Abstract
Morphological variation is often attributed to differential adaptations
to diverse habitats, but adaptations to a similar environment do not
necessarily result in similar phenotypes. Adaptations for water and heat
budget are crucial for organisms living in arid habitats, and in
snails, variation in shell morphology has been frequently attributed to
selection by stressful environmental factors. However, their phenotypic
divergence often is not accompanied by a relevant niche differentiation
and consistent relationships with environmental correlates are lacking.
In the pulmonate genus Albinaria, there is
great size and shape variation between and within species, and there are
two major shell sculpture morphotypes, ribbed and smooth. We used 62
populations of 28 Albinaria species, taking
into account their phylogeny, to examine the variation of shell traits
(sculpture, size, shape), their effect on water and heat budget, and
their association with geographical and climatic gradients. We found
unambiguous size and shape discrimination between the two morphotypes.
Ribbed shells are lighter, taller, and slimmer and have a smaller
aperture than the smooth ones. Moreover, significant correlations
between shell traits and heat/moisture budget and climate/geography were
revealed. Ribbed and taller shells retain more water on their shell
surface, and on the other hand, smooth shells exhibit lower water
permeability. Therefore, two strategies are being used to prevent water
loss, active retention or resistance to loss. Consequently, different
alternative solutions evolved and were retained as responses to the same
stressful factor by the two distinct shell morphotypes. Larger shells
occur in southern latitudes, mostly on islands, and at sites where there
is a shortage of rainfall. Therefore, the variation of the examined
traits is nonrandom with respect to location and to climate and their
evolution can be attributed to selection by environmental factors, with
water availability being the key driving agent of body-size variation.
Keywords
Sculpture, Size, Shape, Adaptation, Tradeoff, Environmental gradients
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