From visual observations of the external features, the difference in sex can be recognized. Males usually have a slender shape. The milt can be observed at the genital opening if slight pressure is applied on the belly of the mature male. The female can be recognized from the big soft round belly with the red-pink papilla extruding out at the urogenital operture. If the female has a fully ripe egg, the egg will be visible when the belly is pressed by hand.
