Genotypic Diversity and Population Structure of the Hawaiian Reef Coral, Porites compressa

Abstract

The assessment of clonal diversity in populations of the endemic Hawaiian coral Porites compressa was undertaken using four independent assays of genotypic identity: colony morphology, immunocompatibility testing by tissue grafting, electrophoresis of soluble proteins, and chromatography of ultra-violet absorbing compounds. All methods were corroborative, but electrophoresis of soluble proteins provided the single most efficacious assay of genotypic diversity, with a 7-locus (21 alleles) system which was estimated to sufficiently resolve approximately 95% of clonal samples. Populations of Porites compressa were demonstrated to have derived substantial contributions from both sexual and asexual modes of reproduction. Sexual reproduction in P. compressa was similar in most life history parameters to other Porites species that broadcast spawn. Age at first reproduction was estimated to be between 1.8 and 2.5 years. Mean oocyte diameters were approxiInately 250 um at spawning, with 10-30 eggs per polyp. Spawning was synchronized with full moon during summer months, and some colonies spawned over more than one night and over subsequent months. Planulae were competent to metamorphose after three days, and sibling juveniles which settled gregariously were often observed to fuse, suggesting that there may be ontogenetic changes in self-recognition responses in this species. Production of asexual propagules by fragmentation was calculated to be between 35 and 96 fragments/m2/year for a patchreef population of Porites compressa in Kaneohe Bay, Oahu. Fragment production was continuous through the year, with a 5-10 fold increase in summer months due pertlaps to increased activity of turtles near sheltering areas on the reef. Asexual propagules were large (1.5-151 cm2) and survived for many weeks or months before "recruiting" or being lost from the fragment pool. Spatial and/or numeric abundance of dominant and rare clones of Porites compressa could be explained, in part, by differences in measurable fitness parameters of growth rate and competitive ability. No significant differences in the propensity of clones to produce fragments were detected. The relationship between clonal abundance and single fitness correlates, however, was not a simple one. Clonal fitness is the sum of many life history and ecological characteristics of a genotype. Clonal abundance and distribution may also be effected by non-selective random physical processes leading to asexual colony replication and recruitment. Genotypic diversity in six populations of Porites compressa was directly related to habitat disturbance histories. Highest diversity was found in populations which had been intensely or recently disturbed. In these populations, space was not limited and mean colony size was small (<500 cm2), suggesting an early stage in population recolonization. In a stable, undisturbed population, low genotypic diversity revealed the extent of clonal replication of established genotypes. Unoccupied substratum was rare in this habitat, and average colony size was larger (>2000 cm2). Single clones were distributed over small or large areas (<1 to >16 m2) or distances (<1 to >90 m), and were numerically (>13% of total number of colonies) or spatially (>15% of total colony area) dominant.