I had looked into writing about a couple of different organisms recently before settling on the Crocodile Icefish. I had been sent a paper from one of my upcoming Professors who told me that “Anything you learn from this paper is likely to be very relevant come exam time” so I thought I could do some (albeit a little early!) exam revision and bring you a really cool Family as well.
Crocodile Icefish belong to the Family Channichthyidae, and with the exception of one species, are found in the Southern Ocean. Although this is a very stable environment, the temperatures of between -1.8°C to -3°C do offer challenges that the Icefish have developed some strange adaptations for. The most peculiar traits that they posses which set them apart from other fish is their lack of haemoglobin, which is used to transport oxygen in the blood, and antifreeze proteins that help prevent damage from the sub-zero temperatures of their environment. A lack of scales and a swim bladder also sets them apart from most other teleost fish, however, those traits can be found more commonly such as in the Frogfish I have talked about before.
Most species of Icefish can be found at a depth of around 700-800m, but some have been found as deep as 1,500m. Their size and age can vary. Age can range from 6-7 years in some while other species can live for 12-15 years. Adults have even been recorded as old as 19. Most Icefish can grow in excess of 55cm, with growth occurring quite rapidly at around 6-10cm in length every year until they reach sexual maturity. With the exception of Champsocephalus which reach this stage at about 3-4 years, Icefish will generally reach sexual maturity between 5-8 years old. The temperature of their environment perhaps begins to play a role at this early stage in their life. Incubation periods contrast the closer to the Antarctic they spawn, with Icefish in the north of the Southern ocean having a 2-3 month period to that of more than 6 months in the furthest south. Predators are few, and except when they were commercially fished in the 1960’s to the 1980’s, penguins and seals pose the main predatory threat. Albatrosses have been known to occasionally dine on Icefish if other sources are scarce.
What makes the Icefish such an interesting topic of study is the lack of haemoglobin, as mentioned earlier, which is believed to have arisen before the genera had diversified. This trait poses a major problem as their blood can only carry 10% of what ‘normal’ fish blood can carry. A number of physiological, morphological, and behavioural changes have evolved to deal with this problem. One helping hand, strangely enough, is found in the cold water of their environment. Oxygen demand for fish that live in temperatures below or close to zero is reduced, and the solubility of oxygen in the water and fish’s plasma is increased.
The most obvious adaptations can be seen in the circulatory system. The size of the heart is larger than would be expected, with each heart stroke pumping 6-15x more blood than other teleosts. The volume of blood is also higher. 2-4x more! This can cause serious problems with high blood pressure. To combat this Icefish blood is thinner and capillaries are 2-3x larger. All this takes its toll, 22% of the oxygen intake is spent on blood maintenance alone. And that’s only when they are at rest!
Icefish are of a keen interest to scientist but when they were first studied there was an unusual incident. In 1844 an expedition set out and documented the first Icefish species. They should have been able to identify two species but it seems someone forgot to feed the ship’s cat and it ate them all before they could be identified!