Structure and function of haemocytes in two marine gastropods, Megathura crenulata and Aplysia californica

Aplysia californica is an important mollusc for neurobiologicalresearch and Megathura crenulata is becoming valuable as thesource of keyhole limpet haemocyanin (KLH) which shows promisein treating cancer, allergy and immunosuppression. The cultureof these animals provides opportunities to assess their physiologicalresponses to stress and disease. Molluscan haemocytes are knownto be involved in a variety of physiological responses, yetthe haemocytes of this opisthobranch and vetigastropod havenot been characterized. The purpose of this study is to describethe morphology of these cells and summarize their functionsbased on a suite of assays previously developed on other speciesof molluscs. Using morphology, differential centrifugation andstaining reactions, we identify a single type of circulatinghaemocyte in the blood of both animals. All haemocytes lackgranules and contain glycogen and vesicles that react as lysosomes.In A. californica, three to four wing-like lamellipodia extendfrom the ovoid cell body, whereas in M. crenulata haemocytesare simple ovoid cells. The haemocytes are actively phagocyticand rates of phagocytosis are higher when the assays are performedin the presence of plasma, relative to tests with washed cells.When haemocytes engulf yeast, peroxidase and superoxides areproduced. Phenoloxidase activity was not detected. When bloodis removed from these molluscs, the plasma does not clot andthe haemocytes rapidly adhere to one another in suspension,or settle on substrates, migrate and form nodules. Cell spreadingand aggregation involves microfilaments and microtubules, andcan be inhibited by EDTA, cytochalasin B, caffeine and, to alesser extent, RGD and colchicine. All cells show immunoreactivityagainst a polyclonal antibody to ACTH which is consistent withprevious studies suggesting molluscan haemocytes contain moleculessimilar to those involved with vertebrate stress responses.These results should be useful in future studies evaluatingthe physiological status of these animals in the wild and inculture. (Received 15 December 2006; accepted 26 July 2007)