The evolution of shell apertural barriers in viviparous land snails (Gastropoda, Pulmonata, Clausiliidae)

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shell morphology, reproductive strategy, adaptation, X-ray microcomputed tomography, embryo, Gastropoda, Clausiliidae, door snails
  • A. Sulikowska-Drozd
  • M. Walczak
Canadian Journal of Zoology
Apertural barriers in the snail shell were frequently associated with adaptation against predators or with reducing water loss. Yet formation of teeth occluding the aperture is costly and potentially precludes life-bearing reproduction. In viviparous species, a trade-off between the shell aperture size and the embryo shell size at birth is expected. This hypothesis was tested in clausiliids, land snails with strong apertural barriers, that displayed a range of reproductive strategies (oviparity, egg retention, viviparity). We assessed their three-dimensional internal shell morphology by means of X-ray microcomputed tomography (XMT). The data were later analysed with a specially designed algorithm that mimics the movement of the spherical embryonic shell in the lumen of the parental shell. It appeared that viviparous reproduction required the widening of the passage through the ultimate whorl, but that did not necessarily lead to a reduction of apertural barriers. This provides an example of evolutionary compromise between enhanced fitness of the offspring owing to a long gestation period and survival of the parent. Clausiliids might be selected for viviparity only if other selective forces were released, e.g., in areas where desiccation was not a major threat. This hypothesis is in congruence with the distribution of viviparous clausiliids in Europe.

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