Behavioural Changes of Anisakis Simplex (s.s) Third-Stage Larvae Induced by Biotic and Abiotic Factors in the Fish and Mammalian Hosts: in Vitro Studies

The marine parasitic nematode Anisakis simplex has a complex life cycle including marine mammals (mainly cetaceans) as definitive hosts, invertebrates (e.g., copepods and euphausiids) as the first paratenic hosts, and a wide range of fish species acting as second paratenic hosts. During the life cycle, the larva encounters a wide range of biotic (host immune factors and microelectric activity) and abiotic conditions (temperature and pH). We hypothesized that these factors may explain the differential behaviour of the nematode, recognized during the different life cycle stages. In this study, third-stage larvae (L3) of A. simplex were isolated from freshly caught Atlantic herring (Clupea harengus) from FAO zone 27. We exposed nematodes to different pH values (pH 2 to 9) at different temperature levels (4, 14, 21 and 37 °C), electric currents (6 mA, 12 mA, 18 mA) and different concentrations of fish immune cells. The nematode larvae exhibited significantly differential behaviour (stretched non-aggregated, spiral non-aggregated and aggregated) and activity levels when exposed to the different physicochemical conditions. We recorded negative correlations between activity and pH (maximum at pH 2) and positive correlations between activity and temperature (maximum at 37 °C). The nematode larvae were affected when exposed to electricity and fish immune cells. Electric currents at 6 mA induced minor changes, but at 12 mA and 18 mA, the majority or all nematode larvae aggregated and rolled up into spirals. Exposure to leukocytes, isolated from rainbow trout head kidney and spleen, induced a similar concentration-dependent spiralling process in larvae. We discuss these behavioural patterns of A. simplex as adaptations to conditions encountered by the worm larvae during the different stages of their complex life cycle.