by Peter Palma (Aquaculture and Marine Studies tutor)
Animals reproduce in order to ensure the survival of their species. Reproduction, a biological process that yields offspring, requires the participation of both male and female members of the population. In the marine environment, juvenile fish grow up to become sexually mature either as male or female, enabling them to participate in spawning events. However, a small proportion (~5%) of fish display a rather peculiar pattern of sexual development.
Some fish can change
Sex Groupers are coral reef-dwelling fish found in tropical and temperate regions. They consist of the members of subfamily Epinephelinae, family Serranidae. All juvenile groupers attain female sexual maturity capable of producing eggs during spawning season. After spending some time as functional females, they undergo a process called “sex reversal” where they change sexual characteristics into male (e.g. restructuring of ovary to testis, increased aggression). While sex reversal occurs at a certain age or body size of groupers, it is highly influenced by social conditions. For instance, unavailability of a male stimulates the largest female to sex reverse thereby maintaining optimum sex ratio within the population. This pattern of reproductive development is called protogynous hermaphroditism.
In addition to sex reversal, recent studies have shown that a number of species of groupers are also capable of maturing directly as male without passing through female phase. This type of reproductive development is called diandric protogynous hermaphroditism; “di” means two and “andry” means man or male, suggesting the two ways to become male.
The worth of sex reversal
The ability of groupers to sex reverse provides some ecological advantages. For instance, the development of groupers from juvenile to female and then to male stage means that males are the oldest and largest in the population. Thus, they are able to perform a better job at guarding their territory. Death of a male does not require females to search throughout the reef for an alternative male mate. Instead, one of the females become male through sex reversal. On the other hand, groupers that exhibit strict protogynous hermaphroditism are highly vulnerable to fishing pressure. In a population where the number of mature individuals have been exhausted because of over fishing, groupers take time to mature first as female and then reverse to male before they can reproduce successfully. Thus population recovery is slower than fish that have separate sexes, called gonochoristic.
Opportunities for Aquaculture
Because of their unique way of reproduction, groupers have been studied quite extensively as a model species for understanding the mechanism of sexual development in fish. Knowledge derived from these research activities have been translated to technologies that are now applied in the aquaculture industry.
For fish farmers who are conditioning groupers for captive breeding, early male maturation is induced by hormonal manipulation involving the administration of androgenic hormone (e.g. Methyltestosterone). Sex reversal technique is also used in the monosex farming of some fish species such as tilapia to avoid undesirable breeding during grow out stage, as well as to enhance uniformity of fish size during harvest. Moreover, this method has been extended to ornamental fish to obtain males which command a much higher market value thus increasing the business profit.
Image courtesty of Amada44 – Own work, CC BY 3.0, https://commons.wikimedia.org/w/index.php?curid=16161753