Morphological variation along intertidal gradients in a population of black abalone Haliotis cracherodii Leach 1814

Abstract

Temporal patterns of morphological variation in multivariate size and shape in a population of black abalone Haliotis cracherodii Leach 1814 in an intertidal boulder field in southern California was examined. From July 1983 to May 1985, 707 individuals were tagged and measured for six shell traits. The extent and pattern of morphological variation in allometric size, tremata (= respiratory pore) number, and relative tremata size was similar among surveys. Shore-level gradients in morphology varied temporally and involved independent variation in both shell size and shape. In the summer and fall, individuals with relatively fewer smaller tremata occurred higher on the shore than individuals with relatively more larger tremata. Seasonal variation in intertidal distribution and the incidence of Octopus-drilled abalone shells indicated that declines in abalone abundance were due to predation by Octopus in the summer and fall. Shore-level gradients were thus associated with opposing selective regimes, predation in the low intertidal and desiccation in the high intertidal. These results support the hypothesis that shore-level gradients in morphology result from movement and differential growth and are a dynamic response of individuals to temporal changes in physical stresses and predation which vary along intertidal gradients.