Morphological Variability of Phorcus lineatus (da Costa, 1778) Shells along the Coast of Portugal and the Significance of the Environment on Shell Morphology

Abstract

Phorcus lineatus (da Costa, 1778) is a marine intertidal gastropod occurring in the Eastern Atlantic, from Northern Ireland and Wales to Mauritania. Along the Portuguese continental coast, it displays marked shell variability. Northern shells have a worn spire, but southern specimens display good preservation. At most sites, the majority of shells show no umbilicus, but at sites scattered along the coast, most shells display a distinct or conspicuous umbilicus. In order to understand these discrepancies, I carried out analyses of the mean grain size of the beach sand where live animals were collected and estimations of the annual mean wave height near the sampling sites by using Portuguese Institute for Sea and Atmosphere (IPMA) forecasts. Sampling, experiments and data loading were carried out in the period 2012–2015. Whereas no direct correlation was found between umbilicus prominence, shell size and studied environmental variables, abrasion and wear of the spire were found to be primarily dependent upon sea wave height and on beach sand grain size. It was concluded that current malacological literature, particularly field guides, sub-represent the morphological diversity of P. lineatus shells omitting eroded shells, and that worn shells occur along the distribution range of this species. Abrasion and wear of P. lineatus shells appear to increase with increasing sea wave height, and also with increasing beach sand grain size, reflecting the intensity of the hydrodynamics at a local scale.