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Islands are often considered natural laboratories and thus excellent models for test- ing 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 239254), 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 Stra- monita haemastoma from both study areas). Pleistocene S. haemastoma fossils from the Azores were also analyzed. Specimens were measured for defense-related shell characteristics and com- pared (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. haemas- toma fossils and extant specimens showed significantly narrower apertures and thicker lips in Pleistocene shells. Despite some evidence supporting the arguments used by Vermeij, our analy- sis failed to conclusively support this hypothesis as a whole.

Defense in shallow-water invertebrates at oceanic islands vs. the mainland coast