Gill-cleaning mechanisms of Paratya curvirostris (Caridea: Atyidae) and comparisons with seven species of Japanese atyid shrimps

Abstract

Fresh-water atyid shrimps are an ancient group of carideans that uses 'passive' cleaning mechanisms to protect their gills from fouling. We studied gill cleaning in Paratya curvirostris from New Zealand, and P compressa, Caridina japonica, C. typus, C. leucosticta, C. serratirostris, Neocaridina denticulata, and Atyopsis spinipes from Japan. Gill cleaning in atyids involves epipod-setobranch complexes, associated with their pereiopods, and multidenticulate setae on the posterior end of the scaphognathite. Multidenticulate scaphognathite setae are particularly well developed in atyids and show a negative correlation with the number of setobranch setae in different species. At one extreme is A. spinipes, lacking setobranch setae but having the largest number of multidenticulate scaphognathite setae, which are relatively longer and have the maximum number of digitate scale setules. At the other extreme are P. curvirostris and P. compressa, with the largest number of setobranch setae but the smallest number of multidenticulate scaphognathite setae, which are relatively shorter and have a smaller number of digitate scale setules. In atyids, there is a compromise between these two gill-cleaning mechanisms. Mapping gill types and gill-cleaning mechanisms on a decapod cladogram suggests that phyllobranchiate gills represent a convergent derived state that evolved independently from trichobranchiate ancestors, in carideans, thalassinids, anomatans, and brachyurans. There are no particular associations between the type of gills and the mechanism used to clean them. 'Active' gill cleaning, using pereiopods, has evolved independently in several lines. If the primitive gill-cleaning mechanism is setiferous epipods, then epipod-setobranch complexes must have been independently derived in carideans and the astacid-thalassinid group. Multidenticulate scaphognathite setae occur with both setiferous epipods and epipod-setobranch complexes. The multidenticulate scaphognathite setae may be the ancestral state for the Pleocyemata, or a synapomorphy of the Caridea, and the Homarida-Astacida-Thalassinida-Brachyura group, having been lost in the Anomala which have 'active' gill cleaning.