Channichthyidae

Some fish live in very cold water.

These fish live near Antarctica. The water there is very cold.

Their blood is clear. It has no red color. This is because they lack a special protein.

This protein helps blood carry air. But the cold water has lots of air in it. The fish can get air through their blood easily.

These fish like to hide and wait. They eat other fish and small shrimp. They are very special fish!

Icefish live in the cold Southern Ocean near Antarctica.

Most fish have red blood. This is because of a protein called hemoglobin. Hemoglobin helps blood carry oxygen. But icefish are different. They are the only known vertebrates to lack hemoglobin as adults. This makes their blood look clear instead of red.

How do they live without it? The water near Antarctica is very cold. Cold water holds a lot of oxygen. The icefish can take oxygen from the water into their plasma, which is the liquid part of the blood.

To help move this oxygen, icefish have special bodies. They have much larger hearts than other fish. Their blood volume is also four times larger. They have bigger blood vessels to help the blood flow fast.

Most icefish eat other fish or krill. They are ambush predators. This means they hide and wait to strike. They can even eat a fish that is half their own size!

Scientists found a huge group of icefish nests in the Weddell Sea. One area had 60 million nests.

Icefish live in the freezing Southern Ocean near Antarctica.

Most animals have red blood because of a protein called hemoglobin. This protein grabs oxygen to carry it through the body. However, icefish are very special and unique. They are the only known vertebrates to lack hemoglobin as adults. This means their blood is clear and colorless instead of red. They live in very cold waters that stay at stable temperatures.

Since they lack hemoglobin, icefish move oxygen in a different way. They use their blood plasma, which is the liquid part of the blood, to carry oxygen. This works because cold water can hold a lot of oxygen. To make this work, icefish have developed several amazing body changes. They have blood volumes that are four times larger than other fish. Their hearts are also five times larger to pump more blood. They have larger blood vessels to help the liquid flow quickly.

Scientists are still learning how these fish first changed. Some believe icefish are only about 6 million years old. Other ideas suggest they might be 15 to 20 million years old. A large ocean current called the Antarctic Circumpolar Current helped isolate these waters. This current formed about 25 million years ago. It separated the cold Southern Ocean from warmer waters to the north. This isolation helped the icefish evolve in their unique home.

There are at least 16 different species of crocodile icefish.

In February 2021, scientists found something incredible in the Weddell Sea. They found a massive breeding colony of Neopagetopsis ionah icefish. This area covered about 92 square miles at the bottom of the sea. There were an estimated 60 million active nests in that spot. Most nests had one adult fish guarding about 1,735 eggs. This shows how many icefish can live in one area.

Icefish are often ambush predators that wait to strike. They eat other fish or tiny creatures called krill. They can even eat a fish that is 50% of their own length. Their bodies are also protected from freezing by special proteins. These are called antifreeze glycoproteins, which stop ice from forming inside them. Just like how you might use a heavy coat in the snow, these proteins help them survive the deep chill. They are perfectly built for life in the ice.

The Channichthyidae, commonly known as crocodile icefish, are a unique family of notothenioid fish. They live in the freezing waters of the Southern Ocean near Antarctica. These fish are famous because they are the only known vertebrates to lack hemoglobin as adults. Hemoglobin is the protein that makes blood red and carries oxygen. Because they lack this protein, their blood is actually colorless. This biological quirk makes them some of the most unusual creatures in the ocean.

To survive without hemoglobin, icefish have developed a specialized circulatory system. In most animals, red blood cells carry oxygen using hemoglobin. In icefish, oxygen is instead dissolved directly into the blood plasma, which is the liquid part of the blood. This works because the Southern Ocean is extremely cold. As water temperature decreases, its ability to hold dissolved gases like oxygen increases. However, this method is much less efficient. The oxygen-carrying capacity of icefish blood is less than 10% of that of related fish.

To compensate for this low oxygen capacity, icefish have undergone massive physical adaptations. They possess blood volumes that are four times larger than those of other fish. Their hearts are also much larger, providing a cardiac output five times greater than typical fish. They have developed larger blood vessels and capillaries to allow for high flow rates. Their heart muscles are very spongy, which allows them to absorb oxygen directly from the blood they pump. Additionally, they have increased mitochondrial density to help deliver oxygen to their cells more effectively.

Scientists have identified at least 16 different species of crocodile icefish. These belong to several different genera, including Chaenocephalus, Channichthys, and Neopagetopsis. These fish are primarily piscivorous, meaning they eat other fish, but they also consume krill. They are often ambush predators that can survive long periods without food. In some cases, an icefish can eat a prey item that is 50% of its own body length. They also produce antifreeze glycoproteins (AFGPs). These special proteins prevent ice crystals from forming inside their cells.

The history of how icefish lost their hemoglobin is a subject of scientific debate. One theory suggests they are about 6 million years old, appearing after the Southern Ocean cooled. Another theory suggests they are much older, perhaps 15 to 20 million years old. The formation of the Antarctic Circumpolar Current (ACC) played a major role in this evolution. This current formed roughly 25 to 22 million years ago. It thermally isolated the Southern Ocean from warmer northern waters, creating a stable, oxygen-rich environment.

There are several theories regarding why this loss occurred. Some researchers believe the loss of hemoglobin was a neutral mutation that survived because there was no competition in the empty habitats of the Southern Ocean. Others suggest it was an adaptation to conserve iron. Iron is a limiting nutrient in these waters, and hemoglobin requires iron to function. By not making hemoglobin, icefish may minimize their need for this metal. Another factor involves nitric oxide levels. When icefish lost hemoglobin and myoglobin, their total nitric oxide levels rose. This chemical helps regulate their large blood vessels and heart function.

Recent discoveries have shown how massive these populations can be. In February 2021, scientists documented a breeding colony of Neopagetopsis ionah in the Weddell Sea. This colony covered approximately 92 square miles on the ocean floor. Researchers estimated there were 60 million active nests in this single area. Most of these nests were guarded by one adult fish. On average, each nest contained about 1,735 eggs. This discovery highlights the incredible scale of life in the Antarctic deep sea.