Chondrichthyes vs Osteichthyes
Chondrichthyes and osteichthyes are the main two taxonomic classes of the fishes, cartilaginous and bony fish respectively. These two together make up almost all the fish species happen to exist on the Earth. Altogether, there are 28,000 species of bony and cartilaginous fish. They exhibit a range of differences between them that makes it interesting to perform a comparison.
Chondrichthyes (Cartilaginous Fish)
In chondricthyes, or in cartilaginous fish, it is a cartilage skeleton rather than bones as the name indicates. Sharks, skates, rays are the prime example for living cartilaginous fish. There is no connection between their upper jaw and skull so that they can move it independently. The skull comprises of 10 cartilaginous parts, and they have eyelids to protect their eyes. Cartilaginous fish do not have ribs and bone marrow. Therefore, the production of red blood cells takes place in the spleen. Dermal denticles cover the whole skin and those are similar to the structure of our teeth. The mouth is sub-terminal, i.e. located ventrally in cartilaginous fish. They do not have an operculum to cover the gills, and there are five to seven gill slits that are exposed to exterior at all times. Their caudal fin is not symmetric, and the two lobes of the fin are unequal in size.
Another interesting feature is that their pectoral fin is parallel to the longitudinal axis of the body, and which helps them to balance their body rather than providing power to swim through the water column. Their lightweight skeleton along with the oil-filled liver provides buoyancy against the heavy body. Their heavy weight could crush the internal organs outside water (e.g. shark). They excrete urea as the nitrogenous waste product. They are living fossils as cartilaginous fish started to evolve before 420 million years, and presently there are over 970 species living in the sea.
Osteichthyes (Bony Fish)
Osteichthyes is the taxonomic class of fish with the internal skeleton is made up of calcified and ossified bones; hence, they are widely called as bony fish. Their upper jaw connects with the skull, and the skull has 63 tiny bony parts. Bony fish keep their eyes always open as they do not have eyelids. They have scales covering all over the body, and the caudal fin is symmetric. In addition, their pectoral fin is perpendicular to the longitudinal axis of the body. Bony fish have a gas-filled swim bladder, which is useful for buoyancy. They have a flap called operculum to cover the gills. Bony fish excrete ammonia as their nitrogenous waste product.
Bony fish inhabit both freshwater and saltwater, and there are more than 27,000 extant species of them. Moreover, bony fish account for more than half of all the vertebrate species on Earth.
What is the difference between Chondrichthyes and Osteichthyes?
• With more than 27,000 species, osteichthyes is more diversified than chondrichthyes, which is composed of less than 100 species.
• The internal skeleton is made up of cartilages in Chondrichthyes, whereas it is a bony skeleton in osteichthyes.
• Chondrichthyes fish keep their gills open while osteichthyes fish keep those covered by an operculum.
• Chondrichthyes fish can move their upper jaw freely from the skull, but not the Osteichthyes fish.
• Outer covering is scales in osteichthyes, whereas dermal denticles cover the skin in chondrichthyes.
• Caudal fin is asymmetric in chonricthyes fish, but it is a symmetrical caudal fin in Osteichthyes.
• Pectoral fin is parallel to the longitudinal axis of the body in chondrichthyes, whereas it is perpendicular to the longitudinal axis of the body in osteichthyes.
• Chondrichthyes has a sub-terminal mouth while osteichthyes could have any type of mouth depending on the habitat of the water column.
• Chondrichthyes fish excrete urea while osteichthyes fish excrete ammonia.