Anatomy of finfish and shellfish

Unit 12 - Arthropoda

12.3.5. Respiratory System
The gills of crabs differ from those of lobsters, where each gill consists of many filaments arranged, plume-like, around a central axis. In crabs two rows of closely set, leaf-like plates or lamella are attached to the central axis of all or, in same species such as the blue crab, all but one pair of gills. In the blue-crab one anterior pair of the gills has only one row of lamella. Gill of crabs known as phyllobranchs, after the Greek words phylion, meaning leaf, and branch, meaning gill. There are eight gills on each side of a blue crab's body.
Water enters the branchial chambers of crabs primarily through an anterior opening above the base of each claw, or cheliped, and to a much less extent through openings at the base of the other thoracic legs. In the blue crab, when the chelipeds are raised and held forward, the opening at the base of the chelipeds is very large and nearly circular. When the chelipeds are folded against the body, the opening is a wide slit, which becomes narrower when the third maxillipeds are brought close to the midline, for a flange at the base of each third maxillipeds reduces the width of the slit. Bristle-like setae arising from the basal portion of the cheliped filter some of the water entering the slit.
In crabs, the opening at the base of the last pair of thoracic legs may or may not be important in the entry of water. According to Dr. Arudpragasam and Naylor, who studied pathways of gill ventilation in several species of crabs, the more flattened and shortened the body of a crab and therefore the more it diverges from the elongated body and laterally facing gills of a lobster or shrimp, the more important in the entry of water are the anterior openings at the base of the chelipeds and the less important are posterior openings.
As in shrimps and lobsters, the current of water through the branchial chambers of crabs is maintained by the beating of the gill bailer, which lies in the channel at the anterior exhalant opening of each branchial chamber. After entering through the openings at the base of the chelipeds and, to a less extent at the base of the thoracic legs, the water passes under the gills, up between the gills, over the gills, and out through the exhalant aperture. Periodically, as a result of reversal in the action of the gill bailer, the direction of the respiratory current is reversed. This aids in cleaning the gills of debris and tends to divert water back over the gills that lie in the posterior part of the branchial chambers.