Actinopterygii

Most fish have thin fins.

These fins have bony spines. They look like folding fans. This helps the fish move well.

These fish live in many places. They live in the deep sea. They live in high mountain streams. Some live in fresh water. Some live in salt water.

Some of these fish are tiny. Others are very big. The giant sunfish is huge. The giant oarfish is also very long.

Most of these fish are called teleosts. They have thin scales. These scales are see-through. They help the fish grow.

Many fish lay eggs. The males often help. They may guard the eggs. It is a busy life under the water!

Ray-finned fish are a huge group of animals. They make up nearly 99% of all living fish species.

You can find them almost everywhere in the water. They live in the deep sea and high mountain streams. They live in fresh water and salt water too.

These fish get their name from their fins. Their fins have thin bony spines called lepidotrichia. These spines look like the ribs of a folding fan. This shape helps the fish move through water very well.

Most of these fish are teleosts. This is a large group that includes most living fish. Teleosts often have thin, clear scales. As the fish grows, it adds new layers to these scales.

Ray-finned fish come in many sizes. Some are tiny, like the Paedocypris. Others are very big. The giant sunfish and the giant oarfish are both massive. Long ago, a fish called Leedsichthys lived in the Jurassic seas. It may have grown to 16 meters long!

Ray-finned fish are an amazing group of animals. They are members of a class called Actinopterygii. These fish make up nearly 99% of all living fish species.

Most of them are teleosts. This group is so large it includes over 50% of all living vertebrates. You can find them in almost every kind of water. They live in the deep sea and high mountain streams. They live in fresh water and salt water too.

These fish get their name from how their fins work. Their fins are made of skin webs. These webs are supported by thin bony spines called lepidotrichia. These spines look like the ribs of a folding fan.

Because they are like fans, they can change shape easily. This helps the fish move through the water with great power. They can change their orientation or the area they hit the water. This makes them very good at swimming compared to other fish. Their fin rays attach to bony parts called radials. These radials connect the fins to the fish's internal skeleton.

Scientists have studied these fish for a long time. The earliest known fossil of a ray-finned fish is called Andreolepis hedei. This fossil is 420 million years old. It was found in Russia, Sweden, and Estonia.

Other early fossils come from the Triassic period. These include fish like Prohalecites and Pholidophorus. These fish are the early relatives of modern teleosts. The group has changed a lot since the Devonian period. Even the ancestors of land animals evolved from bony fish during that time.

Ray-finned fish come in many different sizes. Some are tiny, like the Paedocypris. Other fish are huge, like the giant sunfish or the giant oarfish.

In the past, a fish called Leedsichthys lived in the Jurassic seas. It might have grown to 16 meters long. Most ray-finned fish have a swim bladder. This is a part used for buoyancy to help them float. Some fish, like the arapaima, even use it to breathe air. Many teleosts also have thin scales called leptoid scales. These scales are clear and grow in new layers.

Many of these fish have interesting ways to grow their families. In most species, the males and females are separate. Most females lay eggs that the male fertilizes outside the body.

Some fish even change their sex during their lives. This is called sequential hermaphroditism. Some start as females and then become males. This is called protogyny. Other fish, like the mangrove rivulus, can even do both at once. This helps them survive in tricky places like mangrove forests.

Actinopterygii is a massive class of bony fish. These animals are commonly called ray-finned fish or actinopterygians. They are incredibly successful and diverse. In fact, they make up nearly 99% of the over 30,000 living fish species.

This group is so large that it includes more than 50% of all living vertebrates. You can find them in almost every aquatic environment. They live in freshwater, brackish water, and the salty ocean. They inhabit everything from the deep sea to high mountain streams. They even live in subterranean waters.

The name "ray-finned" describes how these fish move. Their fins are made of thin webs of skin. These webs are supported by bony spines called lepidotrichia. These spines are arranged like the ribs of a folding fan.

This structure allows the fins to change shape and orientation easily. By changing the wetted area of the fin, the fish can move with great efficiency. They have a high thrust-to-weight ratio per movement. This makes them better swimmers than the lobe-finned fish or sharks. The fin rays attach to bony elements called radials. These radials connect the fins to the internal skeleton, such as the pectoral or pelvic girdles.

Many ray-finned fish possess a swim bladder. This is an internal organ used for buoyancy, which helps the fish stay at certain depths. In most ray-finned fish, this organ develops from a dorsal bud above the foregut. Some early forms used the swim bladder for breathing. This trait is still seen in the Holostei group, which includes gars and bowfins. Some modern fish, like the arapaima, have modified the swim bladder to breathe air. Other lineages have lost the organ entirely. Teleost fish also have unique scales called leptoid scales. These scales are thin and transparent. They lack the hard enamel layers found in other fish. New scales are added in concentric layers as the fish grows.

Ray-finned fish are divided into several distinct groups. The first group is Cladistia, which includes bichirs and reedfish. The next is Chondrostei, which includes sturgeons and paddlefish. These fish often have skeletons that are partially cartilaginous.

The largest group is Neopterygii, which means "new fins." This group includes the Holostei and the massive Teleostei. Teleosts are the most successful group. They make up 96% of all living fish species.

Within the teleosts, one large subgroup is called Acanthomorpha. This subgroup alone contains 40% of all fish species.

Scientists have traced the history of these fish through fossils. The earliest known fossil of an actinopterygian is Andreolepis hedei. It dates back 420 million years to the Late Silurian period.

The crown group of actinopterygians likely began near the Devonian-Carboniferous boundary. Teleosts have diversified greatly since the Mesozoic era. Some teleost lineages have even undergone whole-genome duplication. This process happened about 320 million years ago in teleosts. It also happened in chondrosteans around 180 million years ago. Some groups, like salmon, experienced it 80 to 100 million years ago. Even goldfish experienced it as recently as 14 million years ago.

Reproduction in ray-finned fish is very varied. In most species, the sexes are separate. Females usually lay eggs that are fertilized externally by males.

However, some fish practice sequential hermaphroditism. This means they change their sex during their life. Protogyny is common, where a fish starts as a female and becomes a male. Protandry is less common, where a fish starts as a male and becomes a female. While most teleosts do not provide parental care, about 21% of families do. This includes viviparity, where live young are born. In these cases, male care is often more common than female care.

These fish demonstrate an incredible range of sizes. The smallest is the Paedocypris, which is only about 7.9 millimeters long. On the other end, the giant sunfish can reach 2,500 millimeters. The giant oarfish can reach 8 to 11 meters.

In the Jurassic period, the extinct Leedsichthys was even larger. It is estimated to have grown to 16 meters. This massive variety shows how well ray-finned fish have adapted to the world. They are connected to the history of all land animals. Tetrapods, or four-limbed vertebrates, actually evolved from a group of bony fish during the Devonian period.