World Congress of Malacology Antwerp ... - Unitas Malacologica

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Sex and systematics in the Streptaxidae (Pulmonata: Stylommatophora)
Rowson, Ben
Dept. Biodiversity & Systematic Biology, National Museum of Wales, Cathays Park, Cardiff, UK
CF10 3NP,
Email: ben.rowson@museumwales.ac.uk
Recently, sexual selection (often via sexual conflict) has been implicated in the diversification of
organisms. The Streptaxidae are a little-studied, widely distributed and highly diverse family of
carnivorous tropical land-snails. New data from an ongoing systematic study suggests sexual
selection may have been a major force in the diversification of certain clades. A synapomorphy of the
family is possession of a simple penis studded with sharp conchiolinous hooks. In different major
African lineages this has been modified as follows: (A) taxa having serially repeating hooks, lacking
spermatophores, and having a muscular vagina; (B) taxa having serially repeating hooks and penial
stimulators containing novel, conchiolinous dart-like structures, lacking spermatophores, and having
a muscular vagina; (C) taxa having differentiated, specialised hooks and configurations of penial
pilasters, possibly functioning as stimulators, and possessing calc sacs, stylophores or
spermatophores, but lacking a vagina. Type (A) and (B) taxa are moderately speciose, usually
occurring in allopatry, while type (C) taxa are extremely speciose, often occurring in sympatry. These
divisions are largely congruent with phylogenetic reconstructions based on non-genital morphology,
mitochondrial (16S) and nuclear (actin exon) DNA. The latter also suggests that the majority of type
(C) taxa are the product of a single, recent radiation. I interpret the genital differences between the
three types as evidence of differences in the type and strength of sexual selection, and interpret the
differences in diversity as evidence of the resulting response. Under this scenario, the genital features
unique to type (C) taxa are seen as adaptations to rampant sexual conflict, and may also be key
innovations that have promoted the recent diversification. The evidence for an alternative
explanation, that of adaptive radiation relating to carnivory, is considered strong only for type (A)
taxa. Improved phylogenetic reconstructions will allow a more robust test of these predictions in a
comparative framework.
Streptaxomorph shells: an evaluation and possible explanation
Rowson, Ben
Dept. Biodiversity & Systematic Biology, National Museum of Wales, Cathays Park, Cardiff, UK
CF10 3NP,
Email: ben.rowson@museumwales.ac.uk
A “streptaxomorph” shell form, in which the coiling axis deviates substantially to the right during
growth, is common among the Streptaxidae (Pulmonata: Stylommatophora). It does not occur in any
plausible outgroup. This raises two questions: i) why does streptaxomorphy occur at all? and ii) has it
arisen many times, or been repeatedly lost after being inherited by all streptaxids? This poster
presents methods for describing the phenomenon in morphometric terms, offers explanations for its
occurrence, and suggests a possible scenario for its evolution. Preliminary data indicate that the vast
majority of streptaxid species are in fact axially deviated, either to the right (i.e., streptaxomorphy) or
to the left (as in columnar or barrel-shaped species). Streptaxomorph shells are less frequent, but
occupy a greater region of morphospace than left-deviated ones, including a region exploited by few
other land snails (the “Cain gap” where height is approximately equal to diameter). However,
alternative and more appropriate measurements indicate that this is misleading, with streptaxomorph
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