Giant clams belong to a class of mollusks called pelecypods or bivalves, which contains such members as mussels, scallops and oysters. All bivalves have two shells joined together by tough tissue. Giant clams, such as Tridacna crocea and Trachna gigas, play host to symbiotic algae. The relationship is mutually beneficial. For example, when T. crocea offers hospitality to algae of the genus Symbiodinium, the algae manufacture food by the process of photosynthesis and share the resultant carbohydrates with their host. Though the clam can receive nourishment via filter feeding, algae provide most of its nutrients.
Other clams enjoy a similar relationship with algae, such as freshwater clams of the genus Anodonta. Algae of the genus Chlorella live in the gills and mantle of Anodonta and provide food for their host. Algae also grow and proliferate in the mantle of another bivalve mollusk called Clinocardium nuttallii, the Pacific common cockle. (The mantle of a bivalve is the protective tissue that covers the vital organs.)
Snails belong to a class of mollusks called gastropods. They are also called univalves because they have only a single shell, in contrast to the two-shelled bivalves. Algae live in internal organs of such snails as Strombus tricornis. In spite of the fact that the snail shell and other tissue cover them, these algae receive some sunlight.
Limpets are snails that look like a small Native American tepee. Some limpets tend gardens of red algae in marine intertidal zones. For example, Scutellastra cochlear, the pear limpet, grazes on a small patch of red algae off the coast of South Africa and fertilizes it with its bodily wastes. Microscopic algae also interact with limpets by growing on the outside of their shells. The limpets neither profit nor suffer harm from their presence. Other mollusks, such as chitons, also tend to accumulate algae on their shells.
Slugs are also gastropod mollusks, but they have no shell. Some marine slugs harbor algae in places where light can easily reach them. Others kill the algae and appropriate their chloroplasts, which continue to produce food by photosynthesis inside the slug.