poles still rotating
frozen shards traverse the plains
warming bears naught!

During winter in Ottawa, Canada, temperatures plummet well below 0°C. My nose freezes, my fingers freeze, my legs freeze, and my hair freezes. Thankfully my blood never freezes; the red fluid continues to flow throughout my body. I’m an endotherm after all. My body can generate heat internally by, say, digesting the sugar in a toasted marshmallow hot chocolate with whipped cream.

Fishes are not endotherms. Does their blood freeze when the surrounding temperatures drop below zero?

Sort of.

With ice in their blood, icefish happily live in freezing, Antarctic waters (down to −2.0°C). The ice gets into an icefish when it drinks or eats. Wouldn’t you know, icefish have anti-freeze proteins (AFPs) that stop ice crystals in blood from extending and getting larger. But in an unexpected, evolutionary twist, AFPs also prevent ice in blood from melting. Even when waters warm in the summer to temperatures that should theoretically melt the ice in icefish blood, the ice doesn’t melt. How does an icefish survive if ice is accruing in its blood year after year? The answer to this question remains a perciform-ic puzzlement!

P.S. Icy icefish blood is colourless. Colourless blood initially baffled scientists and Dracula alike. Turns out icefishes don’t have hemoglobin – the protein that gives blood its red colouring.