ARONOWSKY, Audrey; Univ. of California, Berkeley: Evolution of a complex synapomorphy in naticid gastropods

Naticid gastropods are a diverse and ecologically important family of marine snails. Previous morphological phylogenetic analyses of Naticidae have demonstrated the monophyly of the family and its close relationship to more derived caenogastropods such as the cypraeids and lamellarids. Several complex synapomorphies diagnose the clade, including possession of an accessory boring organ on the ventral tip of the proboscis, and the construction of a spiral sediment-impregnated egg mass. The egg masses are of particular interest because they are unique among gastropods. A robust phylogenetic hypothesis and information about construction and function are necessary for elucidating how and why the distinctive egg mass evolved. However, outgroup comparison and the fossil record, two common methods for investigating the evolution of character complexes, provide no insight into the evolution of naticid egg masses. Close sister groups do not share any egg mass characters with naticids, and there is no recognized fossil record of these sedimented structures. Two of the approximately 250 extant naticid species produce sediment-free gelatinous egg masses. Phylogenetic analysis suggests this sediment-free condition is derived in extant naticids and therefore does not shed light on the evolution of this complex synapomorphy. Here, I present new phylogenetic hypotheses obtained with morphological and molecular datasets. Morphological features related to the unique sediment-impregnated egg mass are mapped onto these phylogenetic hypotheses to investigate the evolution and function of egg mass morphology. Characters investigated include the use of sediment, arrangement and packing of eggs within a mass, mean egg diameter, number of whorls, egg-free zones within a mass, and fluting at the basal edge of the mass.