Male and female reproductive systems of the pseudocycnid copepod Cybicola armatus (Bassett-Smith, 1898) (Siphonostomatoida) infecting the commercially important scombrid fish Scomberomorus commerson: a light and electron microscopic study

Abstract

Cybicola armatus (Bassett-Smith, 1898), a host-specific parasitic copepod, massively infects gill filaments of the scombrid fish, Scomberomorus commerson. While the female attaches to the host fish’s gills, the relatively small-sized male prefers to attach to the female genital somite in copulatory position. To our knowledge this is the first report on the structural features of male and female reproductive organs of C. armatus, a pseudocycnid parasite, using morphological, histological, histochemical and ultrastructural parameters. In males, the sac-like testis is followed by a highly coiled vas deferens ending with a bulbous spermatophore sac in which the round-shaped spermatozoa are abundantly seen. Histologically, germ cells in the testis are at different maturation stages and region-specific. In females, the ovaries are comprised of germarium, previtellogenic oocytes and follicle cells. However, interestingly, mature vitellogenic oocytes enveloped by follicular cells are seen only in the oviducts, which are ensheathed by longitudinal and circular muscle bands. The spermathecae of the post-copulatory females contain filiform spermatozoa, while in pre-copulatory specimens, the spermathecae appear as thin, flabby structures. The post-copulatory female carries spermatophore sacs at her genital orfice. The developing eggs in the egg sacs did not display synchrony with respect to stages of embryogenesis, as borne out from histological studies. The currently reported study also demonstrates, for the first time, the copulatory events of C. armatus that help us understand the mode of sperm transfer in this fish parasite. The total of information derived from the currently reported study is pertinent not only from the perspective of understanding the reproductive mechanism and adaptive strategies of a potential, but rather unexplored pseudocycnid copepod. Rather, it throws light as well on the necessity and, until now, inadequacy of investigations on parasitic crustaceans that inflict heavy damage to fisheries and contaminate the aquatic ecosystem.