Whether it be for intentional breeding or by accident, fish reproduction in a freshwater aquarium is inevitable if you have both males and females of the same species.
Freshwater aquarium fish either reproduce by laying eggs which are then fertilized by the males, or they are livebearers meaning they give birth to live fry. Each freshwater aquarium species is different in terms of spawning.
There are multiple different categories of egg layers when it comes to freshwater aquarium fish, as well as different parameters needed for livebearers.
Read on to learn more about how fish reproduce in freshwater aquariums!
Reproductive Anatomy of Fish
It is important to know about fish reproductive organs to fully understand how they reproduce.
Like us humans, for fish to produce an egg from the female, it needs to be combined with sperm from the male.
Most fish are dioecious, meaning females have ovaries and males have testes. Some will even have a small fleshy tube located at the back of the anus where sperm is released, this organ is known as the genital papilla.
However, there are a few species of fish that are hermaphrodites. This means they contain both female and male organs. This will be further explored below.
Some female fish can reproduce by themselves. The females can give birth to female fry without any fertilization from the male. They may mate with the male fish, however, the sperm isn’t used for spawning.
There are three main classifications of how fish reproduce include Oviparous fish, Ovoviviparity fish, and Viviparous fish.
A fun fact is that the reproductive organs in fish are typically the slowest organs to develop as they grow. This is simply because they are not necessary for survival.
Mating Time Frame
Different factors play into when they are mature enough to start mating, this is based on the species, size, and age. Typically the smaller the adult size of the fish, the sooner they will be ready for mating.
Some species may take years to be mature enough for mating, whereas other species can be ready for mating not long after their birth.
Different species also vary on how often they will mate and reproduce. Some may reproduce a few times a year, such as guppies and platys.
Other species will only reproduce in a certain season, and some will reproduce only once and then die after laying their egg or releasing sperm.
Egg Layers (Oviparous Fish)
Over 90% of bony fish are egg layers, meaning like birds, the embryo will develop inside the egg outside of the female’s body.
This means the female will start laying the eggs first, and then the male goes behind her and fertilizes the eggs.
Most female fish lay large quantities of eggs at one time. They can lay a multitude at a time, as opposed to livebearers.
This is because it requires a lot less energy than growing the embryo inside the body.
The fecundity of a fish is also closely related to the length and weight of the female fish. The fecundity of a fish is the number of eggs that a female can produce during one spawning season.
Fertilizing the Eggs
The male fish can fertilize the eggs in a variety of ways by releasing sperm, or by rubbing against the eggs.
The way a male fertilizes the eggs is dependent on where the family lays the eggs in the first place. Here is a list of how the eggs are laid and fertilized:
- Egg Buriers: In freshwater aquarium fish such as a killifish, the female buries the eggs in the substrate at the bottom of the tank. From there, the male will then dive into the substrate to fertilize the egg.
- Nest Builders: Typically the male fish will build a nest made of plant materials or a bubble nest. After the nest has been built, the female will lay her eggs there. The male will then fertilize them by swiping over them. This kind of egg-laying can be seen in bettas and gouramis.
- Egg Depositors: Egg depositors, such as Corydoras catfish, will require the female to lay all her eggs in one spot. The male then swims past fertilizing the eggs.
- Mouthbrooders: Some fish, such as the dwarf Egyptian mouthbrooder, will lay their eggs in one spot and wait for the male to fertilize them. After this, the female will collect the eggs into their mouths.
- Egg Scatterers: The female lays sticky eggs typically undercover, and non-sticky eggs in open waters. The male fertilizes these eggs by swimming through the area and releasing semen. This kind of egg-laying is common in Koi or goldfish.
Livebearers are fish that give birth to fry and bypass laying any eggs. This means that the fertilization of the egg and the embryo development happens inside the female.
The males use their modified anal fin to impregnate the female by releasing sperm from that fin into the female’s body.
Typically livebearers will have fewer, but larger fry than an egg layer does. As mentioned earlier, the egg layers can lay and produce so many eggs because it requires less energy.
This means the livebearers use more energy in developing the fry so they reproduce less than an egg layer. Again, they give birth to live fry which takes more effort.
Ovoviviparous and Viviparous Fish
There are two different kinds of livebearers, ovoviviparous fish and viviparous fish.
The difference between the livebearers is how the embryo receives nourishment while it develops in the female body. After the embryo is fully developed the mother gives birth to the live fry.
The ovoviviparous fish reproduce by the male fertilizing the eggs while they are still in the female.
From there, the embryo will not receive any nourishment from the mother herself. The embryo will receive nourishment by using the egg’s yolk to develop.
In the viviparous fish, like the ovoviviparous fish, fertilization is done by impregnating the female herself.
However in viviparous fish, as the embryo develops they receive nourishment directly from the female fish as she carries.
As previously mentioned, some fish contain both male and female organs. This is also known as being a hermaphrodite.
Fish are born as one sex and as they mature they will actually switch to the opposite gender at a later age.
Even though these fish contain both sex reproductive organs, they still need another fish from the opposite sex in order to reproduce. They can not reproduce on their own.
There are two main different classifications of hermaphrodite fish depending on the starting sex.
A protogynous hermaphrodite is a female switching to a male, and a protandrous hermaphrodite is a male changing to a female.
With some fish that stay in groups, the group will have a large male and female. The rest of the fish in the group are small males.
If the large female leaves that group, her partner, the large male of the group, will actually change into a female.
After this, the next largest fish from the rest of the group will become the partner of the new female fish.
There is however another kind of hermaphrodite in fish, known as synchronous hermaphroditism.
Only a small number of species fall into this classification. This kind of hermaphrodite in fish can actually produce both sperm and eggs at the same time.