MACROEVOLUTION, ECOLOGY AND ENVIRONMENTS OF TROPICAL AMERICAN SCALLOPS (BIVALVE: PECTINIDAE)

Ecological characteristics strongly influence evolutionary patterns, but quantification of these effects is difficult. Scallops exhibit striking evolutionary differences across the Isthmus of Panama. Caribbean species diversity is more than double that in the eastern Pacific and larvae spend less time in the plankton as estimated from the size of larval shells. These differences in larval biology reflect differences in food availability and spatial and temporal environmental variability across the Isthmus and are in agreement with the predictions of life history theory. In contrast, Early Pliocene faunas were similar in diversity and larval characteristics. Their subsequent divergence is closely correlated with changing environmental conditions associated with the final closure of the Isthmus of Panama. The exact timing of evolutionary events is still to be determined but several factors point to higher evolutionary rates in the Caribbean related to decreased larval durations in the plankton and shorter larval dispersal. Early Pleistocene faunas in both oceans are made up of almost entirely extant species so extinction in both oceans occurred in the Middle to Late Pliocene. Extinctions in the Caribbean predominantly affected taxa with Pacific-like larval ecologies. In contrast, most Plio-Pleistocene originations in the Caribbean were bysally attached taxa found in high carbonate areas with large reef-building corals. These taxa all have very short larval durations and include several taxa with lecithotrophic larval development, which does not occur in the eastern Pacific. In addition, all living species with fossil records in the Caribbean, regardless of time of origin, have shorter larval durations than those in the eastern Pacific. The result is an evolutionary pattern of decreasing larval durations and increasing species diversity in the Caribbean. This same evolutionary pattern of larval durations is observed within 3 of the 4 dominant genera, so this pattern reflects varying evolutionary rates within clades as well as the replacement of clades.