Refining molluscan characters: morphology, character coding and a phylogeny of the Caenogastropoda

Midgut morphology of gastropod molluscs has been underutilized as a resource of characters for phylogenetic analysis. The exclusion of these features reflects the inference that they will be uninformative in determining phylogenetic relationships because they are functionally correlated. In general, it has been hypothesized that the style sac form of midgut is an adaptation to microphagy and becomes secondarily simplified in taxa that have adopted a macrophagous/carnivorous habit with a corresponding increase in extracellular digestion (i.e. radular trituration, gizzards and/or foregut glands). This assumption has resulted in the formulation of adaptive scenarios concerning gastropod alimentary systems, which are mapped onto phylogenetic hypotheses derived from other characters. However, any conclusions regarding phylogenetic utility, and therefore homology, must be realized within a cladistic context. For this analysis, a multi‐organ system anatomical data set was assembled for 16 caenogastropods and two outgroups. The data matrix comprises 64 characters and includes many systems poorly represented in previous broad‐based comparative surveys, such as the alimentary and reno‐pericardial systems. In addition, several taxa were included for which no comprehensive anatomical studies have been available (Cyclophoroidea, Ptenoglossa). Phylogenetic analysis with NONA 1.6 resulted in two most‐parsimonious trees with length 188, CI = 0.53 and RI = 0.63, differing only in the placement of Prunum apicinum and Conus jaspideus. The topology of the strict consensus (Macleaniella Theodoxus((Neocyclotus Marisa)((Lampanella((Petaloconchus Strombus)(Crepidula Bithyia)))(Littorina(Neverita(Cypraea(Nitidiscala(Panarona(Prunum Conus(Ilyanassa Urosalpinx)))))))))), is largely congruent with several independent estimates based on both morphological and molecular data, supporting caenogastropod monophyly and monphyly of the Architaenioglossa, Sorbeoconcha and Neogastropoda. To evaluate phylogenetic utility of the midgut, a broad sampling of taxa was included representing a diversity of feeding modes, food preferences and alimentary morphologies. Character optimization revealed that the evolution of midgut structure is highly mosaic, cutting broadly across patterns of feeding, diet and foregut complexity, to a degree previously unappreciated. In addition, features from the foregut (subradular organ, oesophageal folding) and midgut (position of gastric shield) are broadly distributed among large clades of caenogastropods, providing critical basal synapomorphies within the group. This demonstrates that the gut is an unexploited resource for important and informative characters in higher order systematics of caenogastropods. © 2003 The Linnean Society of London, Zoological Journal of the Linnean Society, 2003, 137 , 447–554.