An Overview of the Classification
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.
FAQs on Difference Between Bony Fish and Cartilaginous Fish
1. What is the main difference between bony fish and cartilaginous fish?
The primary difference lies in their skeleton. Bony fish (Class Osteichthyes) have a skeleton made of bone, while cartilaginous fish (Class Chondrichthyes) have a skeleton made of flexible cartilage. This fundamental difference affects many other features like their gills, scales, and buoyancy.
2. What are some common examples of bony and cartilaginous fish?
It's easy to find examples of both in our daily lives.
- Bony fish: This is the largest group of fish and includes common examples like Tuna, Rohu, Salmon, Sea horse, and Catla.
- Cartilaginous fish: This group is smaller and includes predators like Sharks, Rays, and Skates.
3. Can you explain the difference in gills between these two types of fish?
Yes, their gill structures are quite different. Bony fish have their gills covered by a hard, bony flap called an operculum. This cover protects the gills and helps pump water over them. In contrast, cartilaginous fish have 5 to 7 pairs of gill slits that open directly to the outside, without any protective cover.
4. How do bony fish and cartilaginous fish stay afloat in the water?
Bony fish have a special organ called a swim bladder, which is a gas-filled sac that helps them control their buoyancy and stay at a certain depth without needing to swim. Cartilaginous fish, like sharks, lack a swim bladder. They rely on their large, oily livers and must swim continuously to avoid sinking.
5. What are the key features of cartilaginous fish?
Cartilaginous fish (Chondrichthyes) are mostly marine animals. Key features include:
- An endoskeleton made entirely of cartilage.
- Skin covered with tiny, tooth-like placoid scales, which makes it feel rough.
- A mouth located on the ventral (under) side of the head.
- Absence of a swim bladder for buoyancy.
6. Besides their obvious differences, what do bony and cartilaginous fish have in common?
Despite their many differences, both groups share important characteristics as fish. Both are vertebrates with a streamlined body, possess fins for movement, and use gills for respiration in water. They are also both cold-blooded (poikilothermic), meaning their body temperature changes with the surrounding water.
7. Why are the scales of a shark different from the scales of a tuna?
The difference in their scales is linked to their skeletal type. A shark (cartilaginous fish) has placoid scales, which are structurally similar to teeth with a hard enamel covering. This gives their skin a sandpaper-like texture. A tuna (bony fish) has thin, flexible scales (cycloid or ctenoid) that overlap like shingles on a roof, providing protection without adding much weight.
8. How does the position of the mouth affect how these fish eat?
The mouth position reflects their feeding habits. Most cartilaginous fish, like sharks, have a ventral mouth (on the underside), which is well-suited for attacking prey from below. Bony fish typically have a terminal mouth (at the front end of the head), which is versatile for catching prey directly in front of them in the water column.
