One of the traditions at Cape Bird is the annual polar plunge. Swimming in such waters isn’t for the faint-hearted. Sea water freezes at about 1.9°C and when you’re swimming amongst the icebergs (and the penguins), the water can’t be that far from freezing. I do, of course, use the word “swimming” somewhat loosely! Everyone has their own technique: I’m all for the wade out far enough to duck dive, while I’ve seen others leap from icebergs or walk out to ankle depth and lie down. However you do it, it’s a very rapid dunking, typically accompanied by much squealing and swearing.
You need very specific conditions for swimming: it needs to be sunny, dead calm and “warm” – above zero is ideal. But you don’t want it to be too warm or you get a lot of melt-water from the icecap behind the colony, which brings with it a spate of penguin poo. You also don’t want too much pack ice drifting past, bringing with it lurking leopard seals. Christmas Day 2011 dawned calm, clear and perfect for swimming. Well, it didn’t dawn – we have 24 hours of daylight but you get the picture. Three intrepid swimmers set off to commune with the penguins and enjoy an icy dip. The first brave soul stepped out into icy water and began to wade out*. Squeals of horror at the water temperature turned to squeals of something else as my colleague spotted something in the water. A leopard seal? An orca? No, she’s running back with something in her hands!?! A comb jelly!**
“Ctenophores have been described as the most beautiful, delicate, seemingly innocent yet most voracious, sinister and destructive of plankton organisms.” (Mianzan et al., 2009)
Comb jellies (Ctenophora) are gelatinous marine organisms that are similar to jellyfish. They have eight rows of cilia (little hairs) that run along the length of the body and beat rhythmically, allowing the animals to move through the water. Comb jellies are typically transparent and glowing cilia you can see in the video (the cause of all the discussion) are caused by the refraction of light. Unlike jellyfish, comb jellies don’t have stinging cells and engulf their prey of zooplankton whole. They range in size from 2mm to 2m – this fellow was about 10cm in length.
* All three intrepid researchers did indeed go for a Christmas dip. However, proof of this would affect the PG rating of this blog.
** No comb jellies were harmed in the making of this post