Cold water creatures

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I’d like to introduce you to a fishy friend, the white-blooded icefish. First discovered in 1927, this species is unique among vertebrates by having no haemoglobin or red blood cells. Why does this matter? Well, haemoglobin is the iron-containing protein that carries oxygen in the blood around the body, releasing the oxygen to burn nutrients to provide energy.

The icefish instead has to rely on plasma to carry oxygen around its body, and plasma can carry only 10% of the oxygen that haemoglobin can carry. And while the icefish does have a much larger heart, blood vessels etc., this wouldn’t make enough of a difference to allow the creature to survive if not for two other factors.

The first is that, like many creatures that survive on the margins, the icefish had fewer competitors and predators,  particularly during their period of adaptation when their survival was at risk.

The other factor is that they live in the Southern Ocean, off the coast of Antarctica. And these waters have a much greater oxygen content, allowing the icefish to gather enough oxygen to survive. The lack of red blood cells is an advantage here – it allows the blood to flow more easily, since it’s thinner.

Light bulb sea squirt (Clavelina lepadiformis). Image by Pino Bucca, licensed under Creative Commons 3.0
Light bulb sea squirt (Clavelina lepadiformis). Image by Pino Bucca, licensed under Creative Commons 3.0

Of course, climate change will pose an extra threat to these creatures, as they can only survive in the cold waters of the Antarctic.

The Antarctic octopus, too, has evolved to survive the cold. Normally, when a creature gets too cold, the potassium ion channels, and hence the nervous system, get sluggish. But Antarctic octopuses are able to modify their RNA to optimise the function of their potassium channels for the cold environment.

The collapse of the Larsen A and B ice shelves within the past 10 years has revealed a selection of other fascinating Antarctic creatures. Researchers found abundant crinoids (no, not the Doctor Who version), and their relatives sea cucumbers and sea urchins. These are normally found along the sea bed in much deeper waters, where resources are scarce: much like life under an ice shelf.

Initial discoveries included a large population of gelatinous sea squirts, but recent forays have found fewer sea squirts and much larger numbers of glass sponges. These sponges were first thought to grow very slowly, but observations over time seem to indicate that they can undergo growth spurts. Since glass sponges feed on plankton, which take carbon out of the seawater when they produce food, the sponges fix the carbon on the sea floor.

[Featured image: Icefish. Image by Marrabbio2, in Wikimedia Commons]


  1. The ice fish are really amazing, if you haven’t read it I can recommend Sean Carroll’s book “The making of the fittest” where he discusses how they evolved from normal red blooded fish. It’s fascinating!

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