Osteichthyes

Osteichthyes, also called bony fish, are a large group of vertebrate animals. Their skeletons are made mostly of bone tissue. They include the fish you see in the water and, in a science way, all animals that live on land too. Unlike sharks, which have skeletons made of cartilage, bony fish have real bone.

Osteichthyes has two main groups: ray-finned fish, which are most fish today, and lobe-finned fish, the ancestors of all land animals. The oldest bony fish fossils are from about 436 million years ago, in the Silurian time period. These early fish had features like sharks but also had parts that later became swim bladders and lungs.

Today, Osteichthyes is a very big and common group. There are 45 orders, over 435 families, and nearly 28,000 species. They are important in water ecosystems all over the world.

Characteristics

Bony fish have bones in their heads and around their jaws to stay strong. They also have small bones around their eyes and inside their ears to help them hear and balance. Many bony fish have air-filled bags called swim bladders that help them stay up or sink down in the water.

Early bony fish had simple air pockets near their throats to help them breathe in water with little oxygen. Over time, these pockets changed into swim bladders in some fish and into lungs in animals that can live on land, like frogs, reptiles, birds, and mammals. Bony fish usually have bony scales on their bodies instead of the hard, tooth-like scales that cartilaginous fish have. These bony scales come in different types, like ganoid, cosmoid, and teleost scales.

Classification

See also: Evolution of fish and List of prehistoric bony fish genera

Bony fish, or Osteichthyes, are a large group of fish with skeletons made of bone. They were once thought to be just a class of fish, but we now know that land animals, including humans, are part of this group. This means bony fish include both fish in water and all animals that walk on land.

Today, scientists classify bony fish to include all land animals. Most bony fish in water belong to a group called ray-finned fish. This new way of grouping helps us learn how land animals evolved from certain types of bony fish.

Clade	Example	Description
Actinopterygii	ray-finned fish	Actinopterygii, members of which are known as ray-finned fishes, is a class of the bony fishes. The ray-finned fishes are so called because they possess lepidotrichia or "fin rays", their fins being webs of skin supported by bony or horny spines ("rays"), as opposed to the fleshy, lobed fins that characterize the clade Sarcopterygii which also possess lepidotrichia. These actinopterygian fin rays attach directly to the proximal or basal skeletal elements, the radials, which represent the link or connection between these fins and the internal skeleton (e.g., pelvic and pectoral girdles). In terms of numbers, actinopterygians are the dominant class of vertebrates, comprising nearly 99% of the over 30,000 species of fish (Davis, Brian 2010). They are ubiquitous throughout freshwater and marine environments from the deep sea to the highest mountain streams. Extant species can range in size from Paedocypris, at 8 mm (0.3 in), to the massive giant sunfish, at 2,700 kg (6,000 lb), and the long-bodied giant oarfish, at up to 8 m (26 ft) (or possibly 11 m (36 ft)).
Sarcopterygii	lobe-finned fish and tetrapods	Sarcopterygii (fleshy fin), members of which are known as lobe-finned fish, is an unranked clade of the bony fishes. Traditionally, it is a class or subclass that excludes Tetrapoda, a group of typically terrestrial vertebrates that descends from lobe-finned fish. However, under modern cladistic classification schemes, Sarcopterygii is a clade that includes the tetrapods. The living sarcopterygians are the coelacanths, lungfish, and tetrapods. Early lobe-finned fishes had fleshy, lobed, paired fins, joined to the body by a single bone. Their fins differ from those of all other fish in that each is borne on a fleshy, lobelike, scaly stalk extending from the body. Pectoral and pelvic fins have articulations resembling those of tetrapod limbs. These fins evolved into legs of the first tetrapod land vertebrates, amphibians. They also possess two dorsal fins with separate bases, as opposed to the single dorsal fin of actinopterygians (ray-finned fish). The braincase of sarcoptergygians primitively has a hinge line, but this is lost in tetrapods and lungfish. Many early lobe-finned fishes have a symmetrical tail. All lobe-finned fishes possess teeth covered with true enamel.

Phylogeny

A family tree shows how living bony fish, including animals that walk on land, are related. You can see this in the cladogram below. An important change happened early on when the ancestors of these fish had all their genes duplicated.

Biology

All bony fish have gills to breathe. Most bony fish use only their gills, but some, like lungfish, can also breathe with their lungs or special air-filled bladders. A few can even breathe through their skin, intestines, or stomach.

Bony fish are usually cold-blooded, meaning their body temperature changes with the water. But some larger fish like the opah, swordfish, and tuna can keep their bodies warmer. Bony fish eat many kinds of food, like plants, other animals, or tiny particles in the water.

Some bony fish can produce both male and female cells, and a few can reproduce without a mate. Most lay eggs, but some give birth to live young. While many fish don’t care for their young after birth, some parents protect their eggs in different ways. For example, male sea horses carry the eggs in a special pouch until they hatch.

Examples

The giant sunfish is the heaviest bony fish in the world. Fishermen found one near the coast of Faial Island, Azores. It weighed 2,744 kilograms and was very big.

Other large bony fish include the Atlantic blue marlin, the black marlin, some sturgeon, and the giant and goliath grouper. Some of these can weigh more than 300 kilograms. In contrast, Paedocypris progenetica and the stout infantfish are very small, measuring less than 8 millimetres. The beluga sturgeon is one of the largest freshwater bony fish today, and Arapaima gigas is also very large. The largest bony fish ever was Leedsichthys, which was even bigger than many fish alive today.

Comparison with cartilaginous fishes

Osteichthyes, or bony fish, are different from cartilaginous fishes like sharks and rays. While cartilaginous fishes have skeletons made of cartilage, bony fish have skeletons made of bone. This makes their bodies harder and stronger.

Bony fish also usually have a bony flap called a lid over their eyes, which helps protect them. Cartilaginous fishes do not have this.

Another big difference is in how they breathe. Bony fish often have a special organ called a swim bladder that helps them stay at the right depth in the water. Cartilaginous fishes do not have swim bladders.

Bony fish lay eggs with a protective cover called a shell, while many cartilaginous fishes give birth to live young. Both groups, however, are important parts of aquatic ecosystems.