Some littoral bivalves, such as Tridacna, as well as some sea slugs, such as Aeolidia, share an obligatory symbiosis with zooxanthellae (a group of algae). Another metabolic association exists between certain bacteria and several bivalves and gastropods of deep-sea hot vents or other sulfidesystems. There are several parasitic mollusks.

Locomotion

Mollusks have a wide range of locomotory patterns. Solenogasters and various smaller gastropods glide upon cilia that beat rapidly against a pathway of mucus secretions. This pattern of movement is supported or replaced in larger mollusks by the propulsive waves that run along the surface of the foot and are controlled by the actions of the dorsoventral musculature (Figure 1). Burrowing occurs as an interaction between musculature and the hydrostatic skeleton (see below Internal features); it is performed in caudofoveates and several sea slugs by the whole anterior body but is restricted to the foot in scaphopods, bivalves, and some specialized gastropods.

Various bivalves (e.g., cockles) and snails may perform rapid twists or jumps through violent flexion of the foot. Buoyancy floating and jet propulsion are found in cephalopods; floating is also known in gastropods, and swimming of a different kind is practiced by some opisthobranch and prosobranch gastropods as well as in scallops and related bivalves. Octopods use their arms to crawl or even to swim or float with the help of the body skin interconnecting the arms (interbrachiate web). Some bivalve groups bore into hard surfaces by secreting strong chemicals that dissolve the substrate or by drilling, using the shell and radula. A sedentary (sessile) way of life has been adopted by many bivalves and some gastropods.