LATE NEOGENE DIVERGENCE IN LIFE SPAN OF SCALLOP LARVAE ACROSS THE ISTHMUS OF PANAMA

The time larvae spend in the plankton affects larval dispersal, connectivity of populations, and rates of evolution. Larval duration varies among environments but we know very little about changes in larval ecology in response to long-term changes in the environment. To this end, we documented changes in size of larval shells for recent and fossil scallops (Bivalvia, Pectinidae) from both sides of the Isthmus of Panama over the past 15 million years. Oceanographic conditions strongly diverged across the developing Isthmus during this time. We used SEM to measure the PII (prodissoconch II) stage of the larval shell on more than 1300 specimens of 35 species of scallops. Size of the PII larval shell is positively correlated with larval duration. Recent scallops in the Caribbean have smaller PII larval shells than the Eastern pacific (208 versus 245 microns) and therefore spend significantly less time in the plankton than do Eastern Pacific species. These differences are highly significant statistically despite strong variability in PII larval size among taxa and life habits. Size of PII scallop larvae from Late Miocene deposits averages 220 microns. Since that time, size increased 25 microns in the Eastern Pacific and decreased 12 microns in the Caribbean. Changes in size appear gradual in both oceans but more data are required to be confident about rates of change. These trends support models of larval ecology that predict longer larval lifetimes in response to increasing seasonality and productivity.