Difference Between Bony Fish and Cartilaginous Fish

Hundreds of millions of years of evolution have resulted in the formation of two varieties of fish that appear to be identical on the surface. The makeup of their skeletons, however, is the most visible difference between the two species of fish. Bony fish, as the name implies, have a skeleton built completely of bones, whereas cartilaginous fish have a skeleton made entirely of cartilage. There are around 20,000 different types of fish on the globe. Pisces is the superclass of the Chordata class. The explanation for this is that fish makeup more than half of the chordates. Fish are classed as Osteichthyes or Chondrichthyes in this category (bony fish or cartilaginous fish). 

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Although phylum chordata includes a lesser number of species as compared to non- chordates, yet chordates show advanced evolutionary characters. This phylum derives its name from a structure, named notochord, and surely classifies Homo sapiens under its domain. The term chordata originated from two words of the Greek language, the chorda and the ata. The meaning of ‘chorda’ is a thick string and the meaning of ‘ata’ is to have, so an overall meaning of chordata is animals having notochords. Notochord supports the body of these animals. So, chordates are the animals in which notochord is present in any stage of their life-span. Animals that do not have notochord are known as non- chordates. In chordates, species of maximum live animals is Pisces group and minimum live animals is Amphibia group.

Classification of Cartilaginous Fish

Phylum: Chordates

Class: Chondrichthyes

Features of Chondrichthyes

The features of Chondrichthyes are briefly discussed below. 

This class includes Cartilaginous fishes and they are marine in nature. Their endoskeleton is made up of cartilage and notochord is persistent throughout life. The exoskeleton over the skin is made up of placoid scales and these scales are like denticles and originate from the dermis of the skin. Their teeth are modified placoid scales, which are backward in direction. In these fishes, 5 - 7 pairs of gills are present, which open directly outside the body by gill slits. The operculum is normally absent in these fishes. In them, the mouth is present at the ventral surface of the head. 

Jaws and teeth are present. Suspensorium of jaws is Hyalostylic type. Their Jaws are very powerful and animals are predaceous in nature. Air bladders and lungs are absent in them, as they have to swim constantly to avoid sinking. A spiral valve or scroll valve is found in their intestine to increase surface area along with this cloacal aperture is also present. Their liver is mainly bilobed. There is a special structure at the dorsal surface of the head in these fishes, which is called “Ampullae of Lorenzini” that works as a thermoreceptor. The genital duct of these organisms opens into the cloacal aperture. The mode of fertilization is internal as male fishes have “claspers” as copulatory organs, which are developed by the inner edges of pelvic fins. Most of them are Viviparous in nature.

Classification of Bony Fishes

Phylum: Chordata

Class: Osteichthyes

Features of Osteichthyes

The features of Osteichthyes are briefly discussed below. 

This mainly includes bony fish. Fishes of this class are found in freshwater as well as marine water. The endoskeleton of these fishes is made up of bones, so these are called bony fishes. Their exoskeleton is made up of scales, which may be cycloid or ctenoid or ganoid type. Placoid scales are absent. In these types of fishes, respiration occurs by 4- pairs of gills. These gills are covered by an operculum at each side of the body. Their mouth is normally terminal or subterminal, teeth are found in jaws. The Suspensorium of the jaw is autostylic.

Helping respiratory organs in them is mainly air bladders. Lung fishes respire through air bladders. In other fishes, these air bladders are hydrostatic which help in maintaining a balance of the body and provide buoyancy. The spiracle is absent in them along with the scroll valve of the intestine. Cloaca is absent, in place of cloacal aperture annus is present, also no ampulla of Lorenzini. Their liver is trilobed and their tail is normally a homocercal type but sometimes it may be diphycercal type. Fishes are oviparous but may be ovoviviparous or viviparous.

Difference Between Bony Fish and Cartilaginous Fish

To conclude we can say that we have developed an understanding of the differences between bony fishes and cartilaginous fishes. We have also learnt about some of the important features of both the subsets of fish.