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.

No abstract

Free-swimming ensheathed larvae of Anisakis simplex were shown experimentally to be ingested by the copepods Oitona similis and Acartia tonsa and by the nauplii of barnacles Balanus sp. The larvae did not grow in the copepod hemocoel. Experimental infections of various malacostracans were unsuccessful. Sticklebacks Gasterosteus aculeatus are naturally infected with larvae of A. simplex in coastal brackish water. Such sticklebacks may have acquired the infection by eating either a crustacean host or third-stage larvae (L3) from fish. Experimental infections of cod Gadus morhua with L3 from viscera of herring Clupea harengus showed that about one-third of the ingested larvae passed through the cod alimentary tract and were extruded whole but dead. Experiments showed that larvae from herring viscera survived and remained infective after at least 6 weeks in brackish water and seawater and that L3 from herring viscera were ingested by sticklebacks (and flounder Platichthys flesus), where they reencapsulated on the viscera; the L3 were alive 2 years later.

Hatched, ensheathed third-stage larvae of Contracaecum osculatum, 300-320 microns long, were shown to be infective to copepods, to nauplius larvae of Balanus and to small specimens of fishes (sticklebacks, O-group eelpout). Other fishes such as gobies and small flatfishes became infected by ingesting infected crustaceans. Cod were infected by being given infected small fishes. In the crustacean paratenic hosts, little growth of the larvae occurred, if any. In the liver sinusoids of sticklebacks and gobies the length of most of the unencapsulated third-stage larvae had not even doubled within 6 months of infection. The fate of larvae (max. 2 mm long) given to cod via infected intermediate fish hosts was apparently decided by the size of the larvae only. Small larvae became encapsulated and eventually died in the liver and wall of the gastrointestinal tract. Larger larvae migrated to the liver parenchyma, where some grew to a length of as much as 10 mm. The growth of the larvae in sticklebacks was shown not to be affected by an increase in temperature (infected fish being transferred from 8 degrees to 14 degrees and 20 degrees C), by the intensity of infection and, partly, by the age of infection (e.g. some 2-week-old and 6-month-old larvae were of identical size). In the liver and mesentery of plaice the third-stage larvae developed via copepod paratenic hosts to infectivity (i.e. to more than 4 mm in length), showing that the life cycle may be completed with an optional paratenic invertebrate host and only one intermediate fish host.(ABSTRACT TRUNCATED AT 250 WORDS)

Two moults occur during larval development in the eggs of Anisakis simplex (Rudolphi, 1809) and Pseudoterranova decipiens (Krabbe, 1878) from the North Atlantic. Live larvae forced out of eggs in sea water by coverslip pressure shortly before spontaneous hatching were surrounded by the thin cuticle of the first-stage larva. Infective larvae from naturally hatched eggs are loosely ensheathed in the thick cuticle of the second-stage larva. Thus, it is the third-stage larva that emerges from the egg of both species and not the second-stage larva as previously believed. The thin, smooth, fragile cuticle of the first-stage larva remains in the egg. The striated, cocoon-like cuticle of the second-stage larva of A. simplex may increase the buoyancy of the third-stage larva. The tail tip of the cuticle of the second-stage larva of P. decipiens is sticky and adheres the sheathed third-stage larva to the substrate.