On the Evolutionary Significance of the Mantle Margin in Pteriomorphian Bivalves

Abstract

The bivalve mantle margin comprises the free portion of the pallial lobes, and often bears muscular, sensory or secretory structures located on extensions of the mantle, called mantle folds. In the bivalve Infraclass Pteriomorphia, which underwent an extensive adaptive radiation in epifaunal habitats, the mantle margin exhibits an enormous diversity in mantle fold organization and associated structures. The present work reviews the current knowledge of the mantle margin in Pteriomorphia and discusses how the mantle margin can be used as a model for investigating the evolution of the marine epifaunal benthos as many species in this community have undergone similar selective pressures and developed comparable morphological adaptations, such as ciliated tentacular organs. Herein, we stress how additional investigations are necessary to cover the huge diversity seen in this taxonomic group. Given the scarcity of broad comparative studies of bivalves, including Pteriomorphia, the homology of their pallial features is still not clear. Moreover, although it is suspected that mantle margin diversification is correlated with transitions in pteriomorphian life habits, no hypotheses have been tested within a phylogenetic framework. To address these questions requires extensive comparative analyses of mantle margin diversity in Pteriomorphia that focuses on: 1) genetic expression and molecular dynamics in the mantle margin; 2) mantle margin morphogenesis, 3) assessment and test of hypotheses on homology, convergent and parallel evolution of mantle margin characters, and 4) testing possible correlations between mantle margin morphological diversity and lifestyle transitions. In a broader sense, by using Pteriomorphia as a model clade, we will gain insight into the macroecology of the marine epifauna.