All About Reproduction in Fish

Written and verified by the biologist Georgelin Espinoza Medina

Written by Georgelin Espinoza Medina

Last update: 14 June, 2023

Fish exhibit great diversity, which is reflected in their appearance, size, and ecology. In addition to this, reproduction in fish also shows great variety, and we can see all types of vertebrate reproduction among them. Read on to find out more.

Generally, in these animals, there’s a high mortality rate in the early stages of development. Out of hundreds, thousands, or even millions of individuals, only a few manage to reach adulthood.

So, with this in mind, establishing adequate strategies is essential for survival. Here we bring you the aspects, types, and reproductive behavior of these fascinating animals. Don’t miss it, some of them are really amazing!

Characteristics of reproduction in fish

The variety in reproduction in fish is astonishing. These aquatic animals have adapted to make the most of every situation. Let’s take a look at a summary of some aspects related to their multiplication:

• Reproduction is sexual, with a diversity of strategies and behavior.
• Most species are oviparous, that is, they lay eggs, which can be of different sizes and colors. In addition, the eggs are laid on a variety of substrates (on the bottom, among plants or rocks, or suspended in the water). Some have adhesive substances that help them to remain fixed.
• Fertilization is equally variable. It can occur internally or externally to the mother’s body.
• Environmental conditions, such as light and temperature, are important for reproduction. Thus, some species have well-defined seasons for their cycle. On the other hand, others have the capacity to reproduce throughout the year.
• Development can be direct or indirect (when they go through larval stages).
• Some specimens, in order to counteract the low survival in the immature stages, produce a large number of gametes. Others, on the other hand, improve their reproductive strategy by laying few eggs, but incorporating more care. They try to achieve success in this way.

Male reproductive organ

The male reproductive organ consists of a pair of paired gonads, with ducts that end in the cloaca or urogenital opening. Fish produce a whitish substance, which is the sperm or milt. Some species have modified pelvic fins as copulatory structures, as is the case in certain sharks.

Female reproductive organ

The female reproductive organ is variable, depending on the anatomy and gestation or dependence of the embryo on the mother. In general, it’s made up of the female gonads or ovaries (1 or 2), the ducts and the urogenital orifice.

Types and forms of reproduction in fish

As has been stated, reproduction in these beautiful aquatic animals is sexual, as it occurs by means of gametes (male and female). The 3 types of reproduction that exist in fish are the following:

Gonochorism or bisexuality

Gonochorism, also called bisexuality, is the most popular type of sexual reproduction, not only in fish, but in all other vertebrates. It occurs when there are two separate sexes (dioecious), that is, individuals that produce sperm (males) and those that generate eggs (females). Examples of this type of reproduction are sea bass (Dicentrarchus labrax) and Atlantic salmon (Salmo salar).

Hermaphroditism

Hermaphrodite species are characterized by having both sexes in the same individual. Both sexes may develop at the same time in the organism, which is known as simultaneous or synchronous hermaphroditism. In them, self-fertilization is uncommon. An example is the sea bream (Pagellus bogaraveo).

Other specimens change sex during their lifetime, which is called sequential or consecutive hermaphroditism. Thus, they may first mature as males (protandric) or as females (protogynous). Here we can cite the gilthead sea bream (Sparus aurata), which is male for the first two years of life and after 3 years becomes female.

Parthenogenesis

Reproduction by parthenogenesis in fish consists of females laying eggs from unfertilized eggs. However, certain species need to mate with a male of a related species in order for embryonic development to begin. Thus, the offspring are identical to the mother. An example of this is the amazon molly (Poecilia formosa).

Similarly, fish have developed 3 ways to achieve reproductive success:

• Oviparity:  It refers to the laying of eggs, a practice carried out by most fish. For this, fertilization is external, that is, outside the mother’s body, once the spawning is complete. Although, in the same way, there are few oviparous with internal fertilization. However, the first is the most common and occurs in trout (Salmoninae) and tilapia (Oreochromis), as examples of commercial fish.
• Ovoviviparity: In this case, the eggs are retained inside the female until the time of hatching. The nutrition of the embryo depends on the yolk substances of the eggs. Fertilization is internal. Typical examples are guppies.
• Viviparity: In this condition, development and fertilization occur equally within the mother. However, the embryo receives direct nutrients from the mother. Among the viviparous fish, we can mention some sharks.

Reproductive behavior in fish

The reproductive diversity of fish also includes impressive behavior, such as some courtships, nest formation, egg incubation, and migrations. Let’s look at each of them.

Courtship

Some fish have evolved interesting courtship strategies to ensure reproduction. This can be carried out by either, or even both, sexes. For example, the female Pacific salmon (Oncorhynchus kisutch) performs a dance with her mate before spawning. She then goes to the appropriate place, digs with her tail, deposits the eggs, and the male fertilizes them. She then proceeds to cover them with gravel.

Nests and egg incubation

Certain fish prepare amazing nests, either on the bottom of the sea or river, digging in the sand, with vegetation and algae or sticky substances (which they excrete), among other behavior. Parental care of eggs or young may also be present in some organisms. In the Mozambique tilapia (Oreochromis mossambicus) there’s even incubation in the mouth, as an impressive method of not abandoning their offspring.

Migrations

These swimmers can also travel long distances in order to reproduce. Thus, there are two types:

• Anadromous: These migrate from the ocean and large rivers to the gravel spaces where they were born, in order to reproduce. Here we can mention salmon.
• Catadromous: These travel from freshwater environments (where they live) to the sea to lay their eggs. Freshwater eels perform this type of migration.

The seahorse, an example of reproduction in fish

If you’re one of those who think that gestation is only a female thing, in fish the diversity is so impressive that this isn’t always the case. One example is the common seahorse (Hippocampus hippocampus).

This species initiates its reproduction with a courtship, the couple intertwines its tail and “dances” for several minutes. The female is in charge of passing the eggs from her cloaca to the male’s belly pouch, thanks to a 3 millimeters long papilla. For his part, he takes care of the eggs for almost two months.

After this time, they hatch and come out as miniature sea horses, which remain under the protective care of the father. The number of hatchlings is numerous (up to more than 400), so the male has a hard and exhausting job.

The seahorse is a typical example of how reproduction in living beings, and in particular in fish, can involve unusual methods. The animals will establish any strategy in order to guarantee their success and survival.

As you can see, reproduction in fish goes beyond laying eggs. In these animals, we can see some of the most extraordinary behavior and mechanisms. Knowing about these aspects is of great interest, not only out of curiosity, but also because it’s very relevant for those who keep and breed fish.