The gender of fish

In theory, this is a simple matter: there are males and females. In practice, the situation is sometimes less clear, especially in marine fish. In fact, some species can change sex: this is known as successive hermaphroditism, and can be seen in groupers, Anthias, rainbow wrasses, and some Labnds, which are first female and then male. Clownfish present male characteristics when they are young, but they cannot reproduce at this stage. When they mature they become either a fertile male, or a female. As these are species in which individuals live alone, changes in sex increase the probability of forming a pair. This phenomenon has a similar basis in those species which adopt the harem system, i.e. a male living with several females: if he should die, one of the females can become a male to ensure the continuing survival of the group as a whole.

Some fish lay their eggs on a horizontal support, then ventilate and protect them from therapaciousness of other species.

The vitellin sack of these15-day-old larvaehas been practicallyall used up. •

THE VITELLIN SACK

The vitellin sac, or vitellin vesicle, constitutes the fish larva's food reserves in the first days of its life. It derives from the vitellin of the egg - the equivalent of the yolk in poultry -accumulated by the female over the course of the maturing process. It is sent along the blood vessels, which distribute these reserves around the body of the larva. This allows it to survive until it is able to catch prey itself, i.e. a few days after emerging from the egg.

The vitellin sack of these15-day-old larvaehas been practicallyall used up. •

THE REARING TANK

This is normally modest in size, as it is used to isolate a pair of reproducing fish and their future offspring. There is no need for a bed, except for some species (the Cichlasomas, for example). A rearing tank must be heated but only moderately filtered. Avoid any systems which may "swallow" the eggs and the fry. Aeration is not obligatory. Fill the tank with water taken from the parents' aquarium, which can then be gradually modified to obtain the precise characteristics required by the reproducing fish (pH and hardness). A support should be provided for the egg-laying, and this can be either natural (plants, rocks) or artificial (PVC). More detailed information on this subject, family by family, can be found below.

This female Cichlid, which incubates her eggs in her mouth, finds greater peace in a rearing tank. •

Sometimes reproduction takes place in a communaltank, catching the aquarist unawares.

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