The sum of the internal organs accounts for roughly 50 to 60% of the body weight
in a classically shaped fish.
This is fairly simple in fish, when compared to other more evolved animals. The
parts corresponding to sight and smell are particularly well developed, demonstrating
the importance of these two senses.
Obviously, this supports the fish's body, but it is less sturdy than that of
a land animal, as a fish, partially freed from gravity, is "carried" by the water.
Nevertheless, the relative fragility of the skeleton is a handicap and it is not
uncommon to find fry that emerge from their egg "twisted".
The respiratory and circulatory system
This system is highly distinctive. The blood loaded with carbon dioxide is pumped
by the heart to the branchiae, where it is oxygenated. Nature has provided fish
with eight branchiae (four on each side), each made up of two leaves. The total
surface area of these essential organs, when spread out, would be nearly equal to
that of the fish's body. The vivid red color of the branchiae is due to their abundant
irrigation of blood; a darker color is a symptom of a respiratory problem. The branchiae
are fragile organs, susceptible to damage from suspended sediment or parasites,
resulting in a reduced intake of oxygen, with all its unfortunate consequences.
After traveling through the branchiae, the vivid red blood, rich in oxygen, irrigates
the body, and the oxygen goes on to break down the foodstuffs in the organs.
The fish in the Belontiid family, which live in water in which the oxygen is
sometimes rarefied, have a special organ for trapping atmospheric oxygen.
The digestive system
This has no special characteristics, apart from the fact that the stomach can
stretch to hold large prey, especially in carnivorous fish. This is where digestion
starts, and it then continues in the intestine. With large prey the process can
last several days, but in an aquarium, with artificial food, it will not take more
than a few hours.
The excretory system
This allows undigested matter to be evacuated through the anus in the form of
excrement, or feces. The urine is formed in the kidneys, situated under the spinal
column; it is evacuated through the urinary pore. It is worth mentioning that fish
also excrete nitrogenous substances via the branchiae. All excreted substances contain
nitrogen and are toxic for animals, but in a well-balanced aquarium they are eventually
converted into nitrates and thus cause no harm.
The swim bladder
Fish have a swim bladder, also known as an air bladder. This is an organ connected
to the digestive system, which fills up with gas and helps fish to regulate their
flotation when moving between two different types of water. They empty it to dive
and fill it up when they need to come nearer the surface. Bottom-dwelling fish generally
have a smaller swim bladder, or none at all, as they rarely swim in open water.
In Poeciliids (livebearers), the anal fin in the male (below)
is transformed into a coupling organ.
The reproductive organs
Males have two testicles that are linked to the vas deferens. while females have
ovaries extended by the oviducts. In both cases the sexual products - the spermatozoa
and ova - are expelled via the genital orifice. As the fertilization of the eggs
is external and takes place in the water, there are no organs for coupling and fertilization,
except in the case of livebearers.
DO FISH RECOGNIZE THEIR OWNER?
Countless aquarists have noticed that some of their fishes react more enthusiastically
to their presence than to that of strangers. If they do "recognize" the person who
looks after them, how do they do it? They are capable of distinguishing the special
characteristics of the vibrations caused by the footsteps of such and such person,
which are transmitted to the water of the aquarium. Not only that, their sense of
sight, although not perfect, helps them in this task of "recognition."
Which of the two recognizes the other first?
DO FISH SLEEP?
When an aquarium is abruptly switched on in the morning, it is noticeable that
its occupants do not immediately resume their normal activity. Some of them are
on the bottom of the tank, some in the plants, and others remain almost immobile
in the water. It is difficult to speak of sleep in the generally accepted sense
of the word, but it is certain that fish have periods of restfulness, of varying
degrees. This can be verified at night, with the aid of a small flashlight: the
fish are practically stationary (apart from nocturnal species), but their eyes are
not closed as they have no eyelids.
DO THEY FEEL PHYSICAL PAIN?
The sensation of pain is sent to the brain via the sensory nerves. As fish are
endowed with the latter, it can be assumed that they feel pain when they are hit
or wounded, and perhaps even when they are sick.