Streptaxomorph shells: an evaluation and possible explanation
Dept. Biodiversity & Systematic Biology, National Museum of Wales, Cathays Park, Cardiff, UK CF10 3NP
My poster at the World
Congress of Malacology dealt with an aspect of shell form seen among
the Streptaxidae (Pulmonata: Stylommatophora). This work forms part
of my ongoing PhD on the systematics and diversity of the group,
based at the National Museum of Wales and Cardiff University.
A “streptaxomorph” shell form, in which the coiling axis deviates substantially to the right during growth, is found in many species of streptaxids (Fig. 1). Although occurring in a few other isolated gastropod groups, it does not occur in any plausible close relatives of the Streptaxidae. 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? My poster presented methods for describing the phenomenon in morphometric terms, offers explanations for its occurrence, and suggested a possible explanation for its evolution.
|Fig. 1. Four East
African streptaxid species,
illustrating some of the range of form (not to
scale). The coiling axis of the body whorl (red
line) is deviated to the left in species a and b,
and to the right in the streptaxomorph species
c and d.
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; Cain, 1977). However, alternative and more appropriate measurements indicate that this is misleading, with streptaxomorph taxa avoiding the Cain gap by attaining a “streamlined” narrow shell profile. In life this may confer greater freedom of movement in the apertures of prey snails, and hence selective advantage. Size and shape of the main raptorial and feeding organ, the buccal mass, is also positively correlated with the degree of streptaxomorphy, despite this organ showing remarkably weak allometry and considerable conservatism across streptaxids in general. I suggest that selection for a narrow shell and a large buccal mass marks out streptaxomorphy as an ecological adaptation to dealing with large prey. As such it is likely to have arisen repeatedly and homoplasy is to be expected in systematic studies. This could explain the observation by taxonomists (e.g. Schileyko, 2000) that streptaxomorph genera do not form a natural group. My ongoing systematic work aims to address this and other issues in a phylogenetic framework.
|Cain, A. J. 1977. Variation in the spire index of some coiled gastropod shells, and its evolutionary|
|significance. Philosophical Transactions of the Royal Society of London B 277: 377-428.|
|Schileyko, A. A. 2000. Treatise on Recent terrestrial pulmonate molluscs. Part 6: Rhytididae,|
|Chlamydephoridae, Systrophiidae, Haplotrematidae, Streptaxidae, Spiraxidae, Oleacinidae, Testacellidae. Ruthenica Supplement 2, Moscow.|