Interpreting segment homologies of the maxilliped of cyclopoid copepods by comparing stage-specific changes during development

AbstractDevelopment of the maxilliped of 14 species of cyclopoid copepods from 14 genera in the families Cyclopinidae, Oithonidae and Cyclopidae is described. Segment homologies are inferred from the assumption that homologous setae and arthrodial membranes are added during the same copepodid stage, and from a model of development that patterns the endopod proximally from the proximal of two endopodal segments present at the first copepodid stage. An arthrodial membrane separates the praecoxa and coxa of two Cyclopinidae and two of three Oithonidae. The praecoxa of the Cyclopinidae and the Oithonidae has two groups of setae; the praecoxa of Cyclopidae has no more than one group. The coxa of these copepods has only one group of setae; all Cyclopidae share a coxal lobe with a single seta. The endopod of these three families may include as many as five segments. In general, the distal arthrodial membrane of a segment appears to have been more labile during evolutionary history of the maxilliped than has the ventral seta which inserts on that segment. For purposes of phylogenetic analyses, uncoupling the presence of the distal arthrodial membrane of a segment from the presence of its ventral seta and analyzing each separately may provide a better way of understanding evolutionary transformations of the limb than considering the segment as the basic structural unit of the limb