INTERNAL ANATOMY

The sum of the internal organs accounts for roughly 50 to 60% of the body weight in a classically shaped fish.

The brain

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.

The skeleton

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.