Sequential hermaphrodites are animals that change their sex during their lifetime. This natural reproductive strategy is primarily observed in marine species, unlike simultaneous hermaphroditism, where an organism possesses both male and female reproductive organs concurrently. Sequential hermaphroditism involves an individual starting life as one sex and then undergoing a complete transformation to the opposite sex. This shift is genetically programmed and often triggered by social or environmental cues, allowing these animals to maximize their reproductive success within specific ecological niches.
Why Some Animals Change Sex
The driving force behind sex change in the animal kingdom is often explained by the Size Advantage Model. This model suggests that an individual should change sex at the point in its life history when its reproductive success would be greater as the opposite sex. For many species, reproductive output is closely tied to body size, meaning the shift occurs when an individual reaches a certain size threshold.
In social species, the trigger for this transformation is frequently the social structure or a dominance hierarchy. The removal of a dominant male or female from a group can prompt the next largest subordinate to begin the sex-change process. This adaptation ensures that the group can quickly re-establish a functional breeding unit, which is beneficial for species living in isolated groups, such as on coral reefs.
Changing from Male to Female (Protandry)
Protandry describes sequential hermaphroditism where the animal starts life as a male and later changes to a female. This pattern often evolves in species where the reproductive output of the female is correlated with a large body size, as a larger female can produce more eggs. The most widely known example of this is the clownfish (Amphiprion), which lives in strict social groups hosted by a sea anemone.
A clownfish group consists of a breeding pair—the largest female and the second-largest male—and several smaller, non-reproductive males. If the dominant female dies or is removed, the breeding male begins to change sex, growing larger to become the new female. The largest non-breeding male then matures to take the place of the new breeding male, maintaining the hierarchical structure. This socially mediated change ensures that a breeding pair is always present and ready to reproduce, which is crucial since leaving the protective anemone to find a new mate is risky. Other marine organisms, such as certain species of oysters and marine worms, also exhibit protandry.
Changing from Female to Male (Protogyny)
Protogyny is the opposite form of sequential hermaphroditism, where an individual begins as a female and transitions to a male later in life. This strategy is common in species where larger males gain an advantage in defending territories or monopolizing access to a harem of females. Protogyny is the most common form of sex change observed among fish species.
Many wrasses, such as the Bluehead Wrasse (Thalassoma bifasciatum), and parrotfish are examples of protogynous species. These fish often live in social harems dominated by a single, large terminal male that displays vibrant coloration. If this dominant male is removed or dies, the largest female in the group will undergo a rapid sex change, both behaviorally and physically, to assume the male’s role. This transformation involves a physiological shift, where the ovaries degenerate and testicular tissue develops, often accompanied by a change in body color and pattern. The newly formed male, now larger than the remaining females, is better equipped to defend the territory and secure exclusive mating opportunities.
Environmental Influence on Sex Determination
External environmental factors can influence or dictate the sex of an organism. This process, known as Environmental Sex Determination (ESD), is distinct because the environment determines the sex of the offspring during embryonic development, not a sex change in a mature adult. The most common form of ESD is Temperature-Dependent Sex Determination (TSD), seen in many reptiles.
In species like sea turtles, alligators, and crocodiles, the temperature of the nest during a specific period of incubation determines the sex of the hatchling. For example, in many turtle species, cooler temperatures produce males, while warmer temperatures produce females. American alligators exhibit a different pattern, with intermediate temperatures around 33°C producing males, and cooler or hotter temperatures producing females. The underlying mechanism for TSD involves the effect of temperature on the enzyme aromatase, which converts male hormones into female hormones.
Chemical exposure can also disrupt the endocrine system of aquatic animals, leading to sex changes or intersex characteristics. Endocrine-Disrupting Chemicals (EDCs), such as certain pesticides or synthetic hormones from pharmaceutical runoff, can mimic or block natural hormones. Exposure to these chemicals can cause male fish, like smallmouth bass, to develop female characteristics or result in the feminization of male amphibians. This form of environmental influence is generally considered harmful, potentially leading to population decline by skewing sex ratios and reducing reproductive capability.