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 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
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
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,
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.
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.
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.
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.