EGG-LAYING

One encouragement for egg-laying is light, whether it is the morning sun or the aquarium's artificial lighting. Also, if you think that your fish are taking a long time to lay, you can change around 10% of the water volume, and this may provide them with the stimulus they need.

Fish with free or non-adherent eggs

When the eggs fall to the bed, they must be protected from the voracity of other fish - and this can be done in a normal aquarium. There are a couple of ruses that are useful in this situation:

• divide the aquarium into two horizontal sections by means of a grille which only lets through the eggs;
• cover the bed of the aquarium with glass marbles - the eggs will slide between them and can easily be recovered by siphoning them off.

Most fish that produce free eggs do not take care of them, and the parent fish must therefore be removed from the tank to ensure that they do not devour them. There are some species within the Cyprinodontid family whose eggs can survive outside water, provided they are kept within a moist environment.

CARING FOR THE PARENTS

Successful reproduction depends on the good health of the reproducing fish, and to achieve this they have to remain undisturbed, and therefore isolated, in a rearing tank. Their feeding is especially important, as the quality of the eggs, and above all that of the vitellin, depends on it. Fresh food, particularly live prey, must be provided, with an emphasis on variety. Although artificial food can be used as a substitute, it must never constitute the basic diet.

There are several tricks for shielding the eggs from the voracity of their parents: the grille system (A) and glass marbles (B).

SEMI-NATURAL REPRODUCTION

Semi-natural reproduction can be carried out with those fish which normally live in groups and do not form stable pairs, such as the Characins. A group of ten can be inserted into a rearing tank adjusted to their requirements and left to reproduce. There are then two possible scenarios:

• the adult fish are removed and the aquarist looks after the fry;
• the adults and the fry are left together, so hiding places must be provided for the young - plants on the water surface, rocks, etc. The fry are not moved until they measure 5-10 mm, when they are transferred to another tank to grow into adults. The remaining adults can reproduce again as soon as the maturing phase has passed. The advantage of this method is its relative ease: one or several pairs are allowed to form and they can reproduce whenever they want. Leaving them with the eggs and fry allows a type of natural selection to come into play, whereby only the strongest and most agile will be able to escape the predatory instincts of their parents.

The semi-natural reproduction method can be used for neons (Paracheirodon innesi).

The semi-natural reproduction method can be used for neons (Paracheirodon innesi).

Fish with adhesive eggs Fish such as the Characins that do not look after their offspring lay them on fine-leaved plants (like Myriophyllum and Cabomba), which you should make available in the aquarium. Alternatively, it is possible to use a web made of Perlon wool, which is commercially available for use in filters. This provides a very good artificial support. The parents must be removed after the eggs are laid.

Fish which lay nonadhesive eggs deposit them on a support (above), in a natural or artificial cavity (center), or in a depression hollowed out of the sediment (below).

Some species not only look after their eggs, but also sometimes their fry. Some, such as the scalares, lay their eggs on a plant with hard leaves, while others, such as certain American Cichlids, do so on a vertical or horizontal rock face. An artificial laying support can also be provided, by using a PVC tube or plate, for example.

The female lays her eggs on a support which has been cleaned beforehand. They will be immediately fertilized by the male.

Other Cichlids - Africans from Lake Tanganyika or Americans, such as species of the Apistogramma genus - lay their eggs in a cavity or small cave, sometimes even on the ceiling, if this is the only suitable place. In all cases where the eggs are laid on a rigid support, this is generally cleaned by one of the parents before the female deposits the eggs that the male will go on to fertilize.

The male will then defend the site against any possible enemies, while the female ventilates the eggs with her fins. There are some occasions when the parents swap roles, and others when one of the two even separates out dead eggs, distinguishable by their opaque white color.

A In the case of Cichlids that use oral incubation, the fry can return to the shelter of the female's mouth (here Geophagus steindachneri).

The fry are then supervised by their parents until they are independent.

Mouth brooding

This reproduction technique, used in particular by the African Cichlids in Lake Malawi, ensures a noteworthy survival rate for the eggs and fry, both in the wild and in the aquarium.

The males, generally more intensely colored than the females, have small, brightly colored patches on the anal fin. The female lays the eggs on the bed and immediately gathers them up in her mouth. Nearby, the male flaunts his anal fin and rolls around on the spot. The female then tries to grab the patches at the same time as the male ejects his sperm, which the female inhales. The fertilization therefore takes place inside her mouth.

The embryos develop for 1-3 weeks and during that time the female does not eat. The swelling formed by the eggs in her "throat" is clearly visible. She moves the eggs around in her mouth in order to help them to hatch.

The fry gradually emerge from their mother's mouth, but dart back inside at the slightest sign of danger, and the mother continues to take care of them until they can look after themselves. This technique means that these fish can reproduce in a mixed tank. However, it is possible to remove the female delicately and, with great care, make her "spit out" her fry in order to raise them in another tank separately.

The fry can return to the shelter of the female's mouth, although this is very unusual if she is solated in a rearing aquarium.

The bubble nest

Fish from the Belontiid family construct a nest of bubbles under the surface of the water, where the oxygen concentration is higher, compensating for the low oxygenalion in their living environment. This refuge also affords the fry a certain degree of protection.

Bubble nest built by a male Colisa lalia (Belontiids) on the surface of the water.

The male builds the nest by taking in air at the surface and forming bubbles that are stuck together with his saliva. Then he sets out to seduce the female, clasping her near the nest and fertilizing the eggs she lays, before putting them into the nest with his mouth. This operation is repeated several times. The male then repulses the female, sometimes with great violence - in which case she must be removed from the aquarium to protect her from his aggression - and he watches over the nest. After hatching, the fry stay inside for a few days, and then they disperse. The male must be removed, as he is capable of eating them.

Livebearers

These do not lay eggs; instead, the latter develop and hatch in the female's abdomen. The fry are therefore born alive and completely formed, and they immediately start to search actively for their own food. This type of reproduction is found in only a few fish families, such as the Poeciliids, so these are sometimes considered to be more highly evolved than other families of fish.

The different phases of reproduction in Belontiids: construction of the bubble nest by the male (the female is in a separate compartment), the laying of eggs, the removal of the female.

Marine fish

Few marine species reproduce regularly in aquariums. Apart from the clownfish and other related species, any success is an exception. Scientific research in this field still has a great deal of ground to cover. The use of hormones, widespread in fish farming, is proving promising, but unfortunately it is not as yet an option open to amateurs.