Variable feeding behavior in three species ofMacoma (Bivalvia: Tellinacea) as a response to water flow and sediment transport

Many species of tellinacean bivalves mainly deposit-feed but are also known to vary their feeding behavior in response to predation and to the availability of suspended organic matter relative to that of sedimentary organic matter. This study showed that three species of the Pacific genusMacoma (Macoma nasuta Conrad, 1937,Macoma secta Conrad, 1837, andMacoma inquinata Deshayes, 1855, from the San Juan Islands, Washington, USA) varied their deposit-feeding behavior in response to water flow near the sediment-water interface and to sediment transport. In the summers of 1987, 1988 and 1989, water velocity was varied in a large racetrack flume, in order to change both water velocity and bottom sediment transport. In quiet water, the inhalant siphon was protruded far from the siphon hole. As water velocity increased, with little or no sediment transport, the deposit-feeding radius decreased. Qualitative observations suggested that this was related to the drag on the siphon. InM. secta, the distance of siphon protrusion was not related to body size. Under conditions of higher near-bottom water velocity, combined with bottom sediment transport, some individuals ceased to deposit-feed, while others sustained feeding by ingesting sediment within the siphon hole, at least 1 cm beneath the sediment-water interface. These results suggest that hydrodynamic conditions are a major determinant of feeding behavior, and previous explanations of variable feeding behavior as a response to predation may have to be adjusted to accommodate this expanded set of responses.