More than a gut feeling: utility of midgut anatomy in phylogeny of the Cerithioidea (Mollusca: Caenogastropoda)

Abstract

The Cerithioidea is a large and diverse superfamily of marine, brackish, and freshwater snails. The complexity and variety of midgut features within this group have long been known, but have been underutilized as a resource of characters for phylogenetic analysis. Consequently, this group is an ideal system for assessing the utility of midgut anatomy at lower systematic levels. Midgut anatomy of 46 cerithioidean gastropods and two outgroup taxa (Campanilidae, Plesiotrochidae) is described. A subset of these taxa, combined with published and unpublished descriptions for several additional species, were coded for 28 midgut characters. These characters were integrated with a large multi-organ system data set for 56 taxa, comprising 123 characters from the shell, radula, foregut, kidney, nerves, sperm ultrastructure, and reproductive system. The morphological matrix was analysed using parsimony, and was combined with a molecular data set of 16S and 28S rRNA sequences, and then analysed using parsimony and Bayesian methods. Optimization of midgut characters on the resulting cladograms indicates that midgut characters provide conservative and informative sources of variation at all hierarchical levels, from basal nodes to within family-level taxa. Exploratory analyses including and excluding midgut characters demonstrate the impact of midgut characters in elucidating phylogenetic relationships of cerithioideans, particularly among and within freshwater clades. Statistical analyses of retention index values for characters mapped on both the morphological and morphological plus molecular topologies confirm that midgut characters have significantly higher phylogenetic signals than other morphological characters. In the case of cerithioidean phylogeny, the importance of midgut data as a resource of characters cannot be overstated. © 2011 The Linnean Society of London, Zoological Journal of the Linnean Society, 2011, 162, 585–630.