Life Cycle: Some species of Galactosomum sp. use sea snails as first intermediate hosts (Beuret et al., 2000; Lloyd and Poulin, 2011). Fishes are used as intermediate hosts where the parasites encyst in the brain and can cause whirling disease. The definitive hosts are species of birds who eat the infected fish (Database in Fish and Shellfish).
Elongate forms in which the forebody is sometimes broadly spatulate and the hindbody more or less cylindrical. Cuticle beset with very small scale-like spines, which extend posteriorly to middle of body or beyond. Oral sucker simple, subterminal; prepharynx relatively long; pharynx well developed; oesophagus short or absent. Excretory vesicle tubular, straight or somewhat sigmoid. Genital sinus spacious, complex, with a strongly muscular spiny pad attached to its right dorsal wall, and a gonotyl or tongue-like organ to its left wall. Seminal vesicle well developed, usually constricted into two portions. Testes rounded or somewhat oval, situated directly or obliquely one behind the other in posterior half of body. Ovary rounded or oval, median or lying to right of median line, anteriorly to testes. Receptaculum seminis large. Vitelline follicles lateral or intercaecal, confluent behind testes, extending anteriorly to foremost testis or beyond. Uterus descending and ascending between testes.
Sources: Beuret, J., Scott, D.A., and Pearson, J.C. 2000. The cercaria of Galactosomum bearupi Pearson, 1973. (Trematoda: Heterophyidae) at Heron and Masthead Islands, Great Barrier Reef, Australia. Systematic Parasitol. Vol. 46, No. 1, p.69-77.
Lloyd, M.M. and Poulin, R. 2011. In vitro culture of marine trematodes from their snail first intermediate host. Experimental Parasitology, Vol. 129, p. 101-106.
Database in Fish and Shellfish. http://fishparasite.fs.a.u-tokyo.ac.jp/Galactosomum%20sp/Galactosomum-eng.html