HOW DO PRODUCTIVITY GRADIENTS INFLUENCE MOLLUSCAN SIZE-FREQUENCY DISTRIBUTIONS? PRELIMINARY RESULTS FROM A PILOT STUDY ON THE PACIFIC AND CARIBBEAN COASTS OF PANAMA

Spatial and temporal gradients in individual body size, both within and across species, are a prominent feature of the marine invertebrate fossil record. However it is not always clear what environmental factors, if any, control such size gradients. Primary productivity differences are commonly invoked, but due to the lack of reliable proxies for local primary productivity this hypothesis can be difficult to test. Moreover, modern data on marine invertebrate local body size distributions are relatively scarce, especially in the benthic shelf environments that dominate the continental fossil record. To elucidate environmental controls on local body size distributions we have begun to tabulate molluscan size-frequency data from modern subfossil death assemblages. We take advantage of a large productivity gradient in the Central American tropics: the Pacific coast of Panama, especially the Gulf of Panama, is a strongly seasonal high-productivity setting, whereas the Caribbean coast is dominated by oligotrophic conditions with little seasonal variation. Initial analyses comparing Pacific and Caribbean dredge samples are surprising: mean individual size of both gastropods and bivalves is much larger in the Caribbean than in the Pacific. The pattern holds up at the whole-community level, within most trophic groups and even within congenerics. This contrast does not appear to be explained by differences in shell destruction rate, but may be influenced by higher mortality rates and faster turnover in the Pacific due to natural predation and/or recent human impacts. Despite the larger mean size of Caribbean individuals the overall abundance of shells is much higher in Pacific samples so that estimates of whole-community energetic demand are positively correlated with productivity. To better understand the controls on body size gradients, future work will focus on sampling different environments along each coast, dating shells to assess degree of time-averaging, and comparing growth rates within those genera that occur in both Pacific and Caribbean samples.