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 BONY FISHES AND CARTILAGINOUS FISHES
Fishes belong to two different classes of vertebrates: the cartilaginous fishes (or Chondrichthyes) and bony fishes (or Osteichthyes).
The Chondrichthyes include stingrays, torpedo fishes, common stingrays, eagle rays, mantas and sharks. All the other sea and freshwater fishes, not mentioned above, are bony fishes.
Main feature of the Chondrichthyes is their cartilaginous skeleton, which is not a sign of primitiveness, but of a secondary adaptation.
These fishes maintained a cartilaginous skeleton as they were able to swim also without a bony skeleton. No earth vertebrate developed from these animals, as a cartilaginous skeleton could not bear the body weight.
Therefore all the vertebrates, such as amphibians, reptiles, birds, mammals and even the humans derive from a old fish ancestor.
The majority of the primitive Chondrichthyes estinguished 245 million years ago and the current cartilaginous fishes appeared between the Jurassic and the Cretaceous, between 245 and 65 million years ago. Today they still have many of the past features; some fossils witness the perfect adaptation of the sharks to the environment.
 DIFFERENCES BETWEEN OSTEICHTHYES AND CHONDRICHTHYES
• There are approximately 20.000 species of bony fishes, while cartilaginous fishes are about 600. Bony fishes are the most numerous group of vertebrates, e.g. mammals are about 4.000 species
• Size: the biggest fish is the whale shark, cartilaginous, that can reach a length up to 18 m., but there is also another shark living in depth which is only 10-15 cm. long. Usually, the size of the cartilaginous fishes is bigger; the small species are less numerous in comparison to the bony fishes, which have a few big species (tunas, swardfish, moonfish, etc) but many average-small size species.
• Mouth position: the majority of the bony fishes has a terminal mouth, that is the snout terminates in a mouth, even if sometimes it can have extravagant shapes. The cartilaginous fishes have often a pectoral mouth, located under the snout.
• Tail shape: the bony fishes have a “homocercal” tail, with two lobes of the same length or joined together; the cartilaginous fishes have an “eterocercal” tail, with a longer upper lobe: this shape counterbalance the boost upwards of the huge and rigid pectoral fins.
• Skin: the skin of cartilaginous fishes is covered by curved scales, made up by flesh, dentin and enamel, just like the mouth teeth. On the contrary, the bony fishes have rough bony scales covered by skin.
• Colour: not many other vertebrates can compete with the amazing colours of the bony fishes, especially of the tropical ones. The cartilaginous fishes, instead, are mainly dark, some of them have spots or designs, but colours go from brown to beige to blue.
• Teeth: the teeth of the cartilaginous fishes are not fixed in the jaws, but to the gums, which can be lost and replaced several times (sharks). They can also have dental plaques. Bony fishes instead have different types of teeth fixed in the jaws.
• Sense Organ: all fishes have the “lateral line”. This organ stretches to the sides and on the head, is made up of many little holes linking the external layer with an internal nerve, which reads the water information: salinity, temperature, dissolved substances, etc. Cartilaginous fishes have also the “ampullae of Lorenzini”, an organ which reads the electrical fields and which allows these animals to find their preys in the dark, under the sand or in water with limited visibility and to read the sea currents. Sharks have also a more developed hearing, olfaction and sight in comparison with other bony fishes.
• Swim bladder: almost all bony fishes have this organ, except those living on the ocean bottom. This organ, when filled with gas, allows the fish to float easily, remaining still at the wished depth without any effort. The cartilaginous fishes have no swim bladder, even the big pelagic sharks. This is the reasons why sharks must continue swimming if they want to remain distant from the ocean bottom.
With no swim bladder they must continuously move to avoid sinking.
• Reproduction: bony fishes have an external fertilization and release thousands or millions of eggs, giving birth to larvae, usually unattended by parents. Cartilaginous fishes have instead an internal fertilization (by means of modified fins, the male transfer the sperm into the female body) and therefore have a real copulation. Thanks to these modified and longer fins, it is possible to distinguish males from females. This is impossible with the bony fishes, as both males and females have identical fins (only in some species, sometimes, fins have a different colours). Cartilaginous fishes release big cheratine eggs, which give birth to completely created small fishes. The majority produces live young, that is: the female keeps the babies in its body and then gives birth once they are completely autonomous.
• Gills: bony fishes have an operculum, that is a bone on the two sides of the head covering the gill chamber; cartilaginous fishes have instead five gill slits at the sides of their head and no operculum.
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