Gill-cleaning mechanisms of a dendrobranchiate shrimp, Rimapenaeus similis (Decapoda, penaeidae): description and experimental testing of function.

Observations on functional morphology and results from experiments demonstrate that setiferous epipods compose the major gill-cleaning mechanism in a penaeoid shrimp, Rimapenaeus similis. Epipods on the second maxillipeds and on pereopods 1-3 are equipped with long setae bearing an array of digitate scale setules. These multidenticulate setae reach to most gills and are jostled among them during limb movements. Experiments were performed in which epipods were removed from the gill chamber on one side (experimental) but not the other (control); treated animals were exposed to fouling in a recirculating water system for 2 weeks. Particulate fouling, measured by reduction in relative gill transparency, was significantly greater on experimental than control gills. The pereopodal exopods, not previously implicated in gill cleaning in any decapod, were similarly identified as important gill-cleaning structures. Equipped with long multidenticulate setae like those on the epipods, exopods sweep back and forth over the gill filaments just under the gill cover, areas not reached by the epipods. Exopod-ablation experiments were conducted that showed that exopods prevent particulate fouling on gill surfaces over which they sweep. The similarity in action of the passive gill-cleaning system of R. similis to that of crayfish (Bauer [1998] Invert Biol 117:29-143) suggests the hypothesis that the epipodal and exopodal cleaning setae of R. similis are ineffective against epibionts. The reduction in epipodal and exopodal cleaning systems that occurs in the Penaeoidea is hypothesized to be compensated for by increased development of gill-cleaning setae on the branchiostegite, scaphognathite, or other structures.