Animals

Bony Fish

by Marcia Malory

Bony fish (Osteichthyes) are jawed fish which have skeletons that are made of bone.

Most fish, and most vertebrates,  are bony fish.

Bony fish can be found in bodies of water all over the world, in many different environments, from arctic seas to tropical rivers to desert springs.

There are two kinds of bony fish: ray-finned fish (Actinopterygii) and lobe-finned fish.

Golden koi fishRay-finned fish make up the majority of bony fish.

The lobe-finned fish and the tetrapods comprise the clade Sarcopterygii.

A clade is a group of organisms, all of which share the same ancestor.

Sarcopterygii and a group of extinct fish known as acanthodians (Acanthodii) make up the clade Teleostomi, sometimes known as teleosts. The acanthodian are sometimes called spiny sharks because they were covered in spines.

Habitat

Bony fish live all over the Earth.

Some bony fish live in freshwater, some live in seawater, and some live in brackish water (water that is salty, but not as salty as seawater).

Some bony fish are diadromous. This means that they migrate between freshwater and seawater.

Fish that are anadromous live in seawater as adults but spawn (breed) in freshwater.

Catadromous fish live in freshwater as adults but spawn in the ocean.

Amphidromous fish spend part of their lives in seawater and part of their lives in freshwater, but they do not migrate between freshwater and seawater in order to spawn.

Other species of bony fish migrate, but remain within either saltwater or freshwater their entire lives.

Physical Characteristics

Bony fish have physical adaptations that help them to move efficiently through water.

They usually have streamlined bodies - large in the middle and tapering at both ends. This shape, which is known as a fusiform shape, helps to reduce drag in the water.

The majority of bony fish have scales that overlap from head to tail. This also helps to reduce water resistance.

Most bony fish have a swim bladder, which is an internal organ that is filled with air. The swim bladder allows the fish to stay afloat even when it isn't swimming.

Cartilaginous fish, such as sharks, do not have swim bladder.

Scientists think that early bony fish had lungs. Swim bladders evolved from these lungs.

Bony fish have two-chambered hearts.

A bony fish has two nostrils. In most bony fish,the nostrils are not connected to the fish's mouth.

Bony fish are usually cold-blooded (ectothermic). Some, such as tuna and swordfish, have the ability to control the temperatures in some parts of their bodies.

Fins

The fins of bony fish are usually flexible and are usually supported by spines and rays.

A bony fish usually has at least one dorsal fin (back fin), a caudal fin (tail fin) and an anal fin, which can be found on the underside of the fish's body, in back of its anus and in front of its tail.

It also has a pair of pectoral fins on the sides of its body as well as a pair of pelvic fins. The pelvic fins, which are sometimes called ventral fins, are below the pectoral fins, closer to the abdomen of the fish.

Bony fish usually use their caudal fins to push themselves forward, their pectoral fins to turn themselves around, their dorsal fins to change direction quickly and their pelvic fins to slow themselves down

A bony fish will also use its dorsal fin (or fins), its pelvic fins and its anal fin to maintain stability.

Respiration

Most bony fish use their gills to breathe throughout their lives.

They breathe by taking in oxygen that is dissolved in the water.

A bony fish's gills are supported by gill arches that are made of bone.

There is only one gill opening on each side of a bony fish's body. Each  gill opening is covered by a hard flap of bone that is known as an operculum.

A bony fish takes in water though its mouth.

The water then passes through the gills.

When in the gills, the water pass through plate-like structures known as lamellae, where gases are exchanged between the water and the fish's bloodstream.

Oxygen that was dissolved in the water is absorbed into the blood and carbon dioxide is released from the blood into the water.

Water then passes out through the gill openings.

A bony fish can pump water through its gills by opening and closing its mouth.

In many bony fish, the operculum closes when the mouth opens and opens when the mouth closes. When the operculum opens, the pressure in the fish's mouth decreases, causing water to flow through the gills.

This allows water to pass through the gills very rapidly, so that the fish can obtain enough oxygen to survive without having to swim constantly.

Cartilaginous fish do not have operculums.

Some bony fish use lungs or their swim bladders to breathe air.

Air-breathing fish often live in shallow waters where the oxygen content of the water may decline at certain times of the year.

Reproduction

Bony fish usually reproduce by means of external fertilization.

However, some practice internal fertilization.

Most species are oviparous. This means that eggs are laid outside of the mother's body.

However, some species are viviparous - the young develop inside the body of the mother.

Some bony fish are ovoviviparous. This means that the young grow inside the mother's body but don't get nutrients from her. They obtain nutrients from egg yolks.

In some species of bony fish, the young  develop inside the mother's body and obtain nutrients directly from the mother.

Most species of bony fish are divided between males and females.

However, in some species, individual fish are hermaphrodites, with both male and female reproductive organs.

Sometimes, a bony fish may be born either female or male and then change sex its during its lifetime.