Intraspecific Shore‐Level Size Gradients in Intertidal Molluscs

Abstract

A synthesis of new data and literature observations indicates that, when rocky intertidal gastropod species (1) shell size tends to increase in an upshore direction in species characteristic of the littoral fringe and in high intertidal limpets, and (2) shell size often decreases in an upshore direction in species typical of lower intertidal levels. These size gradients are considered to be a response to gradients in the intensity and nature of postlarval prereproductive mortality on the shore. In gastropods whose size gradients are of type 1, mortality generally resulting from physical extremes operates from above and is most effective against small individuals. Among snails with size gradients of type 2, mortality often in the form of predation or other biotic intraction is most intense at low levels. Sedentary species can become graded according to size with shore level only through differential mortality of one size group relative to another over the entire vertical range of the species, while mobile forms may become size segregated by active migration of one size group relative to another.