Gill Structure and Gill-Cleaning Mechanisms of the Redclaw Crayfish Cherax Quadricarinatus (Decapoda, Astacidea, Parastacidae)

Abstract

The redclaw crayfish Cherax quadricarinatus is a freshwater species native to northern Australia, but it has been widely translocated overseas for aquaculture. Its gill complement consists of 21 trichobranchiate gills and a rudimentary epipod bearing tubular gill filaments. Gill size decreases from the outer layer of the podobranchs to the inner layers of the arthrobranchs and pleurobranchs. Most gill filaments have a thick epithelium and rounded apex, while a few are thinner and have pointed or hooked tips. The gross structure of the fused epipods and podobranchs from the anterior to posterior limbs suggests a serial pattern of morphological development. The redclaw crayfish has passive gill-cleaning mechanisms using the multidenticulate setae on epipods, setobranchs, scaphognathites, and the inner surface of branchiostegites. Limb motion causes the epipodal and setobranch setae to be jostled, scraping gill surfaces, and agitating the podobranchs whose outer filaments are picked and brushed by the branchiostegal setae. Multidenticulate scaphognathite setae have a minor role in gill cleaning due to a reduced number and limited reach to the gills. The compound epipodal setae contact the adjacent limb coxae and epipodal bases. Small lamellar protrusions from the arthrodial joint of the pereiopods bear setae of various types, which may serve as a filter screen for the respiratory water entering the branchial chamber. The same filtering function is ascribed to the dense pappose setae fringing the limb bases. The redclaw crayfish displays “limb rocking” by frequently moving its limbs when at rest, thereby jostling the setiferous processes and the gills inside the branchial chamber. This behavior is perceived to result in passive gill cleaning. The adaptive value and phylogenetic implications of the gill-cleaning mechanisms are discussed.