Mode of Infection with Larval Anisakid Nematodes in the Japanese Flying Squid, Todarodes Pacificus (Cephalopoda: Ommastrephidae)

The Japanese flying squid, Todarodes pacificus, is generally infected by third-stage larvae of two species of anisakid nematodes. Anisakis sp. is typically observed encapsulated in the outer wall of the stomach and caecum of infected squids, whereas Lappetascaris sp. is found at the anterior end of the mantle musculature. To investigate the infection process, third-stage larvae of these two anisakid species were labelled by injecting a mixture of 5% fluorescein isothiocyanate-dextran and 5% biotin-dextran into their caudal end. The labelled larvae were placed inside the muscle tissue of dead prey fish which were then fed to freely swimming squids held within a large aquarium. Several days later, the squids were removed, killed and dissected to observe the characteristics of infection by fluorescing anisakids. Anisakis sp. larvae were found infecting the outer walls of the stomach and caecum, but no Lappetascaris infections were observed. Squids held inside custom-made plastic packs containing free anisakid larvae were not infected with Anisakis sp. but Lappetascaris sp. larvae were found attached to the gills. We conclude that Anisakis sp. penetrates the squid digestive tract after squids consume Anisakis-infested prey; while Lappetascaris sp. infects squids by direct entry through their gills following inhalation of ambient seawater.