Fish reproduction

Practicing diving can take you to the most special corners, where you can meet different animal and plant species.

Fish have various reproductive strategies, some of them really curious. The objective, in any case, is the same: to perpetuate the species, even if this sometimes costs them their lives.

Types of reproduction

Most fish present oviparous reproduction, that is, expelling eggs outside. Although there are also viviparous fish, such as some sharks and some freshwater fish, in which the embryo develops inside the uterus of the female. Other species, such as the tiger shark, are ovoviviparous; These are fish that are born from eggs that hatch inside the female.

External fertilization is the most common in fish; The females expel the eggs and the males the sperm into the water, where fertilization takes place. In this case, many eggs are at risk of being eaten by predators. Other fish such as coelacanths and cartilaginous fish (sharks and rays) however, present internal fertilization. Male sharks and rays have two copulatory organs, the pterygopods, which function as penises. This type of reproduction is safer and the number of offspring is smaller, since the probabilities of survival are much higher.

Hermaphroditism
Virtually all species of fish have separate sexes, male and female. Many species, however, are hermaphrodites, of which most are sequential hermaphrodites, that is, they present a sex reversal as part of their sexual development. Somereach the stage of maturity as females and later become males, these are proterogynous hermaphrodites, such as parrotfish and wrasses, including the maidenheads (Coris julis) or the long-nosed wrasse. (Gomphosus varius). Species that begin their adult life as a male and then become female are called proterandric hermaphrodites, as is the case with clownfish, gilthead sea bream (Sparus aurata), etc.

Different species, different reproductions

There are very few vertebrates that show simultaneous hermaphroditism, that is, during the laying period they can act as both males and females. An example is found in the mountain tree (Serranus scriba), with a quite spectacular nuptial stop. Two individuals rush towards each other arching and one of them fully deploys its fin dorsal adopting a posture like that with your body. If the other individual responds, they rise a few meters to the surface to expel the eggs and sperm.

Attract a partner

Fish also perform nuptial rituals or dances to attract the opposite sex, especially those in which copulation occurs. During courtship, for example, male sharks swim close to the cloacas of females, nibbling their pectoral fins, a fact that predisposes the females to copulate until finally, the male bites the female's pectoral fin to hold on and ensure copulation. . Sometimes a second male has even been observed following the couple, to prevent the female from escaping, as happens with nurse cats (Ginglymostoma cirratum).

Pipefish (Sygnanthus typhle) also show a particular reproductive behavior: the female performs dances around the male and then, facing her belly with his, she introduces about twenty eggs into the male's incubator bag, which will fertilize them. and it will incubate them, similar to what happens with seahorses.

Some species make sounds with their swim bladder in order to attract their partner. This is quite useful, especially in deep areas where lighting is poor. Some abyssal species have luminous organs, a phenomenon known as bioluminescence, which in some cases serves to attract the opposite sex.

Protect offspring

Although most fish do not protect their offspring by releasing their eggs into the water, there are some that make an effort to build nests in the sand or even collect algae, as does the wahoo (Symphodus tinca). In this case, the male builds a nest of algae where the female attaches her eggs and subsequently fertilizes them.

The garibaldi, an inhabitant of the kelp forests of the California coast, also builds a nest and manages to attract females to it. Then it takes care of the eggs until they hatch, constantly moving the fins so that the water is renewed and thus oxygenates them.

There are fish that incubate their eggs in an incubator bag like the seahorse or in other parts of the body. In the case of the wren (Apogon imberbis) or cardinal fish (family Apogonidae), it is the male who incubates the eggs in his mouth. Other species carry the eggs in the gill chambers.

Metamorphosis

Some fish undergo a major change during their growth. A very clear change is that suffered by flat fish such as flounder, sole or turbot, inter alia. The fry are pelagic, that is, free-living, and look similar to any other fish, they are symmetrical with an eye on each side of the head. As they grow, their body flattens and one of the eyes migrates until it is placed next to the other. It is at this time that these fish begin their benthic life, on the seabed. The two eyes are on the same side of the body, to be able to observe their surroundings while resting on the sand.

Territorialism

For nesting species it is essential to defend the territory. Some species show very pronounced territorialism, like many angelfish. Adult males defend their territory and do not allow any other males to enter. For this reason, it is common for juvenile angelfish to have another livery, that is, another coloration. In this way, territorial males allow other young males to be in the same territory. At the moment when these young people reach sexual maturity and adopt their adult coloration, they will have to look for a new territory where they can attract females and have offspring.

Color changes

There are species that present different liveries throughout their development, such as the student (Chromis chromis), which in the juvenile stage shows an electric blue color and when they are adults they adopt a dark coloration. The trumpeter (Macroramphosus scolopax) lives in coastal areas between 50 and 150 meters, its color is reddish, a color that serves as camouflage at depth. The young, however, are pelagic and in order to camouflage themselves on the high seas, they look blue with a silvery belly.

Some fish, such as the julia (Coris julis), the green fish (Thalassoma turkey), as well as many wrasses, exhibit sexual dichromatism. Males and females are distinguished externally as they show different colored liveries. Normally males wear more striking colors to attract females.

Migrations

Some species make long trips for reproductive purposes. Migratory fish that move between the sea and fresh waters for reproductive purposes can be classified into two groups: anadromous and catadromous.

Salmon are an example of an anadromous species, which lives in the sea, but at the end of its life goes up rivers to spawn. It is a very efficient swimmer, which swims up rivers with strong currents and can even overcome obstacles of four or five meters. Eels, catadromous migrators, make the trip in the opposite direction. They live most of their lives in rivers and, inter alia. The fry are pelagic, that is, free-living, and look similar to any other fish, they are symmetrical with an eye on each side of the head. As they grow, their body flattens and one of the eyes migrates until it is placed next to the other. It is at this time that these fish begin their benthic life, on the seabed. The two eyes are on the same side of the body, to be able to observe their surroundings while resting on the sand.