Clearance effects of the Pacific oyster Crassostrea gigas on the fish-killing algae Chattonella marina and Chattonella antiqua

Abstract

We exposed Pacific oysters Crassostrea gigas to the harmful algal species, Chattonella marina and C. antiqua, at various initial algal cell densities for up to 24 h. None of the oysters died, even after exposure to high cell densities of both Chattonella species. Oysters filtered Chattonella from the aquarium water within 24 h, depending on initial algal cell densities. The oyster clearance rates peaked at a specific cell density for each algal species (about 100 cells ml -1 for C. marina and about 10 cells ml -1 for C. antiqua). However, because of differences in cell sizes between the algal species, the algal biovolume per unit water volume varied widely at the same cell density. The rela- tionships between clearance rates and biovolumes of exposed algae were similar between the 2 algal species; clearance rates by the oysters peaked at about 1.0 × 10 6 μm 3 mm -3 . The filtration activity of the oysters was fundamentally affected by their exposure to algal biovolume and not algal cell density. No histological lesions were found after the oysters were exposed to either Chattonella species. Furthermore, immunohistological observations of the digestive glands of the oysters and analysis of their phytopigment content revealed that Chattonella cells that were cleared from the water during exposure entered the cytoplasm of the oyster digestive glands by phagocytosis.