Why do deep sea creatures glow?

Underwater predators surely would love a neon glowing sign saying 'all you can eat here'.
07 December 2021


Pink jellyfish



What is the most fascinating creature ever seen on underwater expeditions?


To answer this deep sea question, Julia Ravey talks to deep-sea explorer Diva Amon, the director of non-profit SpeSeas, about the wild and wonderful creatures lurking in the depths of our oceans.

Diva - Oh gosh. That's the hardest question because you do literally see, on every single expedition, something that blows your mind. Normally something that blows your mind every single day of the expedition. If I had to choose just one and I will caveat that by saying there are glowing sharks, there are octopus that look like Dumbo with these doorbell fins on the sides of their heads, there are worms that subsist only on the bones of dead whales, there's all kinds of stuff down there. But if I had to choose one, I would say the 'kiwa crab' also known as the 'yeti crab' and it has another nickname which I'll get onto. Basically, it's a blind white crab that lives in the deep ocean off of Antarctica and it has a super hairy chest and super hairy arms. It uses these hairs, by putting itself in close-to-the-chemical rich water, and it uses these hairs to grow bacteria on the hairs. Then when it gets hungry, it just scrapes the bacteria into its mouth. Basically it has like arm farms, if you will. The best part of this, or the worst part of this depending on how you feel, is that because it has a really hairy chest, it's been nicknamed, the 'hoff crab' after David Hasselhoff.

Julia - The crab with the hairy chest and it's almost like it's walking around with it's lunchbox on its front and it's like, "Come here to the lunchbox" and it's like ready to eat it. Oh my goodness that is incredible. So our listener Jane asked, why is it that creatures living deep in the water glow and does that not attract predators?

Diva - Right. So that's a great question. Once you go past about 400 meters depth in the ocean there is no sunlight, and how do you communicate in the absence of light? You can either use sound or the other alternative is you make your own lights. Lots of animals in the deep sea use something called bioluminesce, where they're actually able to create their own light. They do that sometimes with the help of some bacteria, other times it's just the chemical reaction, but they do it for a variety of different reasons. It tends to be that if it's a defensive function, it's a quick sudden burst whereas if it's an offensive function, then it's more glowing. There's a squid that can just drop one of its arms, which will glow, and then the predator goes off to the arm while the squid is able to get away. That's really one of the main ways that the animals communicate in the deep sea and given the vastness of the deep ocean it actually provides about 96% of all space on earth in which life can live. And because it's so vast, it's now thought that bioluminescence could be the most common form of communication on the planet.

Julia - Wow. I love the squid can just drop one of it's fins like, 'I'm not here'. So when predators eat glowing fish do their stomachs glow?

Diva - A lot of animals in the deep sea actually have black stomachs because they don't want to give themselves away if they eat something that's glowing.

Julia - Oh my goodness, so it's like, 'I didn't eat anything. I'm not here. I'm still hidden.'


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