Effects of post-larval abalone (Haliotis rubra) grazing on the epiphytic diatom assemblage of coralline red algae

Abstract

Larvae of the abalone Haliotis rubra were settled on pieces of the non-geniculate coralline red alga Phymatolithon repandum. We measured a mean growth-rate of ca. 30 μmsol;day, 11 days after larval settlement which stayed constant throughout the experiment. The diatom assemblage on the surface of the alga was studied for 53 days after larval settlement using scanning electron microscopy. Two species of the genus Cocconeis were found to dominate the diatom assemblage on the surface of P. repandum. Up to 18 days after larval settlement, the diatom population increased exponentially. Eighteen days after settlement, post-larval abalone started to graze on the diatom Cocconeis scutellum leaving the bottom valves of the diatoms on the surface of P. repandum. As a result, the diatom population decreased markedly. The outermost cells and polysaccharide layer of P. repandum were often missing but we did not find any grazing marks on the thallus surface up to 53 days post-settlement. We discuss the hypothesis that epithallial cell sloughing of non-geniculate coralline red algae is an intrinsic mechanism to reduce microscopic epiphytes and may interact closely with grazing by post-larval abalone. We concluded that the post-larvae must find an additional food source derived from the NCA before grazing on the diatoms, because of the high growth-rate during the first and second week of rearing and the constancy throughout the experiment.