THE RELATIONSHIP BETWEEN LOCAL SPECIES ABUNDANCE AND LARVAL DEVELOPMENTAL MODE IN A REMARKABLE SAMPLE OF CONUS (NEOGASTROPODA) FROM THE EARLY PLIOCENE OF THE DOMINICAN REPUBLIC

It is well established that increased dispersal ability is often positively correlated with geographic range size, which in turn has important consequences for the the process of speciation and the long-term persistence of species. An issue that has been less explored in the fossil record is whether an association exists between dispersal ability and localized abundance patterns. Larval developmental mode in benthic marine invertebrates plays an important role in governing dispersal ability: relative to planktotrophic species, lecithotrophic species spend less time in the water column as larvae before settling and therefore tend to be less widely dispersed. Because they have less opportunity to be scattered by currents, however, cohorts belonging to lecithotrophic species may be predicted to be locally recruited in greater numbers, thereby causing them to be more abundant than species with planktotrophic development.

To investigate the relationship between local abundance and developmental mode, I analyzed a remarkable early Pliocene Conus (cone snail) fauna collected from a gravity flow deposit in the Gurabo Fm. of the Cibao Valley, Dominican Republic. This deposit (Tulane University (TU) locality 1227A) was collected in its entirety in 1976 by a team from TU, allowing absolute abundance data to be gathered. The cone snail fauna from TU 1227A includes over 1000 specimens that tentatively represent at least 17 species, most of which are rare and individually account for 1% or less of the total sample. Most of the shells in the sample are juveniles and many have well preserved larval protoconchs, permitting their developmental modes to be determined. The two most abundant species--respectively representing 61% and 20% of the shells in the sample--had lecithotrophic development, while most of the other species had planktotrophic development. I will also discuss the broader importance of this relationship between developmental mode and abundance for the establishment of networks of genetic connectivity between populations, which may have important consequences for evolutionary differentiation and speciation in the marine realm.