VARIATION IN SNAIL SPECIES WIDESPREAD ON PORTO SANTO, MADEIRAN ARCHIPELAGO

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Abstract
Shell variation has been studied in two widespread land snails from Porto Santo, Madeiran archipelago: the helicid species or species complex Heterostoma paupercula (Lowe) and the clausiliid species Boett-geria lowei Albers. Samples were collected from the mainland and the offshore islets from sea level to the highest altitude. Shell size, colour and presence or absence of penstome teeth were scored in H. paupercula and some individuals were dissected to examine genitalia. Shell size and rib number were scored in B. lowei. In B. lowei size declines and rib number increases with altitude. In H. paupercula samples from sandy locations have a larger shell size and a higher frequency of pale colour than those from non-sandy locations. On average, sandy sites have a lower altitude than non-sandy ones, however, so that these two variables are confounded. The survey shows that shell characters of a kind used in taxonomy are subject to variation associated with ecology, this must be borne in mind when assessing present-day taxa and morphological change through the fossil sequence. The data for H. paupercula support the conclusion that it is a single species variable in shell characters and polymorphic for hemiphally.

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