Ecological Structure of the Molluscan Fauna of the Plio-Pleistocene Caloosahatchee Formation of Florida

Ecological structure of the molluscan fauna of the Plio-Pleistocene Caloosahatchee Formation was investigated for two bulk samples collected by Squires and Heaslip in 1955 near La Belle, Florida (samples on loan from the American Museum of Natural History). The samples contained > 12,000 bivalve and gastropod specimens representing > 8000 individuals. Richness, diversity, and evenness were determined at the generic level for both samples; rarefaction analysis compared sample richness. The larger sample exhibited a lower richness than the smaller sample (49 genera in >6700 specimens compared to 57 genera in >5300 specimens). However, Shannon and Simpson diversity indices as well as evenness, dominance, and equitability were similar between the two samples. Predatory drilling was uncommon; at the assemblage level, the two samples had comparable (based on chi-squared tests) drilling frequencies of ~7% for bivalves and <1% for gastropods. Although some rarer taxa were drilled more frequently (e.g., Stewartia, Dosinia, Tellina; drilling frequencies of ~20%), both samples were dominated by Chione, with a drilling frequency of ~ 7% in each sample. Few incomplete drillholes were documented for bivalves (prey effectiveness < 3% for each sample); none occurred for gastropods in either sample. Life habits such as feeding strategies, mobility, and relationship to the substrate were analyzed at the family level for bivalves and gastropods. Bivalves, dominated by venerids (e.g., Chione), included a large proportion of suspension-feeding infaunal siphonates. Chemosymbiotic lucinids were also common, as were several nonsiphonate infaunal taxa. Epifaunal bivalves were rare. Epifaunal grazers dominated the gastropod fauna in terms of abundance, but carnivores showed the greatest generic diversity. Suspension-feeding gastropods were uncommon. In general, a comparison of paleoecological aspects of these assemblages indicates that drilling parameters, diversity, evenness, and guild structure are similar for the two samples.