Host preference of an obligate fish parasitic isopod, Mothocya parvostis.

The ability to recognize and successfully infest an appropriate host during its early life is under strong natural selection in parasitic organisms, as it directly affects the parasite's fitness. Although many cymothoid isopods that parasitize fishes exhibit high host specificity at the species level, it remains unclear whether the manca (i.e., infective early-life stage) can actively recognize and selectively infest an appropriate host species. We conducted controlled aquarium experiments in which mancae were simultaneously presented with their primary host and a non-primary host species to assess host-selection behavior in the cymothoid isopod Mothocya parvostis, a gill-dwelling species that primarily parasitizes the Japanese halfbeak Hyporhamphus sajori but has also been found at immature stages on several non-primary host fishes. When given a choice between the primary host H. sajori and the non-primary host blackhead seabream Acanthopagrus schlegelii, M. parvostis mancae infested H. sajori significantly more often. When presented with H. sajori and another non-primary host, the largescale blackfish Girella punctata, all infestations occurred exclusively on H. sajori. In contrast, when the two offered host species were non-primary, manca infestation was rare and showed no clear preference between the two fishes. Notably, predation on mancae by non-primary host fishes occurred in a substantial number of trials, whereas no predation by the primary host H. sajori was observed. These findings demonstrate strong host specificity in mancae of M. parvostis toward H. sajori, suggesting that host choice occurs at this early life stage, which likely contributes to enhancing the infestation efficiency.