Cymothoidae are mainly described based on the morphology of adult females, making species identification difficult because of the limited morphological information on male, juvenile, and manca life stages. In particular, most species in the genus Anilocra Leach, 1818 lack morphological information beyond that of females, highlighting the need to accumulate such data. In the present study, we described the aegathoid stage, male, and transitional morphology of Anilocra prionuri Williams and Bunkley-Williams, 1986 collected from Japanese waters and documented changes in morphology with growth for the first time. With growth, the morphology of A. prionuri changed: 1) body shape becomes rounder, 2) eye length becomes shorter, 3) shortest pereonite changes from 7 to 2, 4) antennular article 3 enlarged, 5) length of dactylus relative to propodus in pereopod 1 becomes longer, 6) pleopod peduncle becomes shorter, 7) decrease robust setae of pereopods, coupling hooks and plumose setae of pleopods, swimming setae of pleotelson, pleopod, and uropod, 8) appendix masculina on the pleopod 2 endopod shortens and disappears, 9) rami of pleopods and uropods become rounded, and 10) endopods of uropods become elongated. The aegathoid stage of A. prionuri was distinguished from that of Anilocra clupei Williams and Bunkley-Williams, 1986, also distributed in Japan by 1) 3–5 and 0–5 plumose setae of pleopods 1 and 2, (5 and 8 plumose setae in A. clupei), 2) uropodal exopod longer than endopod (same length in A. clupei), and 3) triangular uropodal exopod (oval in A. clupei).
Aegathoid stage, Fish parasite, Male, Manca, Transitional
