There are fewer dorsal procurrent caudal rays than ventral ones in males and females of Cheirodon pisciculus, C. galusdae, C. australe and Cheirodon spp. (from Argentina and Chile). The shape of ventral procurrent caudal rays of Cheirodon is modified throughout ontogeny, and becomes a dimorphic sexual attribute separating adult females and males. The most anterior ventral procurrent caudal rays are formed by an elongate middle element bearing two lateral expanded bony processes in males, and they form a continuous series of bones, each one adjacent to the following one.