Digestion in Aquarium Invertebrates

How shrimp, snails and corals feed and digest: the crustacean gastric mill and hepatopancreas, the snail's rasping radula, and the coral's gastrovascular cavity working alongside its symbiotic algae.

Aquarium invertebrates digest food in ways very different from fish and from each other. Shrimp grind it, snails rasp it, and corals digest prey in an internal cavity while also farming algae for energy. Most are continuous grazers rather than meal eaters, which is why their food supply and water quality matter so much.

Crustaceans: gastric mill and hepatopancreas

Crustaceans such as shrimp and crayfish first shred food with their mouthparts. The alimentary canal then runs through a gizzard-like gastric mill that mechanically grinds the food, before it reaches the digestive gland, better known as the hepatopancreas. This large organ combines the roles that mammals split between liver and pancreas: it produces digestive enzymes and absorbs the digested food, and it also stores nutrients such as lipids. It is the heart of crustacean digestion.

Snails: the radula

Molluscs feed using a radula, a chitinous ribbon covered in rows of tiny teeth that is unique to the group. A snail uses it as a rake to comb up microscopic filamentous algae from a surface, or as a rasp to scrape directly at a plant; the teeth cut and scoop food and convey it through to the gut and digestive gland. Many aquarium snails graze diatoms and other microscopic algae off rock and glass this way. Bivalves are the exception: they lack a radula and instead use waving cilia to draw in tiny food particles.

Corals and cnidarians: cavity plus symbionts

Corals, anemones and jellyfish digest in a gastrovascular cavity, a sac-like blind gut with a single opening that serves as both mouth and anus. Tentacles capture prey such as zooplankton and bring it into the cavity, where extracellular digestion takes place. This cavity is simpler than a true differentiated gut. Crucially, reef-building corals also host symbiotic algae called zooxanthellae in their tissues, which photosynthesise and can supply up to 90 percent of the host's energy needs.