Genetic structure of black coral populations in New Zealand's fiords

The antipatharian black coral Antipathes fiordensis is endemic to south-western New Zealand, where most of the population biomass occurs in shallow water throughout a network of fiords and inlets. The planulae larvae of A. fiordensis are negatively buoyant, weak swimming and short lived. These factors, coupled with hydrographic observations of restricted water movement in the fiords, suggest that larval dispersal and gene flow will be Limited in this species, and that separate fiords may represent genetically isolated populations. This study tests the hypothesis of limited larval dispersal in A. fiordensis by examining the genetic variation and population subdivision both within and among fiords. Allozyme electrophoresis at 10 polymorphic loci revealed an unusual population genetic structure in A. fiordensis. Significant genetic variation was found among sites (mean F-ST = 0.046, p < 0.01), suggesting larval dispersal is restricted even at distances of 10 to 15 km. Interestingly, most genetic differentiation occurred between sites within fiords (F-ST = 0.034), rather than among fiords (F-ST = 0.02) which is contrary to the pattern expected if the fiord populations were reproductively isolated. Furthermore, regression of gene flow with geographic distance showed no evidence of isolation-by-distance between populations (r(2) = 0.017), as might be expected in a species with limited larval dispersal. Asexual reproduction was apparent in populations of A. fiordensis, as evidenced by genotypic diversity ratios <1, as well as significant departures from random mating associated with a combination of heterozygote deficits and excesses at the majority of sites. I propose that the atypical genetic structure observed for A. fiordensis in Fiordland represents a population that has not yet reached equilibrium due to a combination of the effects of recent colonisation, asexual reproduction and the potential longevity of individual coral genotypes.