Copepodid development of Tegastes falcatus (Norman, 1868) (Copepoda, Harpacticoida, Tegastidae) with a discussion of the male genital somite

Abstract

The lateral face of the cephalosome deepens progressively during the six stages of copepodid development of Tegastes falcatus. A simple aesthetasc on the second from the proximal segment of the antennule is transformed into a bifurcate aesthetasc during the molt to copepodid II. Copepodids II-VI bear eight setae on the caudal ramus, a number unique to copepods. Formation of the arthrodial membrane separating the middle segment and distal complex of the endopod of swimming legs 2-4 is delayed until CVI, one stage later than the exopod. Gender dimorphism initially is observed in antennular segmentation, setation of the maxilliped and rami present on leg 5 at copepodid IV. At copepodid V, males and females differ in antennular segmentation, rami present on leg 5 and the shape of leg 6. Gender dimorphism at copepodid VI also includes the shape of the cephalosome, of the genital triple-somite complex, and of one seta on the caudal ramus. The morphology of the ventral attenuation of the sixth and seventh thoracic somites, the anterior abdominal somite, and the degree of rotation of leg 6 are compared for males of Tegastes falcatus, T gemmeus, T ctenidus, Parategastes conexus and Syngastes sp. Tegastes is composed of species whose females and males have a genital triple-somite (sixth and seventh thoracic somites plus the anterior abdominal somite not separated by arthrodial membranes). Tegastidae are diagnosed as species with the rami of leg 1 unsegmented and basis elongate; male genital somite (seventh thoracic) extended ventrally; male leg 6 present only on one side; protopod of male leg 6 unarmed; female embryo sac with 3-4 embryos; female leg 5 with broad baseoendopod forming embryo sac chamber. Species of the lineage Tegastidae plus Peltidiidae have males with an asymmetrical leg 6 in addition to the rectangular distal basal endite of the maxillule noted by Selfried (2003). A stout ventral spine with hyallne membrane on the distal segment complex of the exopod of swimming leg 4 also may be a synapomorphy for this lineage.