Difference 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, eg. 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 10-15cm long. Usually, the size of the cartilaginous fishes is bigger; the smaller species are less numerous in comparisons 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 have 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 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 different colours). Cartilaginous fishes release big cheratine eggs, which give birth to completely create 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.