Morphological diversity of setae on the grooming legs in Anomala (Decapoda: Reptantia) revealed by scanning electron microscopy

The fifth pereiopods (P5) in Anomala are specialized appendages used mainly for grooming. We studied the articulated cuticular outgrowths, setae, on the distal segments of the P5 in 40 species from 18 Anomala families using light and scanning electron microscopy. Five general classes of setae can be found on the P5: serrate, serrulate and simple setae which all appear bristle-like, and tooth-like and scale-like cuspidate setae. We classified the bristle-like setae according to criteria of shape and the arrangement of distinct outgrowths – denticles and setules – on the shaft of the seta. In this way we were able to distinguish eleven mainly serrate and serrulate types of seta. Some setal types imply homology due to their distinctness and could thus help to solve problematic phylogenetic questions. One setal type, for example, is only present in pagurid hermit crabs and king crabs, which corroborates the theory that these two morphologically very dissimilar groups are, in fact, closely related.