Molecular Phylogenetics and Historical Biogeography of Cockles and Giant Clams (Bivalvia

This study produces a nearly comprehensive large-scale molecular phylogeny for the marine bivalve family Cardiidae (Cockels and Giant clams) and uses this topology to examine patterns of diversification in the marine realm. This study uses maximum-likelihood and Bayesian phylogenetic analyses of two nuclear (Histone 3, 28S) and one (16S) mitochondrial for 110 species representing 37 of the 44 recognized genera and all eight extant subfamilies. Lineage divergence times were estimated using the Bayesian estimation with uncorrelated relaxed rates among lineages (BEAST). To reconstruct ancestral geographic ranges, we used the dispersal-extinction-clodegenesis method (Lagrange) with a stratified paleogeographic model in which dispersal rates were scaled according to area connectivity across three time slices. The resulting topologies are discussed with respect to traditional subfamilial classifications, and previous anatomical and molecular findings. We confirm the monophyly of two subfamilies, Tridacninae and Clinocardiinae as previously defined, but found there to be rampant paraphyly/polyphyly of the other six subfamilies. The Cardiidae seem to have originated in the tropical Indo-Pacific some time in the early Cretaceous and diversified within the tropical Pacific. The extant diversity seen in the Atlantic is derived from species that dispersed from the tropical Indo-Mediterranean region during the Cenozoic via the Tethys sea.