When and How Do Fish Mate? Reproduction Classifications Explained!

How Do Fish Mate

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Fish are fascinating pets to watch due to their beautiful colors or the graceful way of their swimming. For those reasons, many people begin to gain interest and become fish enthusiasts. Some are even really serious about it.

On top of keeping and taking proper care of them, many aquarists try to breed many types of fish in their home aquarium. So if you are new to the topic or just come here out of curiosity, you must be wondering, how do fish mate? Do they always need a partner to reproduce, just like humans?

The majority of fish have two things in common. First, they are vertebrates, which means all of them have backbones. Second, they live in the water. Despite the common things they share, not all fish do the same thing to reproduce.

Unlike us humans, there are many ways for fish to mate. Today, we will find out the various ways of how fish get their babies. Some facts may even be new or surprising to you. Let’s dive right in!

Fish Reproductive Anatomy

Like every living creature, it’s crucial for fish to reproduce so that they do not extinct. But before getting into the ways of how to fish mate, we need to understand their reproductive system.

Fish can create new lives by combining the egg and the sperm, just like most vertebrates. There are several ways how that step could happen, but one factor that most fish share in common is the reproductive organs they use: the ovaries and the testes.

The vast majority of fish are dioecious. Meaning that they have separate sexes, in which the females have ovaries, and the males have testes.

But that is not always the case. Some particular fish species have both sets of reproductive organs, and they are called hermaphrodites (we will get into that later on).

Some other species have a secondary reproductive organ which is commonly known as genital papilla. It’s a small fleshy tube at the back of a fish’s anal fin for the male to release the sperm.

Fish for reproductive organs: necropsymanual.net

Fish Reproduction Classifications

Earlier, we have mentioned that there are a few ways for fish to create new lives. There are three main classifications of reproduction that apply to fish:

  • Oviparity
  • Ovoviviparity
  • Viviparity

Oviparity (Egg Layers)

Oviparous fish develop the embryos within the eggs but outside the mother’s body, similar to birds, reptiles, and insects. Approximately more than 90 percent of bony fish are classified as oviparous. This breeding method occurs with the female laying their eggs, and later those eggs will be fertilized by the male.

During the spawning season, it’s common for the females to lay a large number of eggs at once. The reason is that it requires much less energy than developing the embryo inside the female’s body.

The number of eggs which the females can lay in the spawning season is known as the fenducity. The fish’s fenducity depends on the size of the fish. This includes the length and weight.

For instance, the Silver Arowana can only release about 50 to 250 eggs, while the Ocean Sunfish (Mola) can produce up to 300 million eggs per spawning season!

There are several ways of how the male fertilizes the eggs:

  1. They release the sperm directly into the eggs
  2. Release the sperm into the water to meet the eggs
  3. Or rub their sexual organ on the eggs

An egg-layer fish can be one of the following five categories:


Mouthbrooders mean that the females release the eggs into the water, which then the male will fertilize them. After that, the parents will collect and keep the eggs inside their mouths until they all hatch.

Fun fact, it’s usually the male fish that carries the potential fries. However, some fish species require the teamwork of both the male and female to do the brooders’ duty.

Some species of fish that fall into this category are Arowana, Bettas, Blennies, freshwater Cichlids, Cardinalfish, and Gobbies.

Nest Builders

Several types of fish build a nest where they place their eggs to protect them from predators. The nest is usually made of leaves, bubbles (for Bettas), or even just a simple digging in the substrate.

Oftentimes, it’s the male that makes the nest. If the female likes it and is satisfied with the outcome, she will deposit her eggs there. Then, the male will fertilize the eggs by releasing the sperm to them.

A few nest builders fish are Bettas, Bluegills, Gouramis, and Stickle-backs.

betta bubble nest
Betta bubble nest – Image by commons.wikimedia.org

Egg Scatterers

Fish that fall into this category place their eggs either in open water (sticky eggs) or an undercover place (non-sticky eggs). The way the male fertilizes the egg is a bit different from the previous two categories. The male fish will swim through the eggs while spraying semen on them.

Several fish that belong to this category are Tetras, Koi, Goldfish, Tiger Bards, and Zebra Danios.

Egg Depositors

Egg depositors release their eggs only in one place. The male will then fertilizes the eggs by swimming through them. Some egg depositors fish are Clownfish, Dwarf Cichlids, Killifish, and Rainbowfish.

Egg Burriers

Some types of fish bury their eggs within the substrate to protect them. The mother uses the saliva to stick the eggs onto the surface of the hiding place. The male will then dives into the substrate to fertilize the eggs. Catfish and Killifish keep their eggs in this manner.

Ovoviviparity and Viviparity (Livebearers)

Both ovoviviparous and viviparous are also known as livebearers. What distinguishes livebearers from egg layers is where fertilization and embryo development take place.

In livebearers, fertilization and development of the embryos occur within the mother’s body. This means that the female fish has to take the eggs inside her.

It happens with the male impregnating the female by releasing the sperm into the female’s body. To insert the sperm, the male uses gonopodium, which technically is a modified anal fin.

In ovoviviparous fish, the young grow in the egg inside the mother’s belly, although they don’t get any nutrition from the parent. Instead, the young will feed on the egg’s yolk sac to develop.

As for viviparous fish, the fertilization and development of the young also take place inside the parent body. However, the developing embryos receive nutrition from the mother. The nourishment goes through a placenta-like structure that connects the mother with the embryo. Guppies and Swordtails are bred in this manner.

In most fish, the development of the eggs will be complete, and the mother will expel the live fries in approximately one month. To name a few, Guppies and Platys are some of the livebearers.

The live fries are fully formed and able to swim freely. They will immediately begin to search for food on their own. These fries are already switching to a survival mode.

An interesting fact about the mothers is that they only need the male fish to mate once. After that, the females can reproduce independently every month!

Placenta-like structure: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5033709/

Other Ways for Fish to Mate

Besides the breeding ways above, there are also some unusual manners for fish to reproduce. These unique reproduction manners happen to fish that are classified into the following categories.

Unisex Fish

We know that the vast majority of fish, like other living creatures, need a partner to reproduce. However, that’s not always the case. Some species are independent enough to create their offspring all by themselves!

Unisex fish are always females and only produce female fish. They don’t need the presence of the male. Thus, we can’t really call it mating since the male doesn’t play a role, and the sperm is pretty much unnecessary.

However, unisex fish sometimes mate with male fish of similar species. It’s just the sperm is not used for the reproduction process.


There are two common types of hermaphrodites fish, namely:

Synchronous or Simultaneous Hermaphrodites

The first one in the category is synchronous hermaphrodites (also called simultaneous hermaphrodites). As the name suggests, these hermaphrodites fish can produce eggs and sperm simultaneously. That’s because they are born with both female and male reproductive organs.

In some species of fish, the adults can do self-fertilization to reproduce. They will then keep the eggs within their bodies. Once the eggs hatch, the fries will release by themselves. Even so, the parent will stay close to protect their fries until they mature enough to survive on their own.

Interestingly, these hermaphrodites fish will also give birth to youngsters with both male and female sex organs. So, everyone that belongs to the species doesn’t need to interact with fish of the opposite gender to reproduce.

Some examples of simultaneous hermaphrodites creatures are Mangrove Killifish and Slugs.

Sequential Hermaphrodites

This one is quite different from the previous hermaphrodite type. Sequential hermaphrodites mean a fish can be born either as a female or a male. But when they mature, their gender can alter! When they hit the mature age, their sex can change at any time.

If a female fish switches its gender to male, the process is called protogyny. On the other hand, when the male transforms its gender to female, it’s called protandry.

Unlike simultaneous hermaphrodites fish, the sequential ones still need a partner of the opposite gender to produce their offspring. So why do they need to change genders?

Clownfish is the perfect example to help answer the question. In a school of clownfish, there will always be the two biggest fish. One is a male, and the other one is a female. The rest in the group are small clownfish which consists of all males.

When the female dies or leaves the group, her partner will transform into a new female. The next biggest male fish in the school will then be her new partner.

Another example is the Indo-Pacific cleaner wrasse. Their condition is similar to the clownfish. But instead of a school with the male majority, a group of cleaner wrasse usually made of all females with one male.

So, we can tell that fish with this hermaphroditic type switches their gender due to the lack of the opposite gender within the school. Sounds very practical, right?

Clownfish, yes, they are Hermaphrodites!

When Do Fish Mate?

Earlier, we have answered the question ‘how do fish mate?’. Now, we’ll get into another related topic of fish mating. The age of mature fish and the season when they spawn. So, how old should a fish be to be ready to mate? And when is the perfect time for them to breed? You can find the answers below.

At what age is a fish ready to mate?

The reproductive organs are the slowest organs within a fish to develop. That is because the sex organs are not crucial, and fish don’t need them immediately after birth to survive.

Some fish species can start reproducing within a few weeks after they were born. Whilst other species may take years to be sexually mature enough.

This diversity is closely related to the fish species, size, as well as age. Generally, adult fish of smaller size will reproduce sooner than other species of a bigger size.

How often do fish mate?

Fish’s mating season completely depends on its species. For instance, Clownfish, Guppies, Mollies, Platys, and Swordtails can breed at any time throughout the year, regardless of the season.

The earlier mentioned species can mate whenever they want. However, some other species are only able to reproduce during a particular season. Some fish species need to wait for the perfect water temperature or sufficient food supply to get them into a great mating condition.

Other species like the Pacific Salmon can only reproduce once in their lifetime. They will soon die after the eggs are released.

Breeding Fish in Your Home Aquarium

Mating fish in a tank is not an easy task to complete, especially if you’re a newbie to the aquarium world. But if you are patient and willing to learn, there’s a chance for you to breed your fish successfully. To give you a clue on how to mate your fish in your home tank, here are a few things you need to prepare.

Find a Matching Pair

The first step to breed your fish is to find a matching pair of male and female fish. Before that, you need to be able to distinguish the two genders. The majority of fish are either sexually dimorphic or isomorphic.

  • Sexually dimorphic: the male and female can be distinguished pretty easily. There are apparent differences in their reproductive organs, such as color, shape, and size. Typically, the males have bigger body sizes and more vibrant colors compared to the females.
  • Sexually isomorphic: the male and female are hard to tell apart, which will result in difficulty to find a good pair. Sometimes, you can determine their sexes only by looking at the genital papilla’s shape when they’re breeding.

You can do your own research or ask an expert in the fish store before buying a certain fish species to breed. That way, you can ensure that you get a pair of the right sexes and know the steps to mate them.

Besides genders, you also need to make sure to pick mature fish that are in good condition. Signs of healthy fish are bright colors and lively behavior. The pair of fish also needs to be compatible so that they won’t bully each other until one of them dies.

If you want to breed the fish right after buying them, you need to pick the ones of reasonable size. Some fish need to grow to a certain age and size until they are sexually mature.

Once you have a compatible pair in your tank, you need to condition them by feeding them a balanced diet. This includes a variety of foods such as live foods and pellets. Conditioning will keep your fish in great shape before they’re fully ready to spawn.

Setting Up the Perfect Tank Conditions

The perfect breeding aquarium setup varies according to the species. However, in their natural habitat, most fish begin to mate when spring comes. The reason is that the water temperature gets warmer, and they have a longer time under the sunlight.

In a home aquarium, you can mimic the condition by gradually raising the temperature to 10°F above the usual. You will also need to adjust the water hardness and pH to a suitable level. Again, it differs for each fish species, so make sure to do your research beforehand.

On top of that, some fish species also require certain plants, substrates, or water flow to breed successfully. You also can’t forget the required tank size for your fish since it’s one of the most crucial requirements.

It’s best to separate the fish you want to breed from other species so that they won’t be disturbed. Once the fish pair got into a separate tank, you can separate the male and the female with a glass partition. It will help boost their desire to mate.

One more addition, the female fish needs ample high-protein diet to get them into good breeding condition. So, providing them more high-protein foods will be a good idea.

Fun Facts About Fish Reproduction

  1. Splashing Tetra deposits its fertilized eggs on an overhanging tree leaf by jumping out to the surface. After that, the Tetras will splash water onto the eggs to keep them wet. The eggs will drop back into the water once they hatched and become fries.
  2. Freshwater Gars produce extremely poisonous eggs.
  3. Male fish flirts with other males to get the female’s attention.
  4. Some Killifish produce eggs that can survive drought even when they’re completely dried out and can still hatch when rains come.
  5. The annual spawning Sardines from the South African coast produce a massive amount of eggs and sperm that are visible even from the air.

Final Thoughts

If you have read this article thoroughly, you can conclude that there are various ways for fish to breed. That is basically how to answer the question ‘how do fish mate?’ shortly.

Some female fish species release the eggs outside for the male to fertilize them later. While some other species breed differently. The male invests the sperm inside the female body and lets the eggs develop inside. A few species can even reproduce by themselves!

Breeding fish in your home aquarium may be challenging at first. So if you’re still a beginner, we suggest mating livebearers such as Bettas or Guppies. Again, before you start your mating project, you need to know all the preferred tank conditions and mating rituals. Those requirements very much depend on the fish species.

Once you’re already an experienced fish breeder, you can try to mate different fish species that require more complex care and conditions.

Do you have any experience in fish breeding or more tips you want to share with us? Please leave some in the comments!

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