Incurrent and excurrent siphons are the tubular structures that burrowing bivalve molluscs, such as clams, use to feed, respire, reproduce and expel waste into the water, while the body of the bivalve remains buried beneath the surface of the sediment, where it is hidden from predators.
Many species of aquatic bivalves have siphons that allow them to move and respire, but paired, incurrent and excurrent siphons are found only in bivalves that live a large portion of their lives under the sediment. The siphons reach to the surface for water, food and reproduction, while the bivalve stays buried. The length and appearance of the siphons vary in response to the habitat of each bivalve species, as some species burrow much deeper than others in different types of sediment.
For feeding and respiration, the incurrent siphon protrudes from the surface of the sediment to take in water, small microorganisms and nutrients, which are then filtered through the gills. The gills secrete mucus to catch food particles, allowing excess water and waste to pass through and out of the excurrent siphon. The siphons also function as reproductive mechanisms. In female bivalves, the incurrent siphon receives sperm released by males upstream. Fertilization occurs inside the female, and the larvae develop in the gills before being discharged into the water through the excurrent siphon.