We tested the reactions of one species of parasitic copepod and four species of larval monogeneans to magnetic fields, and of two species of parasitic copepods and four larval monogeneans to gravity and light. All six species reacted positively to light, however, geotaxis of the two copepod species exceeded phototaxis. The larvae of Hatschekia sp. (Copepoda), Polylabroides australis and Choricotyle australiensis (Monogenea, Polyopisthocotylea) showed a significant preference for the magnetic North. Zeuxapta seriolae (Monogenea, Polyopisthocotylea) showed a magnetic South preference under enhanced magnetic intensity. Allomurraytrema spari, the only monopisthocotylean monogenean examined, did not react to magnetic fields. This suggests that the posterior appendage, which is found only in polyopisthocotylean monogeneans, may be responsible for magneto-orientation. Preference of Hatschekia, P. australis and C. australiensis for the magnetic North indicates that the magnetic field induces swimming upward. Preference for the South in Zeuxapta seriolae suggests downward swimming, thus increasing encounters with its deep swimming fish hosts. This is the first study in which magnetotaxis has been demonstrated in monogeneans and parasitic copepods.
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