The giant keyhole limpet, Megathura crenulata, lives in rocky intertidal and subtidal environments along the Southern California coast, where it is exposed to viruses, bacteria, and other potential pathogens. We demonstrate that when exposed to bacteria or latex beads, hemocytes from specimens of M. crenulata in vivo and in vitro immediately become adhesive and form nodules. The rapid activation of hemocytes suggests a role for an array of recognition proteins, and inhibition of nodulation by the tripeptide Arg-Gly-Asp indicates that integrins are involved. The morphological changes involved with nodule formation include the rapid extension of lamellipodia, phagocytosis of particles, and compaction of the hemocyte aggregates. The number of nonadherent hemocytes rapidly decreases as aggregates form. The elimination of bacteria is due to a dynamic hemocyte response, rather than antibacterial factors in the plasma. These findings are compared to work on other gastropods and expand the current knowledge on the immune response of molluscs, such as M. crenulata, which is increasing in importance as they continue to be raised in aquaculture for pharmacological use.

Mucus‐secreting epithelia protect marine gastropods from abrasive particles and microbes in seawater. We studied the morphology and function of the epithelium in the giant keyhole limpet Megathura crenulata. All exposed surfaces of the limpets were covered by a mucus‐secreting, simple columnar epithelium, in which most cells on the sole of the foot bore cilia, while the majority of cells on the side of the foot and three mantle regions bore microvilli. None of the coatings had antibacterial properties. The epithelium of the foot was distinct from that of other regions, in that the mucus secreted was used for locomotion and was left behind as the limpet moved. The glycocalyx bound to the microvilli of the mantle cells appeared to be clean in visual and SEM examinations, and attempts to enhance the binding of inert particles and bacteria were unsuccessful. Because studies have shown that standard tissue processing may cause artifactual shrinkage of mucous layers, cryostat sections and additives to standard fixatives were tested, but we found no change in the thickness of the mucous layers of the limpets compared to routinely processed tissues. Sloughing of the glycocalyx in vertebrate systems removes bound microbes, and alterations of the glycocalyx layers are associated with disease conditions. Sloughing of the glycocalyx on limpets was rarely observed. In one case, the outer mantle of the limpet was covered with silt and the glycocalyx appeared to be detaching. This process could be experimentally induced by dousing the outer mantle with talc particles. The types of secretory cells producing the mucus in each region of the skin were characterized using standard histological stains and lectin staining. Because analysis was hampered by their small size, classification of the types of secretory cells was based on TEM descriptions of granule morphology, which allowed for comparison to the secretory cells described from skin of the abalone. The possible roles of the numerous secretions, and the mechanisms of mucus production and loss, are compared to what is known in vertebrate systems and to our relative lack of knowledge regarding invertebrate systems.