Galactosomum nagasakiense n. sp. (Digenea: Heterophyidae) from the black-tailed gull, Larus crassirostris, with a description of metacercariae from some marine fishes developing trematode whirling disease.

Galactosomum nagasakiense n. sp. (Digenea: Heterophyidae), the causative parasite of trematode whirling disease of marine fish, is described, based on the specimens collected from the black-tailed gull, Larus crassirostris in Nagasaki Prefecture, Japan. The new species belongs to the cochleariformum group, and is different from the three species in the group: G. cochleare (Diesing, 1850) in the larger oral sucker and phaynx and smaller eggs, G. cochleariformum (Rudolphi, 1819) in the smaller oral sucker and pharynx and smaller eggs, and G. spinetum (Braun, 1901) in the uterus which extends anterior to the ventrogenital sac. Metacercariae collected from the brain of several marine fish developing trematode whirling disease are also described. Considering low host specificity of the second intermediate hosts, the endemic nature of the trematode whirling disease may be attributed to the infection of a particular population of the final host and/or a limited geographical distribution of the first intermediate host.