Zonal trend of reef topography and bioerosion process by sea urchins in a fringing reef, Ishigaki Island, the Ryukyus, Japan

Topographic zonation is developed as parallel to the shore in many fringing reefs in the Ryukyu Islands. The distribution of reef organisms is corresponding to the topographic zonation. The process and intensity of bioerosion is also assumed to have some zonal structure. The present paper discusses the bioerosion agency in topographic zones on fringing reefs in Ishigaki Island in the Ryukyus. Two transect lines were set to survey the distribution and bioerosion agency of sea urchins in the different types and scale of fringing reef in Shiraho and Arakawa-shita areas. The distribution of species, types and the habitation density of sea urchins conform to the reef topographic zones: Echinometra mathaei type A dominates in shallow lagoon and inner reef flat of Shiraho, and reef slope of Arakawa-shita, E. mathaei type B dominates in reef crest, outer reef flat and reef edge of Shiraho, and reef flat of Arakawa-shita, E. mathaei type C dominates in reef crest, outer reef flat and reef edge of Shiraho, and reef flat and reef edge of Arakawa-shita, E. mathaei type D dominates in reef edge of these two places, and Echinostrephus molaris inhabits reef slope. Highest density was observed at the outer reef flat in Shiraho and at the reef edge in Arakawa-shita. The habitat selection of E. mathaei probably depends on the tolerance to temperature, salinity and desiccation. Moreover, the erosion form by sea urchins also changes according to the topographic zones. The erosion form was classified into four types. The ditch type is observed at the reef crest and the outer reef flat in Shiraho, and the reef flat in Arakawa-shita. The cave type is seen at the reef edges in each place, and the obscure erosion type is observed at the inner reef flat and the shallow lagoon in Shiraho. The well type formed by E. molaris is seen at the reef slope. These bioerosion types shape the reef surface and form the site-specific erosional topography. The volume of bioerosion at the reef surface by sea urchins is also supposed to be site-specific. Therefore, the bioerosion agency on coral reefs varies according to difference of the topographic zones. It may be an important process to consider the balance of sedimentary budget and coral reef formation.