What Does the Term 'Bony Fish' Mean?

Bony Fish Facts, Characteristics and Examples

Two bony fish species: Atlantic sailfish attacking a sardine baitball, Isla Mujeres, Mexico
Rodrigo Friscione / Getty Images

Most of the world's fish species are categorized into two types: bony fish and cartilaginous fish. In simple terms, a bony fish (Osteichthyes) is one whose skeleton is made of bone, while a cartilaginous fish (Chondrichthyes) has a skeleton made of soft, flexible cartilage.

The third type of fish, including eels and hagfish, is the group known as Agnatha, or jawless fish. 

The cartilaginous fish include sharksskates, and rays. Virtually all other fish fall into the class of bony fish--some 20,000 species.

Characteristics of Bony Fish

Both bony fish and cartilaginous fish breathe through gills, but bony fish also have a hard, bony plate covering their gills. This feature is called an operculum. Bony fish may also have distinct rays, or spines, in their fins.

And unlike cartilaginous fish, bony fish have swim bladders to regulate their buoyancy. Cartilaginous fish, on the other hand, must swim constantly to stay afloat. 

Bony fish are considered to members of the class Osteichthyes, which is subdivided into two main types of bony fish:

Bony fish include both marine and freshwater species, while cartilaginous fish are only found in marine environments (salt water). Some bony fish species reproduce by laying eggs, while others bear live young. 

Evolution of Bony Fish

The first fish-like creatures appeared over 500 million years ago. Bony fish and cartilaginous fish diverged into separate classes about 420 million years ago.

Cartilaginous species are sometimes seen as more primitive, and for good reason. The evolutionary appearance fo bony fish eventually led to land-dwelling vertebrates with bony skeletons. And the gill structure of bony fish gill was a feature that would eventually evolve into air-breathing lungs. Bony fishes are therefore a more direct ancestor to humans. 

Environment of Bony Fish

Bony fish can be found in waters all around the world, both freshwater, and saltwater. Marine bony fish live in all the oceans, from shallow to deep waters, and in both cold and warm temperatures.

An extreme example is the Antarctic icefish, which lives in waters so cold that antifreeze proteins circulate through its body to keep it from freezing. Bony fish also comprise virtually all freshwater species living in lakes, rivers, and streams. Sunfish, bass, catfish, trout, pike are examples of bony fish, as are the freshwater tropical fish that you see in aquariums. 

Below are some other species of bony fish:

What Do Bony Fish Eat?

A bony fish's prey depends on the species but may include plankton, crustaceans (e.g., crabs), invertebrates (e.g., green sea urchins), and even other fish. Some species of bony fish are virtual omnivores, eating all manner of animal and plant life.