14 DAY TRIAL // 1. Today there are about 21,000 species of bony fish, inhabiting all marine and freshwater environments. Their number is larger than the number of all other vertebrates together. Compare this with about 50 species of lampreys and hagfish (jawless fishes) and about 700 species of sharks and rays (cartilaginous fishes). Of the bony fish, about 6,700 species live in freshwater (33.1 %), 1,625 freshwater species that can live for a time in salt water (8.1 %); migratory species make 120 species (0.6 %). Tropical sea fish living down to 200 m (660 ft) m make about 8,000 species (39.6 %), while those inhabiting temperate/cold shallow water make 1,130 species (5.6 %). About 1,280 species are abyssal (6.4 %) while pelagic species (living in open sea) are represented by about 260 species (1.3 %). Bathypelagic fish live in the water mass below 200 m; they make about 1,010 species (5 %).
2. Freshwater species make about 40 % even if freshwater makes a very small percent of water volumes. This is explained by the geographical isolation forming numerous species. Each lake or river can form an isolated biotope. Examples are the African lakes of Victoria, Tanganika and Malawi, where fish of the Cichlidae family formed hundreds of species. The finches of Galapagos, Drepanide birds of Hawaii or rodents appear poor in species. Some insects genera may have thousands of species, but they are spread worldwide and are much older than vertebrate genera (they can be considered equivalent to vertebrate families).
3. Coelacanths are one of the most primitive types of bony fish. Their fossils range from 400 to 60 million years ago. When Latimeria, the living coelacanth, was discovered in 1938 near Comoros Archipelago (close to Madagascar), this was a shock: it was like finding a living dinosaur. The locals knew the fish, sometimes accidentally captured, but considered it too greasy to be eaten. Unlike other bony fish, coelacanths have lobed fins, containing muscles and bones. Latimeria does not "walk" with these fins; the fish stands sometimes on its head, making alternative swimming movements with its fins. The fish stays in deep waters during the day, and ascends to feed on squids and small fish during the night. Still, ancient coelacanths are believed to have given birth to amphibians.
4. Lungfishes (Dipnoi) also have intermediary traits towards the amphibians. They appeared 300 million years ago. Today, just 6 species survive in South America, Africa and northern Australia. When rivers desiccate during the dry season, lungfish bury themselves into the mud. The African species make a cocoon inside the mud and enter into a state of inactivation called estivation, waiting for the rains, for months and even years. They survive because they have a pair of lungs and circulatory system adapted to oxygenate the blood through the lungs and gills at the same time.
The Australian lungfish does not live in rivers that dry, but these rivers, too, get suffocated with plants and devoid of oxygen during the summer. Breathing air from time to time allows the fish to survive.
5. Bichirs and reedfish live in African rivers and lakes. They too have lungs besides gills, and a fleshy base (like in coelacanths) at each fin. They propel themselves using their fins (other fish, using their Tails) to hunt insects and small invertebrates.
6. Sturgeons form a group of about 20 species, some restricted to freshwater, other feeding in the sea and breeding in freshwater. The largest species, beluga, can reach lengths of up to 9 m (30 ft) and weights of one ton. Such an individual, about 75 years old, would produce 180 kg (400 pounds) of caviar and 688 kg (1,600 pounds) of meat. Sturgeons are not related to sharks, but they have a cartilaginous skeleton and a tail with a larger upper lobe, too. Sturgeons have been exploited for caviar intensively, and dams impede their breeding. Ichthyocol, a gelatin-like substance, was achieved from the swim bladder of beluga and used for rinsing the wine.
7.Over 95 % of the bony fish are represented by Teleostei. Their type is varied, from eels to flounders and sea horses. They have flexible skeletons and completely ossified skeletons. The vertebrae have two pairs of ribs, and the radii of fins are bony and sometimes replaced by spikes. The fins have the role of rudders, the tail delivering the propulsion. The flat overlapped scales found in most species protect of external trauma. The scales mark the fish's age just like the tree rings. Swim bladder ensures floatability. This bladder is an ancestral lung turned into an organ that absorbs gases from the blood and, like a balloon, it allows the fish to inhabit average depths or to remain suspended in the water when not actively swimming. Fish living at the bottom, like flounders and soles, lost their swim bladder.
That of the abyssal fish stands 3.5 tonnes per square cm, and when these fishes are pulled up to the surface, they do not have time to absorb the air from it, so that their bladder will swell and go out through their mouth. In many cases, the swim bladder also has an auditive role, transmitting water vibrations to the inner ear.
Many teleostei fish still can breathe air, even without a lung. The weather fish, loaches and other related species swallow air, and the intestine takes oxygen from it. In some cases, the swim bladder still can be used for breathing. Labyrinth fishes (like gouramis and fighting fish) have a labyrinth organ, a complicated network of blades, in the gill area, that serves for breathing air oxygen. In some tropical fish (like mudskippers), the mouth mucosa serves for aerial breathing.
8.How speedy fish can be? A 1.8 m (6 ft) long red tuna can reach 70 km (42 mi) per hour in short bursts. A swordfish can go up to 92 km (57 mi) per hour in such bursts!
A 1.2 m (4 ft) long barracuda reaches 27 km (17 mi) per hour, while herrings swim with 5.5 km (3.5 mi) per hour. A trout can reach 10 km (6.4 mi) per hour, while a goby moves with 0.95 km (0.6 mi) per hour. A salmon reaches 22 km (14 mi) per hour while jumping.
8. Over 1,000 species of abyssal bony fish produce bioluminescence, for camouflage or to hide their contour line. Angler fish use bioluminescence to lure their prey, while lantern fish, for communicating with the others.
Salmons are believed to chemically recognize the spring in which they hatched. They go upwards, and when the water has the right taste, they spawn. After hatching, in five years, most of the salmons migrate to the ocean, but some males mature precociously in the river and even spawn. This is the insurance policy that the species will survive, if no male returns from the ocean.
The scorpion fish has a more powerful venom than that of a cobra.
Sea horse is the only fish whose head makes a 90o with the body.
9. Plants evolve in many cases through polyploidy (the increase of number of chromosomes). Fish are a rare example of something like this amongst animals. In goldfish (Carassius auratus), the number of chromosomes is double compared to the Crucian carp (Carassius carassius).
10. Despite their large eyes, fish have a rudimentary retina and see less clearly than other vertebrates. The round crystalline lens also projects diffuse images. Fish detects blurredly the shape of the objects as they are short sighted. They do not have 3-D vision, due to the lateral positioning of the eyes, thus they cannot detect relief and distance, and the image is flat. Instead, moving objects are rapidly detected. Bottom species may see almost nothing, but in lighted surface water, other fish can detect objects from a distance of 10 m (33 ft). Immobile objects may not be seen by fish.
Fish see in colors and this ability is linked to the luminosity of the water. The movement of shiny objects above the water (like jewels) warns or attracts the fish. Angle nylon must be mat, not shiny.
The hearing allows the fish to detect the vibrations of the nylon thread of an angle or of the moving lead.