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Reproduction
Not a whole lot is known about piranha reproduction, as
observations made in the wild are few in numbers. Almost everything we know
about reproduction is based on observations made in aquariums, and only of a
few species that so far have successfully reproduced in captivity. Pygocentrus
nattereri (including the "Ternetzi" variant), Serrasalmus
maculatus and Serrasalmus spilopleura, and more recently (2002) Pygocentrus
cariba, have reproduced in home aquariums. Additionally, some other
species, like Serrasalmus rhombeus, have been observed to reproduce in
large public aquariums and zoo's.
The description below is based on the breeding of Pygocentrus nattereri,
the species that has been bred most extensively in captivity. But this can also
be applied to the other Pygocentrus species, and perhaps to other species as
well, although Serrasalmus courtship and spawning might differ, since most of
the species are living solitarily and are very aggressive, even towards their
own species. It is suspected that (certain?) Serrasalmus species release a
certain kind of hormone into the water, thus signaling that the fish is ready
to spawn and thus lowering aggression levels.
To trigger breeding, all circumstances must be right. First of all, the tank
must be in a quiet environment. Besides that, the fish must be compatible (that
means: a group that has been together for a long time, with a developed social
hierarchy). Furthermore, the water quality must be perfect (zero ammonia and
nitrates, excellent pH and a temperature of about 82 degrees), and the tank
must be large enough for the fish to be able to pair off and form a territory
of their own, without too much disturbance. And last but not least you just
need a large dose of good luck...
When a pair is ready to spawn, they form a little territory from which other
fish are aggressively chased away. Both fish become much darker in color, and
their behavior becomes more territorial and aggressive. In their territory, the
fish start to build a nest by on the bottom, by clearing all plants and
removing stones etc. with their tails and by blowing it away. When the nest is
ready, the male tries to lure the female over the nest. Here she will lay here
eggs, which will be fertilized quickly by the male. After the spawning, the
male guards the nest, and chases off every fish that comes too close. Sometimes
the female will be expelled, but sometimes she helps with the guarding.
The orange-colored, fertilized eggs will hatch in 2-3 days. The fry's yoke sack
provides nutrients the first couple of day. A few days later they will start
swimming around freely. This may be the best moment to remove the fry, to raise
it in a breeder tank. At this moment, the parents may still aggressively guard
their offspring, so be careful when syphoning off the fry.
 
Young Redbellied Piranha's are silver in color, their body
covered with small black spots. The shape of the head is much more concave than
adults and they have very large eyes. Juvenile Pygocentrus piranha's can be
pretty hard to tell apart from baby Serrasalmus species, because they very much
look the same, making proper identification a difficult task.
Even at young age, piranha's are very greedy eaters. They should be fed newly
hatched brine shrimp the first days, and their diet can be complemented with
flakes, blood worms, mosquito larvae and other small foods when older. And they
should be fed often, at least 2-3 times per day, to keep them healthy and
properly developing. Juveniles grow fast: in their first months the growth
speed can increase to half an inch to an inch per month, and soon their diet
can be complemented with small feeders (I used Neon Tetras), pieces of fish,
shrimp etc. Beef heart should not be fed until the young fish have reached an
age of at least 9 months, because of their high contents of fat and fibres.
This puts a lot of strain on their not yet fully developed digestive system,
and constipation may eventually turn out fatal for the young fish. The same is
true for adult piranha's as well, although an occasional treat of beef heart
(once per month) is unlikely to cause any problems.
In a few months, they start to develop red coloration in their fins and belly,
the black spots start to fade, and the silvery color of their body is gradually
replaced by the more (bluish or greenish) steel grey which is so characteristic
for adult specimen. The growth rate remains high until the fish reach a length
of about 4-6 inches: by then, the fish are about 12 to 14 months old, depending
on the circumstances they live in: tank space, diet, water quality, the stress
factor, the presence of water current from a powerhead. After that, a Red
Bellied Piranha's growth slows down to about 1" per year (and even less
once they hit the 9-10" marker) and reach sexual maturity when then are
between 18 and 24 months old, once again depending on the factors mentioned
above. The older the fish, the darker it will get. Some specimen will almost
completely loose their red belly and turn completely pitch black (see picture
above), sometimes because of old age, sometimes because of stress, and
sometimes to signal they are ready to spawn.
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