Flow Velocity Induces a Switch From Active to Passive Suspension Feeding in the Porcelain Crab Petrolisthes leptocheles (Heller).

A flow-induced switch in suspension-feeding behavior of the porcelain crab Petrolisthes leptocheles was investigated in a laboratory flow tank. Crabs were exposed to two types of experimental water flow to stimulate them to switch from active to passive suspension feeding. In the first experiment, feeding crabs were exposed to a unidirectional accelerating water current, and they switched from active to passive suspension feeding at a mean water velocity of 3.49 cm s-1. In the second experiment, crabs were exposed to flow that was fixed at a constant velocity for at least 10 min, and their feeding behavior in this steady flow was observed. This procedure was repeated, using a range of constant-velocity flows that were successively adjusted to increased velocity levels. Crabs exposed to these different constant-velocity flows fed exclusively actively at flows below 1.5 cm s-1 and exclusively passively at those above 4.5 cm s-1. Switches from active to passive feeding occurred throughout the range of constant-velocity flows from 1.5 to 4.5 cm s-1. Changes in feeding activity rate induced by an increase in water velocity were measured. The mean activity rate of active feeding (1.05 Hz) was 3.4 times higher than that of passive feeding (0.31 Hz). The porcelain crab's ability to switch feeding modes in response to increased water velocity probably enhances energetic feeding efficiency in two ways. First, the passive feeding activity rate is lower than the active one and should reduce energetic expenditure. Additionally, the flux of suspended food increases with water flow velocity, so passive feeders are likely to catch more food per unit time than active feeders do. The ability to switch feeding modes is quite similar to that already described for balanomorph barnacles and appears to represent convergent evolution of flexible feeding behavior in response to variable water flow environment.