Carol J. James Measurements and Sex Determination Adults Males Females Juveniles - 10 to approx. 15 mm This determination is variable. We have found that males acquire their first distinguishing external characteristics (elongated anal fin, black markings on dorsal and caudal fins, and over-all darker body color) from approximately 14-16 mm in total length. Since this varies with individuals, the category "Juveniles" is somewhat indefinite. In addition, it must be remembered that an individual will be designated a "female" usually only after this interval (14-16 mm) has been passed. This of course results in error in proportion of adult males to females because of the fact that males may be recognized when smaller in size. Young - 9 mm and smaller This "smaller" measurement decreases to a minimum of 5 mm. This size may be considered "new born" young.