A review of primary freshwater crab mandible morphology: terminology and systematic importance (Crustacea: Decapoda: Brachyura)

Abstract

Abstract The primary freshwater crab mandibular palp is described as possessing two or three divisions (joints) and its terminal article as being either unbranched (simple) or bilobed. Currently, this provides two important familial characters. Addressed here are the long-standing differences in the number of mandibular palp articles between some freshwater crabs and other brachyurans by adopting a standardized terminology based on a generalized brachyuran limb using homology-based terms that allow comparisons with all brachyurans. As a result, the freshwater crab mandible is redescribed as a four-part appendage comprising the coxa (biting jaw) and basis, from which originates a two-articled inner ramus (endopod). A detailed examination of the joint between the basis and the first endopod article of the palp indicates that its expression varies from distinct to obscure, but it is nevertheless present in all freshwater and marine crab families examined here, including those species currently described as possessing a two-articled palp. The coxa, basis, and first endopod articles of the mandibular palp are all uniform and generally not diagnostic within the Brachyura. The terminal endopod article of freshwater crabs, however, is morphologically diverse and provides a familial diagnostic character. The wider implications of these findings for the Brachyura are discussed.