) of fragments from sets of approximately 40 colonies were made (where possible) within each of five habitats on each of 12 reefs. These reefs, within the central region of the Great Barrier Reef, were separated by up to 90 km and included one inner-shelf continental island and groups of seven midshelf reefs and four outer-shelf reefs. Most collections contained a high level of multilocus genotypic diversity and hence showed little evidence of recruitment through fragmentation, although the majority of collectionsdisplayed large and consistent deficits ofheterozygotes. Allele frequencies varied greatly among collections (Fon: = 0.43),and this variation wassufficientto explain two-thirds of observed deficiencies ofheterozygotes via a Wahlund effect. A hierarchical assessment of F ST values revealed that 45% of allelic variation occurred among reefs(Fon: = 0.20),and only 16%of variation within reefswasexplained by variation among five major habitat types (FST = 0.05). A relatively small component of the total variation among samples was attributable to across-shelf variation among the groups of middle-and outer-shelf reefs (Fon: = 0.03); however, the outer-shelf reefs form a single UPGMA cluster separate from all but 4 of the other 43 collections. These data imply that widespread dispersal does occur but that the direction or magnitude of gene flow may be influenced by the along-shelf movement of major ocean currents and weather-dependent currents on or near reefs. Each reef, therefore, forms a partially isolated and highly subdivided population.

Abstract. -Viviparous, branching corals such as Seriatopora hystrix are expected to generate most recruits through asexual reproduction (fission or fragmentation) but are expected to use sexual reproduction to produce widely dispersed colonists. In this study, allozyme electrophoresis was used to test for variation in the relative contributions of sexual and asexual reproduction to recruitment and to assess the apparent scale of larval dispersal (gene flow) in the central Great Barrier Reef. Fifty-seven collections (within ::s 25 m-) of fragments from sets of approximately 40 colonies were made (where possible) within each of five habitats on each of 12 reefs. These reefs, within the central region of the Great Barrier Reef, were separated by up to 90 km and included one inner-shelf continental island and groups of seven midshelf reefs and four outer-shelf reefs. Most collections contained a high level of multilocus genotypic diversity and hence showed little evidence of recruitment through fragmentation, although the majority of collectionsdisplayed large and consistent deficits ofheterozygotes. Allele frequencies varied greatly among collections (Fon: = 0.43),and this variation wassufficientto explain two-thirds of observed deficiencies ofheterozygotes via a Wahlund effect. A hierarchical assessment of F ST values revealed that 45% of allelic variation occurred among reefs(Fon: = 0.20),and only 16%of variation within reefswasexplained by variation among five major habitat types (FST = 0.05). A relatively small component of the total variation among samples was attributable to across-shelf variation among the groups of middle-and outershelf reefs (Fon: = 0.03); however, the outer-shelf reefs form a single UPGMA cluster separate from all but 4 of the other 43 collections. These data imply that widespread dispersal does occur but that the direction or magnitude of gene flow may be influenced by the along-shelf movement of major ocean currents and weather-dependent currents on or near reefs. Each reef, therefore, forms a partially isolated and highly subdivided population.

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