Phylogeography of western Pacific Leucetta 'chagosensis' (Porifera: Calcarea) from ribosomal DNA sequences: implications for population history and conservation of the Great Barrier Reef World Heritage Area (Australia)

Leucetta ‘chagosensis’ is a widespread calcareous sponge, occurring in shaded habitats of Indo-Pacific coral reefs. In this study we explore relationships among 19 ribosomal DNA sequence types (the ITS1-5.8S–ITS2 region plus flanking gene sequences) found among 54 individuals from 28 locations throughout the western Pacific, with focus on the Great Barrier Reef (GBR). Maximum parsimony analysis revealed phylogeographical structuring into four major clades (although not highly supported by bootstrap analysis) corresponding to the northern/central GBR with Guam and Taiwan, the southern GBR and subtropical regions south to Brisbane, Vanuatu and Indonesia. Subsequent nested clade analysis (NCA) confirmed this structure with a probability of > 95%. After NCA of geographical distances, a pattern of range expansion from the internal Indonesian clade was inferred at the total cladogram level, as the Indonesian clade was found to be the internal and therefore oldest clade. Two distinct clades were found on the GBR, which narrowly overlap geographically in a line approximately from the Whitsunday Islands to the northern Swain Reefs. At various clade levels, NCA inferred that the northern GBR clade was influenced by past fragmentation and contiguous range expansion events, presumably during/after sea level low stands in the Pleistocene, after which the northern GBR might have been recolonized from the Queensland Plateau in the Coral Sea. The southern GBR clade is most closely related to subtropical L. ‘chagosensis’, and we infer that the southern GBR probably was recolonized from there after sea level low stands, based on our NCA results and supported by oceanographic data. Our results have important implications for conservation and management of the GBR, as they highlight the importance of marginal transition zones in the generation and maintenance of species rich zones, such as the Great Barrier Reef World Heritage Area

Unexpected patterns of genetic structuring among locations but not colour morphs in Acropora nasuta (Cnidaria; Scleractinia)

Symbiotic relationships have contributed greatly to the evolution and maintenance of biological diversity. On the Great Barrier Reef, species of obligate coral-dwelling fishes (genus Gobiodon) coexist by selectively recruiting to colonies of Acropora nasuta that differ in branch-tip colour. In this study, we investigate genetic variability among sympatric populations of two colour morphs of A. nasuta ('blue-tip' and 'brown-tip') living in symbiosis with two fish species, Gobiodon histrio and G. quinquestrigatus, respectively, to determine whether gobies are selecting between intraspecific colour polymorphisms or cryptic coral species. We also examine genetic differentiation among coral populations containing both these colour morphs that are separated by metres between local sites, tens of kilometres across the continental shelf and hundreds of kilometres along the Great Barrier Reef. We use three nuclear DNA loci, two of which we present here for the first time for Acropora. No significant genetic differentiation was detected between sympatric colour morphs at these three loci. Hence, symbiotic gobies are selecting among colour morphs of A. nasuta, rather than cryptic species. Significant genetic geographical structuring was observed among populations, independent of colour, at regional (i.e. latitudinal separation by < 500 km) and cross-shelf (< 50 km) scales, alongside relative homogeneity between local populations on within reef scales (< 5 km). This contrasts with the reported absence of large-scale genetic structuring in A. valida, which is a member of the same species group as A. nasuta. Apparent differences in biogeographical structuring between species within the A. nasuta group emphasize the need for comparative sampling across both spatial (i.e. within reefs, between reefs and between regions) and taxonomic scales (i.e. within and between closely related species)