General Features of the External Morphology of Marine Planktonic Copepods
Gynandromorphism and intersexuality
Cases of gynandromorphism are not exceptional in copepods. They most often concern the segmentation of the abdomen, the structure of the fifth pair of legs and the antennule.
Conover (1965) describes a male displaying a reddish-orange mass in the cephalosome, typical of that in gravid females. On the contrary, Dussart (1967) reports in Eudiaptomus gracilis (a freshwater calanoid) a case of hypermasculinisation in a male of which both antennules were geniculate. In this species, some individuals may display a female internal morphology but a masculinised phenotype presenting a geniculate right antenna and a modified fifth pair of legs, with spermatophores attached to the genital aperture.
In Acartia tonsa, genetic males have been noted to become phenotypic females during growth (Heinle, 1970).
Fleminger (1985) observes on the antennules of female Calanidae of the same species a different number and arrangement of sensory setae (aesthetes). The females that display the male phenotype have a gonad that develops into an ovary and are fertilised like the others; they would be derived from genotypic males. The dimorphism as well as the possible sex change are the subject of various hypotheses put forward by this author that cannot be summarised here. In Euchaeta (Calanoida), Hopkins (1982) observes the opposite in juvenile stages (genotypic female -> phenotypic male).
Copepods parasitised by Blastodiniales, in addition to the parasitic castration that is the rule, often show more or less pronounced cases of gynandromorphism in which female characters dominate. According to Sewell (1951), in the castrated female there would be production of a male morphogenetic hormone capable of affecting development in several directions and to varying degrees depending on the stage at which infestation occurs. At the same time there could be a reduction or absence of a female hormone, so that normal development leading to secondary sexual characters cannot continue, the individual retaining certain juvenile aspects. In the genus Corycaeus (Poecilostomatoida), the case of the genus or subgenus Farranula (formerly Corycella) is demonstrative in this regard, since it could be considered as derived from the former by neoteny; the various species of Farranula would be only variants modified to varying degrees of the single species F. rostrata.