How do the fish mate?

Answer

Fish are not only the most species-rich group of vertebrates, they also show the most pronounced variation in all kinds of biological aspects. The way they mate is no exception. Mating in fish can be very simple, but it can also involve very complex mating behavior.

The simplest form of mating in fish is where the female simply releases her eggs into the water, and the male then releases his sperm cells, with fertilization simply taking place in the water. This is the case with many fish that live in seas and oceans. This is also the least efficient way of reproduction, as the chance that an egg will be effectively fertilized and grow into a small fish is relatively small. Such fish also lay very large numbers of eggs (we are talking about millions of eggs here).

Some fish can increase the success of fertilizing eggs by, for example, depositing the eggs on a substrate, after which the male can fertilize them in a more targeted way. In such fish, the eggs are often provided with an adhesive organ, which ensures that they stick. You see this, for example, with clownfish (such as with Nemo). In these fish, the fertilized eggs are often guarded (either by the female, by the male, or by both). Now, there are also spectacular forms of egg deposition on a substrate: some fish lay their eggs outside the water! For example, a female of the South American spatter salmon (genus Copella) will jump out of the water to a leaf that hangs above the water’s surface. While she clings to it for a while, she will lay her eggs that also stick to those leaves. Then the male does the same, fertilizing the eggs.

Yet another way is where a special nest is made by one of the parents. For example, some male rockfish (cichlids) make pits in the sand by digging them out (with their beaks and fins). They then lure females, who lay their eggs in that hole. The male fertilizes it, and then guards it. Asian gouramis use saliva to stick air bubbles together to form a bubble nest. The male then entices a female, who deposits her eggs at the bottom of this bubble nest. Here too, the male fertilizes the eggs and then guards the eggs. Sticklebacks are also known for the nest made by the male. It is a kind of circle of algae, in which the female will lay her eggs. This nest is then also guarded.

Well, these are all ways of mating where the eggs are fertilized externally, so in the open water. Many fish have developed mating behaviors that further increase fertilization efficiency through various forms of internal fertilization. In this area, too, many different strategies have emerged in the course of evolution. On the one hand, there is a form of internal fertilization where cavities other than those of the reproductive system are used. For example, there are African beautiful bass (cichlids) that are mouthbrooders. Females will immediately take in their eggs in the mouth, once they have deposited them. It is then important for the males to be able to fertilize those eggs before the female shields them. Well, in some mouthbrooders one sees a spectacular example of evolutionary specialization: males have yellow spots on their anal fin (the fin on the ventral side, at the very back) that are very similar in shape and size to eggs. This is why they are called ‘egg stains’. When a female has laid her eggs, she will therefore take her eggs in her mouth. If the male now swims right in front of the female, the female will also start biting at those egg spots. At that point, the male will release his sperm cells, which are released at an opening just in front of his anal fin. In this way, the sperm cells end up in the female’s oral cavity, where all the egg cells are located. This greatly increases the chance of fertilization.

Another special way is found with seahorses. That’s where the males will get pregnant! Seahorses exhibit complex mating behavior, in which the female will deposit her eggs in the male’s pouch. Once in the pouch, the male will fertilize it, and will hold it there for a long time. As a result, not only is the chance of fertilization increased, but the chance that the eggs will hatch into small seahorses is also very high. After a while the male will also empty his pouch, so that hundreds of seahorses of a few millimeters are born (see photo of newborn seahorse).

But the ‘classic’ way of internal fertilization can also be found in fish, albeit not with the help of a real penis (as in mammals), but thanks to other copulation organs. Copulation organs are found in cartilaginous fish (such as sharks and rays) and in bony fish. Male sharks and rays have a special structure on their pelvic fins (called claspers), which they use to fertilize the females internally. In bony fish, especially the viviparous tooth carp (Poeciliidae) are known as internal fertilisers. Here the males have a reshaped anal fin, which is mobile. During mating, a male will then constantly swim next to a female, bending that fin to insert them into the female’s genital opening. In this way, the sperm cells are transferred directly into the female’s genital tract and the eggs are fertilized internally.

And this is just a selection of the numerous variants that can exist in the realm of fish.