Evidence for Initial Infestation by Mothocya parvostis (Isopoda: Cymothoidae) on Body Surface of Juvenile Japanese Halfbeak, Hyporhamphus sajori (Beloniformes: Hemiramphidae)

Mothocya parvostis Bruce, 1986 (Isopoda: Cymothoidae) is a parasite that mainly infests the branchial cavity of Japanese halfbeak, Hyporhamphus sajori (Temminck and Schlegel, 1846) (Beloniformes: Hemiramphidae) (pelagic marine fish). Although it has generally been considered to parasitize the branchial cavity throughout its life, early studies suggested that mancae (first free swimming stage) initially attach to the body surface of the host. In this study, H. sajori juveniles were collected off Ohkurokamishima Island, Hiroshima, Japan, and examined for cymothoid infestation. Of the 57 individuals that were examined, 5 were parasitized, all on the ventral surface of the fish bodies. One manca was genetically identified as M. parvostis based on the sequence of cytochrome c oxidase subunit I (100% or 99.81% identity with known sequences). These findings support the earlier claim that mancae infest the body surface of H. sajori. Due to the host’s slender body and small branchial cavity, early-stage parasites may remain external in the initial phase of infestation.