Some will change sex in both directions, and others will be both sexes at the same time. Most species will only change once, but there are some species that change numerous times. The hamlet, a tropical fish, can shift back and forth between the sexes in just 30 seconds. That speed comes in handy during the hamlet's marathon sex sessions; hamlets spawn an average of 14 times in one day. Sometimes, all it takes is an environmental factor to alter one's gender. Humans don't need survival strategies – we are now more than 7 billion – but every other species does. For many species, it is certain that the majority of their offspring will not survive to adulthood. The solution, then, is to simply have more offspring. But this is where nature finds a limitation. While males of almost every species can produce sperm in very short order, the females, once impregnated, must wait until they give birth (or lay eggs) before they can reproduce again. For coral reef fish, evolution found an interesting solution.
Not only do the size, colour, and markings of fish change as they change sexes, so do their sexual organs and sex cells. A red female stoplight parrotfish can turn into a green male.
Female-to-male fish which once produced eggs are able to instead produce sperm. This is conversely true for male-to-female fish. While some fish can only undergo a sex change once in their lives, others can go back and forth many times, or even have both sexual organs at once. There are two kinds of hermaphrodites in the plant and 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. That is a huge reproductive advantage, because it's easy to have a large number of children -- your mating options are doubled. Snails, slugs and earthworms are simultaneous hermaphrodites. Sperm is exchanged between both snails and both will lay eggs after a period of gestation. Banana slugs go one step better. The male sexual organ of a banana slug is large in proportion to its size. If, during mating, the banana slugs get stuck together, one slug will bite off the other's male organ. If a banana slug has lost its male sexual organ, it can still mate as a female, making its hermaphroditic quality a valuable adaptation.
Sexual reproduction occurs when two worms meet, exchange sperm and bury the fertilized eggs just below ground. Sequential hermaphrodites are born as one sex, but change into the other sex as they grow. The beautiful yellow striped fish, Anemone fish, or clownfish, are one of the most prominent examples. Clownfish live on the reefs in a group. There is strict hierarchy based on size: the largest is the female, next largest is the male, and then the non-breeding males. If the female dies, the male will change sex and become the female and she begins mating with the next male. This mating strategy is advantageous for reproduction, particularly since this fish never ventures very far from home. If the males look for new female partners rather than change sex, the process can take months. During this time the fish would be unable to reproduce, and might miss his only opportunity to mate.
In coral reef fishes, sex change is a normal process. In the wrasses, sex change is from female to male. As soon as the male is gone, the largest female in the harem begins to behave like a male: chasing the girls, swaggering around, and being macho. Inside her ovary, within 24 hours the female's immune system is reabsorbing her eggs and turning on the testosterone. Within a week she/he will produce sperm. Limpets which spend their lives on rocks are hermaphrodites who start life as males but after a couple of years become female. The small-eyed goby is ready to dance to any tune. Put her with a larger female, and she becomes male. With a bigger, meaner male, this goby turns back into a female. Sheepheads start off life as females and then turn into males. Overfishing leads to sheepheads maturing early and changing sex earlier. Other gender changers are shrimps, swordtails, guppies, mollusks basses, sea breams.
Some species start out as one sex and then transform into hermaphrodites. The peppermint shrimp matures as a male, and sometimes turns into a hermaphrodite with both male and female sexual organs. Most peppermint shrimps stay male if they are successful at finding mates. The decision to change is determined by the size of the social group. To maximise their chances of being able to mate, shrimp living on their own always turn into hermaphrodites, even though they end up growing slowly because of the energy spent on making eggs. However the male finds it tough going ( he has to chase a mate for up to 8 hours and this costs him 15% of his physical growth). This means that males end up smaller than hermaphrodites, which often out-compete them for food. So being a hermaphrodite makes more sense. Both maleness and femaleness have a lot of costs – females must produce food-rich eggs, males must engage in courtship. So, slow-moving animals that rarely meet a mate are the most likely to be hermaphrodites. Corals may look plant-like, but they are animals.
Mushroom corals change sex from male to female and back on an annual basis. When mushroom corals are small, it makes more sense to be male because it takes less energy to produce sperm than to produce eggs. When the coral size increases it's better to be female. However corals sometimes switch back from female to male when that they are in distress and need to conserve resources. If pollution to climate change pressures push too many mushroom corals towards maleness, a skewed sex ratio could threaten their future. Water fleas are freshwater crustaceans that hop through the water on fan-like fins. When algae are just enough, everyone is female and asexual. But if the food increases, suddenly males are hatched. They mate with females, which produce eggs. For these water fleas, sex can be turned on or off as needed. Being double-sexed often leads to orgies. Sea hares are snails without shells that are male in front and female behind, leading them to form erotic daisy chains that can involve a dozen or more animals at once.
Slipper limpets are male while free-swimming and female when they settle down on a rock. When a male swims by a female slipper limpet, he flops onto her to mate. If another male lands on top of him, the lower male turns female, and they clasp. And so on upto 14 limpets at a time pancaked into towers of lust. In his book The Red Queen, science writer Matt Ridley explains that baby-making is more business than pleasure. 'Sex is not about reproduction,' he writes. 'Gender is not about males and females, courtship is not about persuasion, fashion is not about beauty and love is not about affection.' It is about getting your genes into the next generation, escaping predators, parasites, and the neighbors.
According to scientists , while the main sex changers are now fish, gastropods, worms and plants, it is possible that more species will start changing sex due to environmental factors. A 2004 study details the changes taking place in polar bears and alligators due to manmade pollution. Atrazine, a common pesticide, given even at very low levels has been found to turn male frogs into hermaphrodites. The limit for atrazine contamination for drinking water is 3 parts per billion (ppb) but 0.1 ppb causes sexual abnormalities. Who knows what is happening to the human body.