Article: Variation of Recent and fossil Crassostrea in Jamaica
D. Timothy J. Littlewood and Stephen K. Donovan
Biological studies have indicated that the oysters Crassostrea virginica (Gmelin) and C. rhizophorae (Guilding) may be a single species. This is surprising as they are morphologically dissimilar, C. virginica being far larger and thicker than C. rhizophorae. We postulate that this variation may be ecophenotypic in origin, a cause of gross variation in form in other oysters. To test our hypothesis, we have compared the palaeoecology and ecology of Plio-Pleistocene C. virginica and Recent C. rhizophorae from Jamaica. A spectacular Plio-Pleistocene deposit is dominated by C. virginica, other organisms being almost absent. One exceptional bed, over 3 m thick, is dominantly composed of oysters. This sequence appears to have been near-shore marine, or possibly estuarine, but, somehow, the environment was obviously highly favourable for C. virginica. Conversely, modern C. rhizophorae mainly attach to mangrove rhizophores and may compete with a very broad variety of organisms. Physical factors, such as salinity, can vary rapidly within this environment. In consequence, C. rhizophorae seems to grow fast, reproduce early and die early, whereas Plio-Pleistocene C. virginica grew to a large size which probably indicates considerable maturity. Environmental stress necessitates a rapid life cycle for C. rhizophorae. Therefore, ecophenotypic variation may indeed be the cause of morphological variation between C. virginica and C. rhizophorae. However, detailed studies on living populations of both species are considered essential to test this hypothesis further.