How Do Fish Have Sex?

A Stanford study reveals that the key to female fish success at mating lies in a single brain receptor.

Most fish are sequential hermaphrodites, which means they possess both male and female reproductive organs. This arrangement benefits species that form harems, like wrasses and parrotfish.

These fish start out as males, and switch to females at a later age and size. The process is called protogyny.


In most fish species, fertilization is achieved through the standard sexual process of sperm and eggs. Female fish release their eggs into open water and male fish release sperm, which then fertilize the eggs to form new fish. Most of the time, this happens without any physical contact between the male and female. This method of reproduction helps to protect the eggs from predators that would eat them.

The eggs are formed in the ovary, a reproductive organ located within the female body. The eggs are released into the water when ovulation occurs, a process that can vary among species. Ovulation can be triggered by a change in hormone levels, the occurrence of sexual intercourse, or both. Female fish produce a type of sperm called spermatozoans, while the males produce a sperm known as milt.

To fertilize the egg, the sperm must first be delivered to the ovary. The female uses a sticky substance known as mucus to attract sperm and to hold it in place. This enables the sperm to penetrate the egg and fuse with it, creating a fertilized egg known as a zygote.

In the case of most fish species, this is the only way that fertilation can occur. However, there are some fish that use a different method. Some fish like the killifish, a genus of bony fish that lives in mangroves, use a technique called “DIY fertilization.” The females hide in rotting tree stumps, and the males crawl inside to gather sperm from their gonads. This is a less risky way of getting the job done than going in search of a mate, and it has proven to be successful.

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Unlike many other animals fish don’t go through courtship, which is the behavior leading up to mating. Instead, if a male and female are compatible they will just pair up and start breeding. This is why it’s so important to select a pair of fish that have similar personalities, size, and temperament.

Most fish are dioecious (males have testes and females have ovaries), but some have both sets of organs and are hermaphrodites. These fish, called polyploids, can reproduce either through external fertilization (releasing sperm into water) or internal fertilization (transferring sperm directly to the eggs). The fertilization process is usually quick and straightforward.

For example, some fish lay non-sticky eggs in open or undercovered areas and the male fertilizes them by swimming past. Other fish build a bubble nest to protect the eggs and then the male fertilizes the eggs by digging into the substrate. Males of some species even use a specialized fin to pierce the female’s eggs and fertilize them internally.

On the flip side, some fish form long-term monogamous relationships. These pair bonds often involve elaborate courtship rituals and include building a nest for eggs and caring for the young. This is especially true for seahorses and freshwater cichlids. The males of these fish even possess a bony organ, the priapium, that resembles our reproductive organs and can be used to grasp the genitals of a female with the help of a special clasper.

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Sexual Behavior

Of all the animals on Earth, fish are perhaps the most fluid when it comes to gender. Scores of reef fish can change from male to female and vice versa—a phenomenon known as hermaphroditism. Typically, hermaphrodites are born one sex and switch to the other at an older age or larger size. However, a few fish—like the Asian sheepshead wrasse (Semicosyphus reticulatus) featured in Blue Planet II—have a much more extreme approach to hermaphroditism. This enigmatic species of killifish can transition from male to female and back again, but only after it spends many months hiding in a lair as a female. Afterward, the fish emerges a larger male with testes and a massive head. Scientists call this form of hermaphroditis “protogyny.”

Most fish reproduce by standard sexual behavior, depositing eggs in ovaries and releasing sperm into the water. A few species, including the sexy-looking Phallostethidae (translation: chest-penis fish), have penetrative sex. The male uses a bony copulatory organ, the priapium, to grasp the female’s genital opening—which is on her head!

Other fish rely on more sexy displays to attract and seduce potential mates. Guppies, mollies, and other members of the platyfish family display gonopodia—flamboyantly colored organs that they wave around to attract females. If the female is receptive, she may adopt a quivering posture and allow the male to twirl her with his dorsal and anal fins. If not, she will assume a rejection posture, raising her head or moving away.

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Many fish species engage in standard sexual reproduction involving the mixing of sperm from their testes and eggs from their ovaries. However, this method of reproduction isn’t always the only way that fish breed.

For example, some fish practice external fertilization whereby they release their eggs into the water. The eggs can then drift far on sea currents and be deposited in different places, like on plants or under rocks. Other fish, like freshwater Garfish, lay sticky eggs that stay put on plants until they hatch. Still others, such as a slick of spawning sardines in the open waters or the complex nesting behavior of sea lampreys, practice internal fertilization.

Some fish are hermaphrodites, which means they produce both sperm and eggs at the same time. They usually begin the spawning process by flashing their colors, signaling to other fish that they are ready to mate. Hermaphrodites may also reproduce internally through a process known as internal fertilization whereby the male passes his sperm directly into the female’s body to fertilize her eggs.

Hermaphrodites can also switch their sex, which is called Budding. They start out as one sex and then convert to the opposite at some later stage in their lives. This is the same phenomenon that happens in amphibians and reptiles, but it is believed that in fish this change is controlled by an enzyme, aromatase, that responds to temperature changes.

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