By Maneka Gandhi
Anthias are coral reef fish. Small with elongated bodies, they range from orange, pink, purple, yellow, and they live in large schools with mainly females and a few males. Anthias have an unusual adaptation. They are sequential hermaphrodites. This means they are born one sex, but can change to another.
Anthias are all born female. When a male dies, one of the larger female anthias changes into a male. The change from female to male takes about two weeks. It includes a new reproductive system and a change in size, shape, and colour. However, if a school of anthias forms, with too many males, the male anthias change their sex again—and return to being female.
With the removal of the dominant male from the harem, the next most aggressive female in line will change her sex. In a domestic aquarium, however, the change in anthias is sometimes not complete. In the wild, the dominant male has a harem and mates with numerous females under his control. In an aquarium this is not easy to replicate, and females turning into males often get stuck in a male-female transition zone.
Have you never wanted to be a member of the other sex? As society becomes more elastic in thought, so many people say that they feel they are in the wrong bodies. Thousands of plastic surgeons are in business to change human sexual organs. It takes dozens of painful operations.
It is much easier for animals and plants.
Sequential hermaphroditism occurs when the individual changes sex at some point in its life, and produces eggs or sperm at different stages in life. Either the change is influenced by its society, or when it reaches a certain age or size.
In animals, the different types of change are male to female (protandry), female to male (protogyny), female to hermaphrodite, and male to hermaphrodite.
Protandry occurs in many fish, mollusks, and crustaceans. Clownfish live in a society where one breeding pair lives in a sea anemone. The female is the largest and the male is the second largest. The rest of the group is made up of smaller non-breeders with no sexual organs. If the female dies, the male gains weight and becomes the female for that group. The largest non-breeding fish then sexually matures and becomes the male of the group. Protandrous hermpahrodites start out life as males, and, where social pressures dictate, are capable of sex changing into fully functional females. If the Disney film Finding Nemo were accurate, instead of trying to find a wife for his son, after his wife was eaten by a barracuda, Marlin would have partnered up with another male and then proceeded to change his sex.
Other examples of protandrous animals include: comb jellies, flatworms, the Mormon Fritillary butterfly – small orange, black and silver. Laevapex fuscus is a small, freshwater, air-breathing limpet. First the males emerge and produce sperm in late winter, then a certain number turn into females and copulate with the males after which the eggs mature.
Marine sea star species, like the Common Cushion Star, change gender from male to female like clockwork. The first three years as a male and the next three as a female.
One of the reasons for this change could be the size advantage. Eggs are larger than sperm, so larger individuals are able to make more eggs. The group can enlarge faster by increasing their reproductive potential, by beginning life as male and then turning female upon achieving a certain size.
Protogynous animals are born female and, at some point in their lifespan, become male. All female protogynous species possess germ cells for both sex organs and, when the social situation calls for a change in sex, are capable of suppressing the female gonads and developing male ones. As the female ages and grows bigger she will turn into a male. In these systems, large males use aggressive territorial defence to dominate female mating. Which is why it makes sense for a large female to become male. Parrot fish are mostly born female, but can become male at any point in their lives. After their transformation, some will become super-males — larger than the typical male parrot fish and exhibiting extra-vibrant colouring.
Wrasses are one of the largest families of coral reef fish. Large males hold territories. In the California Sheephead Wrasse, sex change is age-dependent. All Sheepheads are born female, and stay female for four to six years, before changing sex. Other fish in which this happens are Groupers, Porgies, Angelfish, Gobies, Emperors and Swamp Eels. Wrasses are born as males and females and then change sexes. This is unlike angelfish where the males are exclusively derived from females, i.e., there is no such thing as a male born angelfish.
It sometimes occurs in the frog Rana temporaria, where older females will sometimes switch to being males.
Perhaps one of the reasons that beings can change sex is that, if they don’t move very far out enough as adults, the risk of inbreeding exists – which Nature abhors. Some groups change their sex at the same time. If siblings are of similar ages, and if they all begin life as one sex and then transition to the other sex at about the same age, then siblings are likely to be the same sex at any given time. This reduces the likelihood of inbreeding, and they have to go out to look for mates. The change in sex also allows for organisms to reproduce, if no individuals of the opposite sex are already present.
Giant Limpets commonly change from male to female when they live together in small patchy habitats of water. In fact, female limpets, like Patella ferrugina, can turn back into males, depending on the group size and the number of each sex. Which means that limpets can count better than humans!
Bearded dragons are reptiles that can change their sex from male to female while still in the egg. Researchers at the University of Canberra have found that these reptiles (which are still genetically male but take on the role and reproductive capabilities of the female) are fertile and even lay more eggs than their originally-female counterparts. This phenomenon is triggered by changes in climate: the hotter it is, the more likely that the male will change to a female.
Some male cuttlefish have devised an ingenious way to “get the girl”: become female. To avoid confrontation with other males, while trying to woo a partner, a male cuttlefish can change one side of its body to look female, researchers at Macquarie University found. When positioned between a rival male and a female potential mate, the male cuttlefish can appear female on the side seen by the male, all the while appearing in its true male form to lure in the female. The male rival simply sees two females, and has no idea what’s happening right in front of him.
Blackfin Goby change their sex, dependent on need. Though the transformation is from female to male (when the resident male dies), the tiny fish can transform back when they want. As with other sex changing animals, when gobies physically change their sex, their behaviour changes as well. Females that become males are no longer submissive, adopting the jerky movements of natural males. Predation pressure on reefs makes movement for mate selection very dangerous, especially for small and sparsely distributed species like gobies. It would be advantageous therefore, for them to simply stay at home and switch genders as the need arises, such as when a mate dies, or when the sex ratio is skewed strongly toward a specific gender.
The herbicide, Atrazine, has been shown to cause sex change in frogs. It affects the amphibian’s hormone levels and ten percent changes them into females. However, while the females can mate, their children will all be males.
Hydras are small, fresh-water organisms that never die. They have a tubular body with one foot at one end and a mouth opening, surrounded by one to twelve thin, mobile tentacles, which fire at prey. They move by somersaulting just a few inches a day. When a Hydra is cut in half, each half regenerates and forms into a small Hydra. If the Hydra is sliced into many segments then the middle slices will form both a “head” and a “foot”. Hydras living together change their sexes regularly, sometimes all together.
In the marine worm species Ophyrotrocha puerilis, a pair of individuals will spawn multiple times, with the larger individual as the female. When the faster-growing male becomes larger than the female, both members of the pair change sex, spawning in their new roles, until again the male becomes larger, at which point they both change sex again.
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