Neither leg nor jaw—nor always the same: a critical revision of the eumalacostracan maxilliped

Within Malacostraca, legs are diversified variously throughout the groups. Most conspicuous is the transformation of anterior thoracopods, especially the first, into maxillipeds involved in feeding. However, the concept of a maxilliped is not precise, because it relates to a vague combination of morphological and functional deviation from a locomotory limb. Although general homology of the first thoracopod (maxilliped or not) is beyond doubt, special homology (synapomorphy) of the anteriormost maxillipeds remains uncertain. For better insights, we studied the musculature and exoskeletal structures of the first thoracopods in Anaspidacea, Euphausiacea, Lophogastrida, Mysida and Stygiomysida, using three-dimesional reconstruction of laser scanning microscopy and micro-computed tomography data. Our analysis shows high muscular and skeletal complexity of the first thoracopods. We herein reject the term ‘maxilliped’ for Anaspidacea and Euphausiacea, because their first thoracopods differ little from the posterior limbs, although specific correspondences between these taxa might represent synapomorphies. The ‘mysidacean maxilliped’ is morphologically well derived from the posterior thoracopods and appears synapomorphic for the mysidacean subtaxa. A comparison with other Peracarida additionally shows correspondences indicating a homologous ‘peracaridan maxilliped’. In contrast, we consider the peracaridan maxilliped not to be homologous to the decapodan maxilliped. As a distinction, we propose the term ‘unguiped’ for the peracaridan first thoracopod.