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

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 haemastomaResale or republication not permitted without written consent of the publisher Aquat Biol 15: 159-166, 2012 160 In this context, Vermeij believes that the major difference between terrestrial and marine biota lies in the abundance of high-level predators, which in the sea are little constrained by dispersal. Vermeij formulates a hypothesis, which we will call 'Vermeij's hypothesis', wherein marine shallow-water organisms possess fewer defenses against predators around oceanic islands than along mainland coasts, probably due to lower predatory pressure.This hypothesis was based on the comparison of structural shell characteristics such as length and projection of the shell spire, thickness of the shell, and size and degree of occlusion of the shell aperture. Vermeij noted that, in general, shallow-water molluscs from oceanic islands had: (1) a higher and more protruding spire, (2) a thinner shell, and (3) a larger and less occluded shell aperture compared to their counterparts on the mainland. These results were interpreted as a reduced defense capacity of island inhabitants, which he suggested to be caused by 'a reduced selection due to predators breaking or entering gastropod shells' (Vermeij 2004) of island species when compar...