Aspects of the life cycle and morphology of Hysterothylacium aduncum (Rudolphi, 1802) (Nematoda, Ascaridoidea, Anisakidae)

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Abstract
Gravid Hysterothylacium aduncum from the intestine of eelpout, Zoarces viviparus, were used as the source of eggs for study. The two first moults occurred in the egg, which does not normally hatch spontaneously. The third-stage larva within the egg was not infective to either fishes or non-crustacean invertebrates, but developed into a typical third-stage larva in Acartia tonsa, harpacticoid copepods, various amphipods, isopods, and mysids. Further development in the fish host was apparently dependent only on the length of the larva. Thus, larvae from harpacticoids (< 1 mm long) and larvae less than 1.5 – 2.0 mm long from other crustaceans did not survive in the fish; larvae between about 2 and 3 mm in length remained as third-stage larvae in the fish. Larvae longer than 3 mm moulted into fourth-stage larvae in the intestinal lumen of the fish. Thus, a two-host cycle occurs when fishes ingest crustaceans harbouring third-stage larvae longer than 3 mm, and a cycle of three (or more) hosts when fishes ingest third-stage larvae less than 3 mm long. Ctenophores, chaetognaths, polychaetes, and ophiuroids, which become infected by ingesting infected crustaceans, may act as obligate intermediate hosts or transport hosts.

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