The largest groups of fish are bony fish. Most of these, making up 95% of fish species, are known as teleosts. They have skeletons made of bone and are usually covered with small overlapping bony plates called scales. They also have swim bladders, filled with gas, to help them remain buoyant. Cartilaginous fish include sharks, skates and rays. Their skeletons are made of flexible cartilage but, as they do not have swim bladders, they must keep moving all the time to keep their position in the water. They usually have tough, leathery skins and fleshy fins.
Bony fish, also known as Osteichthyes, is a group of fish that is characterized by the presence of bone tissue. The majority of the fish in the world belong to this taxonomic order, which consists of 45 orders, 435 families, and around 28,000 species. This class of fish is divided into two subgroups: Actinopterygii (ray-finned) and Sarcopterygii (lobe-finned).
Cartilaginous fish, also known as Chondrichthyes, is a group of fish that is characterized by the presence of cartilage tissue rather than bone tissue. This class of fish is divided into two subgroups: Elasmobranchii and Holocephali. Common names of cartilaginous fish include sharks, skates, sawfish, rays, and chimaeras.
The principal difference between bony fish and cartilaginous fish is in the skeleton makeup. As previously mentioned, bony fish have a bone skeleton whereas cartilaginous fish have a skeleton made of cartilage. There are, however, several other differences between these two classes of fish. These differences are listed below.
The vast majority of cartilaginous fish survive in marine, or saltwater, habitats. These fish can be found throughout the world’s seas and oceans. Bony fish, in contrast, are found in both saltwater and freshwater habitats.
Fish gills are tissues located on the either side of the throat. These tissues ions and water into the fish’s system, where oxygen from the water and carbon dioxide from the fish are exchanged. In other words, fish gills act as lungs. In bony fish, the gills are covered by an external flap of skin, known as the operculum. In cartilaginous fish, the gills are exposed and not protected by any external skin. The majority of fish, whether bony or cartilaginous, have five pairs of gills.
Bony and cartilaginous fish are also different in their reproductive behaviors. Bony fish reproduce in what is considered a primitive form of reproduction. These fish produce a large number of small eggs with very little yolk. These eggs are released into the open waters, among rocks on the river or seabed. Male fish then swim over the laid eggs, fertilizing them with sperm which may or may not reach all of the eggs. The eggs hatch into larvae, which are essentially defenseless. The larvae must then develop in the wild, where they are vulnerable to external threats. In this method, the survival rate is low.
In cartilaginous fish, reproduction occurs internally. The sperm is deposited inside of the female in order to fertilize a small number of large sized eggs with a significant amount of yolk. Cartilaginous fish embryo may develop in one of two manners. In one, the embryo develops within a laid egg, relying on the large yolk for nutrients. In the second, more advanced manner, the embryo are able to develop in the secure and protected environment of the mother’s uterus. These fish are born as fully functional organisms, rather than as developing larvae. After delivery or hatching, baby cartilaginous fish are able to hunt and hide from predators. This development process ensures a higher rate of survival.
In both classes of fish, the heart is divided into 4 chambers. In the hearts of cartilaginous fish, one of these chambers is known as the conus arteriosus, a special contracting heart muscle. In place of this chamber, bony fish have a bulbous arteriosus, a non-contracting muscle.
Another difference between the bony and cartilaginous fish is in how each class produces red blood cells. In bony fish, the red blood cells are produced in the bone marrow, the central part of the bone. This process is known as hemopoiesis. Cartilaginous fish lack bone marrow for hemopoiesis. Instead, these fish produce red blood cells in the spleen and thymus organs.
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