Crocodile Icefish Facts

'Evolution is a tinkerer'. Though originally quoted by François Jacob, this line was again made popular by H. William Detrich III, Principal Investigator of the United States Antarctic Program, after he came across the surprising living mechanism of the crocodile icefish.

Before we jump to facts related to the crocodile icefish, let's first understand what an icefish really is. An icefish is endemic to the cold waters of Antarctica and South America. It belongs to the perciform family, a family of vertebrates that accounts for 40% of all the fish in the world, and falls under the larger suborder Notothenioidei. There are over 132 species of icefish known till now, with high possibilities of new ones being discovered.

Kingdom :Animalia
Phylum :Chordata
Class :Actinopterygii
Order :Perciformes
Suborder :Notothenioidei
Family :Channichthyidae

It was in 1927 that icefish were first sighted. Zoologist Ditlef Rustad, who was looking into the possibility of making Antarctica a whaling outpost was the first to notice these weird fish. While circling around the coast of Antarctica, he found a type of fish that had no scales, was pale in color, had a crocodile-like jaw, and translucent body parts.

Even stranger, when cut open, the fish bled colorless blood. Because of its crocodile-like jaw and colorless blood, Rustad called this the 'white-blooded colorless fish', making the entry Blod farvelöst, which means colorless blood, in his notebook.

In 1954, after many years of research, biochemist John Ruud published a paper that confirmed the absence of red blood cells and hemoglobin in icefish. Among all the known species of vertebrates, these fish are the only examples that lack hemoglobin and red blood cells.

Many biologists believe that the absence of red blood cells is in response to the cold temperatures of the environment they live in.

The crocodile icefish can be found mostly in the Antarctic waters, where temperature falls below -2 to -4°C. Their diet consists of krill, copepods, and other small species of fish. Crocodile icefish can reach a length of 30 to 70 cm.

The crocodile icefish attains sexual maturity between 5 to 8 years, and the mating season starts in autumn and winter. The female is capable of laying almost 10,000 to 20,000 eggs, and the incubation period lasts from 2 to 6 months, depending on the geographic location.

As the blood of a crocodile icefish contains no hemoglobin (the oxygen-binding protein that gives blood its red color), it is transparent and colorless. Red blood cells are rarely found, and if present are defunct.

The body's low metabolic rate, and the high solubility of oxygen in the freezing Antarctic waters help the fish to survive without hemoglobin. However, the oxygen-carrying capacity of their blood is 10% less than their distant cousins with hemoglobin.

Instead of hemoglobin, the crocodile icefish contain larger blood vessels and capillaries, bigger hearts, and great blood volume as compared to other fish. Coronary arteries are missing, and ventricles are extremely squishy, helping them to absorb oxygen directly from the blood they pump.

Despite being in such freezing waters, the blood of a crocodile icefish never freezes because its body produces its own antifreeze. Trypsinogen, a pancreatic digestive enzyme that breaks down proteins in food, found in other vertebrates has been transformed to be distributed throughout the body to prevent the blood from freezing.

Even though the icefish has survived the harsh stages of evolution, it now faces a threat which has the capacity to make it extinct - man-made climatic changes. Due to the excessive use of fossil fuels and rising pollution levels, the Southern Ocean is getting warmer, more acidic, and less nutritious. Icefish are extremely sensitive to high temperatures, and if the temperature of the oceans continue to rise, future generations might not witness this amazing oceanic species.