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Sex and darts in slugs and snails (Mollusca: Gastropoda: Stylommatophora)

Published online by Cambridge University Press:  29 November 2005

Angus Davison
Affiliation:
Institute of Genetics, School of Biology, University of Nottingham, Nottingham, NG7 2UH, U.K. Institute of Evolutionary Biology, School of Biological Sciences, University of Edinburgh, Edinburgh, EH9 3JT, U.K. Graduate School of Life Sciences, Tohoku University, Aoba-ku, Sendai 980 8578, Japan
Christopher M. Wade
Affiliation:
Institute of Genetics, School of Biology, University of Nottingham, Nottingham, NG7 2UH, U.K.
Peter B. Mordan
Affiliation:
Department of Zoology, The Natural History Museum, London, SW7 5BD, U.K.
Satoshi Chiba
Affiliation:
Graduate School of Life Sciences, Tohoku University, Aoba-ku, Sendai 980 8578, Japan
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Abstract

In the final stages of an elaborate courtship, many slugs and snails shoot calcareous ‘love’ darts into each other. While darts improve the reproductive success of the shooter, by promoting sperm survival in the recipient, it is unclear why some species have darts and others do not. In fact, dart use has barely been studied, except in the garden snail Cantareus aspersus (Helix aspersa). An evolutionary approach was therefore taken to attempt to understand the origin and use of darts, by investigating mating behaviour in a wide range of species. The prediction was that, because darts could have arisen out of an escalating cycle of sperm digestion and investment in sperm, then darts should be found in taxa that enforce simultaneous reciprocity during mating. Likewise, they should not be found in taxa that mate unilaterally, because the co-evolutionary cycle is absent or reduced. Mating behaviour in 60 genera (28 families) of land snails and slugs was recorded, and compared against dart use across the whole of a stylommatophoran phylogeny. ‘Face-to-face’ simultaneous reciprocal-mating behaviour is restricted to three monophyletic groups of snails and slugs, and dart-bearing species are a subset within the same clades, which suggests a link, though not necessarily a causal one. As yet, we are unable to quantify the extent to which darts or mating behaviour, as well as several other correlated characters, are determined by common ancestry or regimes of natural or sexual selection, because the current phylogeny lacks resolution. However, the results emphasize that to understand the use of darts, then data are required from a wide range of species. The realization that several characters are correlated may stimulate further research, and could eventually lead to some testable models for dart and mating behaviour evolution.

Type
Research Article
Copyright
2005 The Zoological Society of London

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