The skeleton of cartilaginous fish is made up of cartilage, while the skeleton of bony fish is made up of small bones.

Comparative chart

What are cartilaginous fish?

Fish are divided into different classes about 400 million years ago. One class is called Chondrichthyes in which cartilaginous fish are included. The skeleton of fish of this class is made up of cartilage. Sharks, rays, skates are some examples of this class. There are still almost 1,100 species of this class. There is no connection of the upper jaw of cartilaginous fish with their skull, so they move independently. The skull consists of 10 cartilage elements and the eyelids protect your eyes. They also have a third membrane for eye protection called the nictating membrane. All cartilaginous fish have four chambers of the heart and the fourth chamber is called the cone arteriosus, which is a contractile heart muscle. The intestine of this class is short but has an internal spiral structure for the absorption of nutrients. The kidneys and genitalia of these fish empty into an opening called the cloaca. Cartilaginous fish lack ribs and bone marrow. So, red blood cells are made in the spleen. Dermal denticles cover the entire skin and are similar to the structure of human teeth. These enamel-coated denticles are called placoid scales. The mouth is subdermal because it is located ventrally. There are 5 to 7 gill slits that remain open and visible at all times, and no operculum covers the gills. The tail fin is not symmetrical and the two lobes of the fin are unequal in size. Another interesting feature of this class is that their fin is parallel to the longitudinal axis of the body, which keeps them in balance instead of providing energy to swim through the water column. Its lightweight skeleton along with the oil-filled liver provide buoyancy against the massive body. Its heavy weight could crush internal organs out of the water. Cartilaginous fish excrete urea as a nitrogen waste product. They are living fossils because cartilaginous fish began to evolve before 420 million years and there are almost 970 species that live in the sea. Its heavy weight could crush internal organs out of the water. Cartilaginous fish excrete urea as a nitrogen waste product. They are living fossils because cartilaginous fish began to evolve before 420 million years and there are almost 970 species that live in the sea. Its heavy weight could crush internal organs out of the water. Cartilaginous fish excrete urea as a nitrogen waste product. They are living fossils because cartilaginous fish began to evolve before 420 million years and there are almost 970 species that live in the sea.

The skeleton of this class is made up of calcified and ossified bones. The bony scales are called cycloid or ctenoid depending on their edges, whether they are smooth or spiny. The upper jaw of bony fish is connected with the skull, and the skull has 63 small bony elements. The eyes of bony fish do not have lids, so the eyes are always open. The bony fish heart has four chambers, and the fourth chamber is known as bulbous arteries which are made of non-contractile muscles and elastic fibers. Its tail fin is symmetrical and the entire body is covered with scales. Its pectoral fin is perpendicular to the longitudinal axis. Bony fish have a gas-filled swim bladder that is useful for buoyancy and performs hydrostatic functions. They also have a flap to cover the gills called an operculum. Bony fish excrete ammonia as nitrogenous waste. Bony fish show rapid growth and reach maturity younger, so they reproduce more. They inhabit both fresh and salt water and there are almost 27,000 species. Furthermore, bony fish account for more than half of all vertebrate species on earth.

Cartilaginous fish versus bony fish

• Cartilaginous fish have open gill slits, while the gill slits of bony fish are covered with an operculum.
• The upper jaw of cartilaginous fish moves independently as it does not connect to the skull, whereas in bony fish the upper jaw is linked to the skull.
• In cartilaginous fish, the caudal fins are asymmetrical, while in bony fish, the caudal fins are symmetrical.
• Cartilaginous fish have an oil-filled liver which is used for buoyancy, while in bony fish, the gas-filled liver is used for buoyancy.
• In cartilaginous fish, the body has been covered with dermal denticles, while in bony fish, the body has been covered with scales.
• In cartilaginous fish, the pectoral fin is parallel to the longitudinal axis of the body, while in bony fish, the pectoral fin is perpendicular to the longitudinal axis of the body.
• In cartilaginous fish, the mouth is always sub terminal, while in bony fish, the mouth can be terminal or sub terminal, depending on the habitat.
• In cartilaginous fish, the tail fin is heterocercal, while in bony fish, the tail fin is homocercal or differmic.