FishHermaphroditism

Hermaphroditism in Fish

Protogyny

• Sex change from female to male
• Displayed as normal part of life history in 14 fish families, including wrasses and parrotfishes

Protandry

• Sex change from male to female; less common than protogyny
• Observed in 8 different families of fish; coral reef examples including porgies, damselfishes, and moray eels

Sequential Hermaphroditism

• Sequential hermaphrodites function as one gender and then switch to the other, often as a result of changing population dynamics

Size Advantage Model

• In a mating system in which large males monopolize the spawning of females, larger males have higher mating success than smaller males. Size is less important for females. In such a mating system, it can be beneficial to begin life as a female when small and then change sex to a male once a large enough size is reached to maximize mating ability. A mating system in which large males monopolize the spawning of females thus favors protogyny.
• In other mating systems where no monopolization occurs, there is no size advantage for males to be large. However, in such systems where random fertilization occurs, it is beneficial for females to be larger because larger females are able to hold more eggs. Such a mating system would favor protandry, or the the changing of sex from male to female, once the male grows large enough to change sex.
• In general, individual fish should change sex when the other sex would have a higher reproductive value at that stage in life.

Common Hermaphroditism in Wrasses and Parrotfishes

• Dominant male defends group of female fish from other males; only dominant male mates with multiple females (polygny)
• When dominant male dies, largest female in harem changes sex and becomes the new dominant male (sequential hermaphroditism)

Simultaneous Hermaphroditism

• Simultaneous hermaphrodites have both female and male gender roles at the same time
• Simultaneous hermaphroditism is only advantageous when population density is very low. If it is difficult to find a mate, it is advantageous for an individual to be able to mate with any other fish it comes into contact with. Simultaneous hermaphroditism is most common in deep-sea fishes.
• Can be evolutionarily adaptive, but is only sustainable if male-type "cheating" can be avoided. "Cheating" is when individuals fertilize the eggs of multiple simultaneous hermaphrodites without expending energy to produce eggs themselves.

Anti-Cheating Strategies in Small Sea Basses (Serranidae)

• Hamlets undergo a practice called "egg trading" in which a pair of fish alternate sex roles over the period of mating. This behavior forces both individuals to expend energy into egg production so that neither individual is forced to produce all of the eggs alone.
• Other basses in the genus Serranus undergo mating just before nightfall so that simultaneous hermaphrodites functioning as males will not have enough time to fertilize multiple females without expending considerable energy themselves in egg production.