DYNAMIC CLAMS - CHANGES IN THE BIVALVE FAUNA OF PACIFIC ISLANDS AS A RESULT OF SEA-LEVEL FLUCTUATIONS

Sea-level fluctuations continually alter the distribution and nature o f shallow-water environments, although not all habitats are equally af fected. Shallow-water habitats on coral reefs around oceanic islands c an be divided into markedly different inner- and outer-reef systems. D uring regressions, the former are stranded while the latter persist. P reviously I showed that numerous species are restricted to inner-reef habitats; I predicted that these would undergo local extinction across most of the central Pacific Ocean during regressions, and would expan d back into the region during high sea stands. An examination of the f ossil record of bivalves on Niue and other central Pacific islands pro vides support for both of these hypotheses, and shows that the range o f some inner-reef specialists can vary substantially among high sea st ands. Despite such unstable ranges, limited data do not indicate highe r global extinction rates for inner-reef specialists. Sea-level fluctu ations can provide vicariant opportunities for speciation, but also im pede the potential for geographic differentiation of populations of in ner-reef specialist taxa, because the lifespan of insular populations is often limited to the duration of single high sea stands.