Is the Japanese skeleton shrimp Caprella mutica a filter feeder? I. Head morphology and kinematics

An active filter feeding organism strains suspended matter from the surrounding water using a sieving structure. Small marine filter feeders however, have to cope with the physical problems of the high viscosity as indicated by the low Reynolds number. In a highly viscous regime, organisms' appendages are surrounded by a thick boundary layer, which makes movement and filter feeding difficult. In this study, we test whether Caprella mutica, a small marine amphipod frequently described as a filter feeder, is indeed capable of filter feeding by using its antennae as sieves. This small crustacean feeds by swaying its body through the water. The forward movement of the head drags along its two pairs of antennae of which the second pair of antennae possesses two rows of long setae. Juvenile C. mutica, being up to five times smaller then the adults, need to move their appendages faster to reach a similar critical Reynolds number to make the antennae act like sieves. In addition, in adult C. mutica the first two pereonites are elongated, which may influence their filtering movements. Detailed head morphology was studied under a light microscope, while the filtering movements of both juvenile and adult C. mutica were recorded using a digital high-speed video camera. The resulting kinematics of antennae, head and body segments showed that the antennae of both juvenile and adult C. mutica indeed move through the water at a Reynolds number around three. Alternative methods of gathering food in caprellid amphipods will be discussed.