There are approximately 34,000 known fish species throughout the world which live in either fresh or saltwater environments. They are cold-blooded animals but only the opah (Lampris guttatus) is warm-blooded fish which is also known as cravo, kingfish, moonfish, and Jerusalem haddock.
There are two main groups of fish, namely Chondrichthyes and Osteichthyes based on the composition of the endoskeleton. The endoskeleton of Chondrichthyes is made up of cartilages and they mainly live in marine habitats while the endoskeleton of Osteichthyes is composed of bones which are found in both marine and freshwater habitats.
It is a diverse group which contains more than 700 species, found throughout the world's marine environments. This group includes the sharks, rays, and skates, and their skeleton is made up by rubbery cartilage, which is very light and flexible. The world`s largest member of Chondrichthyes is the plankton-feeding whale sharks (Rhincodon typus) which can grow up to 18 meters (60 feet) in length with 21.5 tonnes in weight and live up to 130 years. The main characteristics of chondrichthyans fish include jaws, paired fins, paired nostrils, placoid scales, and two-chambered hearts.
General Characteristics of Chondrichthyes
The class Chondrichthyes is divided into the following two subclasses:
Characteristics of subclass Elasmobranchii
Examples: Sharks, rays, and skates.
Characteristics of Subclass Holocephali
Examples: Rat fishes (Chimaera), rabbit-fishes (Hydrolagus) and elephant-fishes (Callorhynchus).
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It is the largest class of subphylum Vertebrata which contains approximately 28,000 known species that comprise 96% of all fishes. At the embryonic stage, the endoskeleton is cartilaginous but in the adult stage, endoskeleton is replaced by a bony skeleton. They live in both marine and freshwater environments. The largest bony fish is the ocean sunfish or common mola (Mola mola) in the world, which grows up to 2.3 meters in length and 2300 kg in weight while the smallest fish is the dwarf pygmy goby (Pandaka pygmaea) which can grow up to 9 mm (female) and 15 mm (male) in length. Class Osteichthyes has two groups: ray-finned and lobe-finned fish where the ray-finned fish bears a single dorsal fin while lobe-finned fish bears two dorsal fins.
General Characteristics of Osteichthyes
Examples: Cyprinids, Perches, flying fish, globefish, sea horses and eels, etc.
Living Osteichthyes are divided into the following three subclasses:
Characteristics of subclass Dipnoi
Examples: Australian lungfish (Neoceratodus forsteri) South American lungfish (Lepidosiren paradoxa), West African lungfish (Protopterus annectens), etc,
Characteristics of subclass- Crossopterygii
Example: Coelocanth (Latimeria chalumnae)
The members of the subclass Actinopterygii are known as ray-finned fishes. They are the most diverse and the largest group of fishes with a wide variety of shapes, sizes, and colors which comprises half of all the living vertebrates. There are approximately 24,000 known species under 42 orders with 431 families. Among them, Teleosts comprise approximately 23,000 species and 96% of all living fish species.
Characteristics of subclass Actinopterygii
Examples: The most familiar fish species of Actinopterygii includes, sturgeons, anchovies, gars, eels, bass, carp, herrings, cichlids, catfishes, oarfish, goldfishes, pickerel, piranhas, seahorses, salmon, trout, etc.
Difference Between Chondrichthyes and Osteichthyes
They are also known as cartilaginous fishes.
They are also known as bony fishes.
They inhabit in the marine environment. Sometimes many cartilaginous fishes enter into freshwater environments.
They inhabit in both marine and freshwater environments
Endoskeleton is made up by the cartilage.
Endoskeleton is made up by bones.
The body is covered by very small denticles, known as placoid scales.
If present, scales are various types such as ganoid and laptoid scales (cycloid and ctenoid).
Generally, they have a large-sized body.
Comparatively, they are small-sized fish.
Their mouth is located at the anterior tip of the body. This type of mouth is known as terminal mouth.
Numbers of Gills
They have 5-7 pairs of gills.
They have 4 pairs of gills.
Gill cover or operculum
The gills are not covered by gill cover or operculum.
The gills are covered by an operculum on each side.
They do not have swim bladders.
They have swim bladder for buoyancy. It is also known as air bladder or gas bladder.
They have heterocercal type caudal fin. In this case, lobes are not equal.
They have homocercal type caudal or tail fin. In this case, both lobes are equal in size.
In many cases, internal fertilization occurs.
In most cases, external fertilization occurs.
Their excretory products are urea (CH4N2O)
Their excretory products are mainly ammonia (NH3).
Generally, they are carnivores in nature.
They may be omnivores, carnivores, herbivores, filter-feeders or detritivores.
Skates, sharks, and rays.
Tuna, Dolphin fish, Barramundi, Salmon fish, trout, rohu, seahorse, etc.
Fish are the cold-blooded aquatic organisms which belong to the subphylum Vertebrata of phylum Chordata. There are two main groups of fish based on the endoskeleton, namely Chondrichthyes and Osteichthyes. The endoskeleton of the members of Chondrichthyes is made up of cartilage while the endoskeleton of fish under the class Osteichthyes is composed of bones.
Fish are very important to human beings because they provide nutrients and micronutrients. These nutrients are essential for the physical development of children. It is also an important part of a healthy diet of all stages of people and controls many diseases. They are an important source of income and employment for the people of many developing countries.
At present, 1245 (8.1%) fish species are listed as vulnerable (IUCN). Among them, 121 species belong to the cartilaginous fish and 1114 species are ray-finned fish. Fish become vulnerable due to pollution, habitat destruction, diseases, over-exploitation, etc. To protect our fish species, we should take proper conservation measures.