Fish Mixing It Up

Some of the weirdest activities take place throughout our world, including the ability of some animals to change their sex in their environment. Hermaphroditism occurs in about one-third of animal species, providing numerous opportunities for the evolution of selfing. These animals get to experience the best of both worlds really, many of these hermaphrodites have both male and female sex organs and can switch between the two depending on numerous situations in their environment. We as humans do not have the ability to change our sex at will but some animals do in fact have the ability to perform this phenomenon. This strange practice of hermaphroditism is more common among species of fish. Some fish change sex in response to hormonal cycles, survival mechanisms and environmental changes. There is also a change in hormones or chemical messengers in the blood of these animals that cause them to change their sex. Scientists have questioned what causes these changes and why exactly do they need to change their sex?

There are two kinds of hermaphrodites in the animal world:  simultaneous hermaphrodites and sequential hermaphrodites. Simultaneous hermaphrodites have both male and female reproductive parts for their entire life and can mate with any other member of their species (Ho, 2002). This is a big reproductive advantage because it is easy to have a large number of children within this species. Sequential hermaphrodites are born as one sex, but change completely into the other sex during the course of their lives. These fish live within a hierarchal structure in which the sole female and the largest male do all the mating. They also change in order to adjust to their environment they are present in. Their mating strategy is helpful for reproduction since they stay relatively close in their structures. The most commonly known hermaphrodites in the fish population are the Parrot fish and Starfish which express simultaneous and sequential hermaphroditism.

Parrot fish (Picture source -

Parrot fish (Picture source –

Simultaneous hermaphrodite are the species of fish known as Parrotfish. These hermaphrodites start out as females but as they grow up some become males. Either way it goes, the sex change works with the lifestyle of that particular fish to produce the largest number of offspring. These fish live in small packs that consist of one male and several females. When that one male dies, the strongest and biggest female will change into males and take over for the pack.This female will take on the male coloration, develops testes, and will even produce sperm, so becoming the main male in the group (Hamilton, 2008). Another thing to note is when another male comes along the female that has turned male can revert back to the female and have the new male lead over her.

Sequential hermaphrodites are the species known as Star fish. Some starfish are born male but throughout development they change to females. Others can change repeatedly in their lifetime depending on their environment they are brought into (Sharpiro, 1986).They also can change different sexes due to food availability and water temperatures they are subject to. Starfish can change their gender when it’s convenient for them and can easily change back to their original sex.

Scientists don’t know exactly how the sex change really occurs, but in either direction, the ability to change from one sex to the other seems to have distinct evolutionary advantage by enabling the species to produce the largest number offspring possible.What remains a puzzle is why the phenomenon is so rare, since a detailed analysis shows the biological “costs” of changing sexes rarely outweigh the advantages (Yale University 2009). One explanation is that the considerable time or energy it takes to change sex makes hermaphroditism unfeasible for most animals. It has been shown that fishing pressure can change when the switch from male to female occurs, since fishermen usually prefer to catch the larger fish. The populations are generally changing sex at a smaller size, due to natural selection. Scientist believe reproductive success and continued survival are the main reasons that species change sex. These changes could occur due to environmental factors or chemical triggers but more research needs to be done on this. More hormones being present in our water waste and chemicals being produced will affect the changes in other species on how they survive and reproduce as well as the ones that already change their sex. Scientists have seen before in numerous studies done that endocrine disruptors are causing species to show both male and female sex organs. The more abnormal sex changes that occur in parrot fish and star fish as well as other species will make it more difficult for them to breed and survive. This phenomenon of hermaphrodites still needs to be understood more so that biologist can grasp the concept of how and why these animals change sexes.

Works Cited

Hamilton RJ, Adams S, Choat JH (2008) Sexual development and reproductive demography of the green humphead parrot fish (Bolbometopon muricatum) in the Solomon Islands. Coral Reefs, 27: 153-163

Ho L (2002) “Hermaphroditism: A Tale of Two Sexes.” Reefscape (Online)

Shapiro D (1986) Intra-group home ranges in a female-biased group of sex-changing fish. Animal Behaviour 34: 865-875

Yale University. “Why Don’t More Animals Change Their Sex?.” ScienceDaily. ScienceDaily, 4 February 2009

About the author: Kathryn Ryan is a senior Biology major at the University of St. Thomas


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