Growth of whaleworm (Anisakis simplex, Nematodes, Ascaridoidea, Anisakidae) third-stage larvae in paratenic fish hosts.

The growth of Anisakis simplex L3 larvae in the three paratenic hosts saithe ( Pollachius virens), cod (Gadus morhua) and redfish (Sebastes marinus) was studied based on material collected off the island of Vega on the west coast of Norway over a period of 1 year. In all three fish species, a positive correlation between the length of larvae and the age of the fish was shown. The number of large larvae, here defined as L3>28 mm, increases with increasing age of the host. These findings clearly indicate an accumulation of large L3 larvae and larval growth in the fish host. The larvae were additionally grouped according to the tissue--muscle, liver or other viscera--they were located in. When the average lengths of the three groups of larvae were compared, the gadoids saithe and cod had somewhat higher values for L3 larvae found in the liver. Redfish, on the other hand, which store fat to a relatively higher degree in the viscera, had the highest average length of larvae in this tissue. The lowest average values in all three species were found in the musculature. These trends suggest that the growth of A. simplex L3 larvae is positively correlated with the fat content of the tissue in which it is encapsulated. Of the three host species included in this study, saithe seemed to be the best suited for A. simplex L3, as the growth rate and average length of the parasite was greatest in this species. The growth and average length of L3 was least pronounced in redfish while cod held an intermediate position.