Sequential Hermaphroditism in the Animal Kingdom
A phenomenon observed in various species where an individual transitions from one sex to another during its lifespan. This is distinct from simultaneous hermaphroditism, where an organism possesses both male and female reproductive organs concurrently. The transition is typically irreversible once completed.
Types of Sequential Hermaphroditism
Protandry
The individual starts as a male and later transforms into a female. This is often driven by size or environmental cues. Larger individuals may have greater reproductive success as females, prompting the transition.
Protogyny
The individual begins as a female and later transforms into a male. This is often related to social structure, where the largest or most dominant individual becomes male.
Examples of Protandrous Species
- Clownfish (Amphiprioninae): Live in anemones in social groups. The largest individual is the female, the next largest is the male, and the others are non-reproductive. If the female dies, the male becomes female, and the next largest individual becomes male.
Examples of Protogynous Species
- Wrasse (Family Labridae): Many species are protogynous. For example, the California sheephead (Semicossyphus pulcher) has a social structure where the largest female transforms into a male if the dominant male disappears.
- Parrotfish (Family Scaridae): Similar to wrasse, various species within this family exhibit protogyny linked to social hierarchy and male dominance.
- Moray Eels (Family Muraenidae): Certain moray eel species demonstrate this phenomenon, although the triggers are less well-understood than in wrasse and parrotfish.
Triggers and Mechanisms
Social Cues
The loss of a dominant male or female in a social group can trigger hormonal changes that initiate sexual transformation.
Environmental Factors
Changes in temperature, food availability, or other environmental conditions can influence the probability of sex inversion.
Hormonal Regulation
Hormones, particularly steroids like estrogen and androgens, play a crucial role in mediating the transformation. Changes in hormone levels can induce the development of gonads appropriate for the new sex.
Genetic and Epigenetic Factors
Underlying genetic predispositions and epigenetic modifications influence the capacity for and regulation of sexual transformation.
Evolutionary Significance
Sequential hermaphroditism is thought to evolve when an individual's reproductive success varies with size or age and differs between the sexes. For example, if larger size is more beneficial for egg production than sperm production, protandry may evolve. Conversely, if larger size is more beneficial for male competition, protogyny may be favored.