The life cycle ofContracaecum osculatum (Rudolphi, 1802) sensu stricto (Nematoda, Ascaridoidea, Anisakidae) in view of experimental infections

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Abstract
Hatched, ensheathed third-stage larvae ofContracaecum osculatum, 300–320 μm long, were shown to be infective to copepods, to nauplius larvae of {jtBalanus} and to small specimens of fishes (sticklebacks, 0-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° to 14° and 20°C), by the intensity of infection and, partly, by the age of infection (e.g some 2-week-old and 6-month-old larvea 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. In combination with earlier results showing that the ensheathed third-stage larva (not the second stage) emerges from the egg and with literature data on the occurrence of larvae in fishes and the presence of fourth-stage larvae and adults predominantly in the stomach of grey seals, the life cycle ofC. osculatum is shown experimentally for the first time.

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