Low genetic variability within and among populations of the brooding sponge Rhopaloeides odorabile on the central Great Barrier Reef

The population genetic structure of the brooding sponge Rhopaloeides odorabile (Dictyoceratida) was examined at 3 polymorphic allozyme loci and the partial mitochondrial cytochrome oxidase sub-unit 1 gene (CO1) sequence (528 bp) from multiple sites across the central Great Barrier Reef (GBR), Australia. Distances between sampled sites ranged from a few 100 metres to 140 km. Observed levels of allozyme variation were generally low, and there were only minor differences in allozyme allele frequencies evident between sites. Likewise, levels of polymorphism for mtDNA CO1 were also low, with only 3 haplotypes found at this locus and with 1 of the 3 haplotypes present at all sites with frequencies ranging from 0.55 to 1.00. There was no obvious partitioning or structuring of the observed allozyme or CO1 genetic variance with spatial positioning of populations and no evidence in this sponge species for genetic differentiation between inner- and mid-reef sites. However, there was evidence against complete genetic panmixia across the central GBR, with 2 pairs of inner-reef sites sampled in 2004 genetically differentiated from most other pairwise site comparisons, and both pairs were also different from single sites within a few 100 metres sampled in the previous year. When viewed together, nuclear and mtDNA markers indicate large-scale genetic admixture in this species, although there is some evidence for small, localised, genetic differences between some populations that might be determined by reef-specific hydrodynamics. This pattern is consistent with the endogenous recruitment expected from a brooding species and with dispersal that is infrequent enough to prevent divergence among populations.