Polar Sea Extremophiles

. CROCODILE ICEFISH

Where some fish just evolve antifreeze proteins to survive the deep and dark waters of the Antarctic, other fish give up “normal” blood altogether. The crocodile icefish do not transport oxygen to their tissues like other vertebrates. In fact, the crocodile icefish family (a scant 16 species in all) are the only vertebrates in existence that have evolved a circulatory system that does not use hemoglobin. They once had it—the coding is still partially present in their genome—but along the way, ancestors lost the ability to make red blood cells (RBCs).

Oddly, this is not actually a beneficial adaptation for the fish (shown in the larval stage above). It just so happened that during a species crash in the Tertiary period—when there were few predators to pick off the weak—the well-mixed, highly-oxygenated Antarctic waters allowed animals with no hemoglobin to survive, using inefficient direct oxygenation. Millenia later, they still exist, survive well in their environment, and have radically adapted their bodies to live with no RBCs: their blood vessels are huge, they have twice as much blood volume, and their heart output is more than 5 times higher than fish of comparable size. Their body is still inefficient and a lack of hemoglobin is still not a benefit to them.

 ANTARCTIC SEA SPIDERS

Though sea spiders (no relation to true spiders) live throughout the world, most are tiny, so small that their muscles are often only one cell long. In the Antarctic, however, sea spiders can reach up to 10 cm long, with legs sometimes 40 cm across. This is another instance of Antarctic gigantism, which isn’t fully understood. Sea spiders do not have a respiratory system, even when they get to sizes that would seem to require one for oxygenation. They eat primarily soft-bodied invertebrates, such as sea anemones, using a pointed proboscis to suck out part of the fleshy body. Unlike most invertebrates, the male is the sole caretaker of sea spider eggs. After mating, the female deposits her eggs and leaves, and the male will carry them next to his body until they hatch.