<P> There are cases where males are substantially larger than females . An example is Lamprologus callipterus, a type of cichlid fish . In this fish, the males are characterized as being up to 60 times larger than the females . The male's increased size is believed to be advantageous because males collect and defend empty snail shells in each of which a female breeds . Males must be larger and more powerful in order to collect the largest shells . The female's body size must remain small because in order for her to breed, she must lay her eggs inside the empty shells . If she grows too large, she will not fit in the shells and will be unable to breed . Another example is the dragonet, in which males are considerably larger than females and possess longer fins . </P> <P> The female's small body size is also likely beneficial to her chances of finding an unoccupied shell . Larger shells, although preferred by females, are often limited in availability . Hence, the female is limited to the growth of the size of the shell and may actually change her growth rate according to shell size availability . In other words, the male's ability to collect large shells depends on his size . The larger the male, the larger the shells he is able to collect . This then allows for females to be larger in his brooding nest which makes the difference between the sizes of the sexes less substantial . Male - male competition in this fish species also selects for large size in males . There is aggressive competition by males over territory and access to larger shells . Large males win fights and steal shells from competitors . Sexual dimorphism also occurs in hermaphroditic fish . These species are known as sequential hermaphrodites . In fish, reproductive histories often include the sex - change from female to male where there is a strong connection between growth, the sex of an individual, and the mating system it operates within . In protogynous mating systems where males dominate mating with many females, size plays a significant role in male reproductive success . Males have a propensity to be larger than females of a comparable age but it is unclear whether the size increase is due to a growth spurt at the time of the sexual transition or due to the history of faster growth in sex changing individuals . Larger males are able to stifle the growth of females and control environmental resources . </P> <P> Social organization plays a large role in the changing of sex by the fish . It is often seen that a fish will change its sex when there is a lack of dominant male within the social hierarchy . The females that change sex are often those who attain and preserve an initial size advantage early in life . In either case, females which change sex to males are larger and often prove to be a good example of dimorphism . </P> <P> In other cases with fish, males will go through noticeable changes in body size, and females will go through morphological changes that can only be seen inside of the body . For example, in sockeye salmon, males develop larger body size at maturity, including an increase in body depth, hump height, and snout length . Females experience minor changes in snout length, but the most noticeable difference is the huge increase in gonad size, which accounts for about 25% of body mass . </P>

When does the male-female difference in the level of aggression become apparent