Développement Post-Embryonnaire Du Copépode Ascidicole Notodelphyidae Pachypygus Gibber (Thorell, 1859)

Abstract

The larval development of P. gibber includes 6 nauplius stages and 6 copepodid stages, the last stage being the adult. The copepodid 2 is the infesting stage. Sexual dimorphism of the copepodids appears at the fifth stage. External morphology of each stage (from the first nauplius to the adult copepodid) is described. Comparison of the development of P. gibber with that of other Notodelphyidae is presented. The importance of characteristic moults is pointed out. This is the first time that a sixth nauplius is described for a notodelphyid. In its first naupliar instar, P. gibber cannot be distinguished from the other 5 genera and 8 species of Notodelphyidae of which the post-embryonal development is known; for naupliar stages II to VI, however, P. gibber is characterized by the absence of the aesthete at the basis of the large apical seta on the terminal segment of the anterior antenna. Furcal hooks are present from copepodid 3 onward. Sexual dimorphism is apparent from copepodid stage 5 onward. The species is furthermore characterized by the disappearance of the setae along the internal and apical margins of the distal exopodal segment of P2 and P3 in copepodid stage 4. These setae reappear in the adults, normal in the males but somewhat coalescent in the females. The rudimentary P6, persistent in the adult female, has not been described earlier for P. gibber. It is the only notodelphyid for which this phenomenon is now known. However, since so far P6 in this species has escaped attention, obviously because of its minute size, one may question whether the same might not also be the case in other species.