LATITUDINAL GRADIENTS OF EPIFAUNAL BIVALVE SHELL MORPHOLOGY AND THEIR RELATION TO PREDATION INTENSITY

Marine epifaunal bivalves face a large array of predators, and several species produce spines that are thought to provide protection against predation. It has been assumed conventionally that predation pressure has increased through geological time and along present-day latitudinal gradients toward the tropics, although neither of these patterns is accepted unanimously. If these trends exist, spined taxa should constitute a large percentage of epifaunal bivalves in the Recent, with an increasing percentage towards the tropics. Previous examinations of current latitudinal gradients recognized morphological distributional patterns consistent with the above hypotheses, but latitudinal gradients were determined through broad geographic regions limited to the coasts of North America, and the results were based upon ambiguous categories of shell morphology that may not have adequately delineated structurally similar groups. To conduct a more rigorous test, a database was constructed to assess the relative, present-day latitudinal distributions of spined, textured, and smooth species by employing exact categorical definitions of morphology based on bivalve shell structure. Distributional and morphological data were gathered for 442 epifaunal bivalve species, ranging from 90ºN to 70ºS latitudes, inclusive of all major ocean basins. Results show that spined bivalves do not constitute more than 25% of epifaunal bivalve species at any latitude. Additionally, no latitudinal gradient in morphology could be detected between tropical and temperate waters (0º to 50º, N and S). These results are counterintuitive, assuming that predation is a major selective pressure in the history of this group and with the stipulation that spines are truly defensive structures. If spines were effective defensive structures, clades capable of producing them would have proliferated, especially in the tropics. This is not the case. Possibly, spines are not effective anti-predation structures for epifaunal bivalves. Alternative functions should be considered and more work is needed to investigate the obviously effective protective strategies of the more copious, non-spined groups. However, it may also be that the underlying assumptions of increased predation pressure should be reexamined, particularly given the paucity of empirical tests that support it.