AB 15:159-166 (2012)  -  DOI: https://doi.org/10.3354/ab00411

ABSTRACT: Islands are often considered natural laboratories and thus excellent models for testing theories on adaptive radiation. To date, most studies have focused on terrestrial ecosystems. In a comparative study between island populations and their mainland counterparts, Vermeij (2004, Frontiers of biogeography, p 239−254), using published data, argued that shallow-water marine shelled molluscs are better defended against predation along continents than on oceanic islands, possibly due to a lower predation pressure on islands. Here we evaluate this hypothesis, using shallow-water marine gastropods from the Azores and the Portuguese mainland as models (the island species Columbella adansoni and the mainland species C. rustica, and populations of Stramonita haemastoma from both study areas). Pleistocene S. haemastoma fossils from the Azores were also analyzed. Specimens were measured for defense-related shell characteristics and compared (island vs. mainland populations) using principal component analysis and Mann-Whitney tests. The results revealed that Azorean C. adansoni possess significantly thicker and wider shells with a narrower aperture than mainland C. rustica, which present a lighter shell and a higher spire. Comparisons of island and mainland S. haemastoma indicate that the Azorean population is lighter, with a wider aperture, thicker lip, and lower spire. The comparison of Azorean S. haemastoma fossils and extant specimens showed significantly narrower apertures and thicker lips in Pleistocene shells. Despite some evidence supporting the arguments used by Vermeij, our analysis failed to conclusively support this hypothesis as a whole.

KEY WORDS: Evolution on islands · Predatory impact · Island biogeography · Columbella adansoni · Columbella rustica · Stramonita haemastoma