Biogeography of coral reef shore gastropods in the Philippines

The aim of this thesis is to describe the distribution of coral reef and shore gastropods in the Philippines, using the species rich taxa, Nerita, Clypeomorus, Muricidae, Littorinidae, Conus and Oliva. These taxa represent the major gastropod groups in the intertidal and shallow water ecosystems of the Philippines. This distribution is described with reference to the McManus (1985) basin isolation hypothesis of species diversity in Southeast Asia. I examine species-area relationships, range sizes and shapes, major ecological factors that may affect these relationships and ranges, and a phylogeny of one taxon. Range shape and orientation is largely determined by geography. Large ranges are typical of mid-intertidal herbivorous species. Triangular shaped or narrow ranges are typical of carnivorous taxa. Narrow, overlapping distributions are more common in the central Philippines. The frequency of range sizes in the Philippines has the right skew typical of tropical high diversity systems. This shows that there are many species with small range sizes, and suggests a tendency for these ranges to overlap. The species area curves are consistent with predictions of basin isolation on species richness. The central Philippine basins (Visayas and, Sibuyan) have a z estimate (a parameter of the Species Area relationship or SPAR) close to unity (0.59-1.30). This contributes to biogeographical provinciality (a measure of faunal uniqueness) in these basins. The basin that is most provincial is the Sibuyan Sea basin. However this provinciality may also be due to a small-area effect or the decoupling of species richness with area as a result of habitat heterogeneity within the basin. Endemicity of taxa is observed mainly in the central, as opposed to the peripheral, oceanic basins. A regression approach was applied to test the effects of larval duration and habitat availability on range size and species richness of Conus. The results suggest that habitat is a more significant factor in determining species ranges and species richness than larval duration. This supports the suggestion from basin isolation for an important effect of habitat heterogeneity on range size and species richness. . Extinction rate estimates are negative for the Philippines and other areas in the Indo-West pacific (IWP). This suggests that species in the Philippine basins, and the IWP in general, have been accumulating in these areas over the past 18,000 years. In Conus, the mode of speciation was inferred from a published molecular phylogeny, coupled with data on modern ranges. This study also tried to infer Conus speciation within the IWP. The relationship of modern ranges and phylogenetic information is not informative, and does not provide inferences on the mode or location of speciation. The ranges and phylogenetic patterns of Conus suggest that changes in range extents have been large during the evolution of the genus. This may be due to the long larval duration, that allows for wide dispersal, being largely conserved during Conus evolution. In the sand-dwelling coral reef genus Oliva, the ranges and species area curves were similar to those of Conus.. The central Philippines basin of the Sibuyan Sea has the highest degree of provinciality. The area of the OGU (geographical regions) affects species richness of Oliva significantly This observation is consistent with results of a PCA ordination of the frequency of occurrence of Oliva. The presence of sandy habitats affect Oliva species richness significantly. The morphological diversity of two widely distributed species of Oliva was studied. Monotopic species (species that are found only in a single substrate type) tend to show morphologies that are found only in certain oceanic basins. Modern ranges suggest basin isolation as an historical process that has maintained and possibly caused the high taxonomic diversity of intertidal and shallow water gastropods in the IWP. There is evidence that high species diversity in IWP is likely related to the existence of numerous habitats. The geological histories of the Philippine oceanic basins may provide important information in future biogeographic studies of patterns of species richness. The evidence is considered with respect to current molecular phylogenetic studies of gastropods. The study highlights the paradox of low endemism in a highly diverse region. Suggestions are made for future research that could provide insight into the nature of endemism and species persistence of marine organisms in the IWP