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 ta nk. Crabs were exposed to two types of experimental water flow to stim ulate them to switch from active to passive suspension feeding. In the first experiment, feeding crabs were exposed to a uni-directional acc elerating water current, and they switched from active to passive susp ension 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 ve locity for at least 10 min, and their feeding behavior in this steady flow was observed. This procedure was repeated, using a range of const ant-velocity flows that were successively adjusted to increased veloci ty levels. Crabs exposed to these different constant-velocity flows fe d exclusively actively at flows below 1.5 cm s-1 and exclusively passi vely at those above 4.5 cm s-1. Switches from active to passive feedin g 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 i n water velocity were measured. The mean activity rate of active feedi ng (1.05 Hz) was 3.4 times higher than that of passive feeding (0.31 H z). The porcelain crab's ability to switch feeding modes in response t o increased water velocity probably enhances energetic feeding efficie ncy in two ways. First, the passive feeding activity rate is lower tha n the active one and should reduce energetic expenditure. Additionally , the flux of suspended food increases with water flow velocity, SO pa ssive feeders are likely to catch more food per unit time than active feeders do. The ability to switch feeding modes is quite similar to th at already described for balanomorph barnacles and appears to represen t convergent evolution of flexible feeding behavior in response to var iable water flow environment.