Life in the deep: how animals live in the eternal dark

by Melissa C. Marquez, Sci Chic's July Marine Biology STEM Star

ocean marine biology sci comm

I think the first time it really hit me that the ocean was bigger than I could comprehend was when I was swimming alongside a tiger shark (Galeocerdo cuvier) in the middle of the Gulf Stream. Looking down, the thought, “There are things in the deep that you can’t see that can see you” ran through my head and I stared longingly at the inky blue, wondering what could possibly be there (besides the obvious tiger shark I was holding on to). Since then, I’ve had a fascination with the deep sea, part of the reason why I jumped at the chance to study deep sea chimaeras in New Zealand.  

As the largest habitat on earth, it’s also the least explored. As you can imagine, the challenges to scientists are many when faced with such a harsh environment. In the western Pacific Ocean, east of the Philippines is where the planet’s deepest point lies: the Challenger Deep in the Pacific Ocean’s Mariana Trench gets as deep as nearly 7 miles (11 kilometers). Sunlight disappears at around 1,000 m, and the rest of the ocean descends into total darkness.

Technological advances has allowed for mere glimpses of these mysterious habitats, large lights illuminating an otherwise pitch black world. NOAA’s Okeanos live streams the wonders it discovers, often showcasing animals that just seem… bizarre. Keep in mind these animals withstand total darkness (except for the bioluminescence many of these animals have adapted), extreme cold, and pressure. Here are some of my favourites…



A form of chemiluminescence, bioluminescence is the production and emission of non-solar light by a living organism. Most bioluminescence is blue (or blue-green) colour because these colours travel the farther in water; because of this, most animals no longer can see red light. Some animals, like the dragonfish, have evolved the ability to produce red light and use it as a spotlight to scope out unsuspecting prey who remain oblivious until it’s too late.

Bioluminescne has a variety of uses. The ones most often seen on documentaries are the lures of anglerfish, which attract curious fish that end up as prey. Counterillumination is also showcased in some nature shows, where rows of photophores (the light organs) on the bellies on these fish produce light that matches the faint sunlight from above, making these animals practically invisible from below. Other uses are using bioluminescence as headlights (like the lantern fish) or to confuse predators/prey via bright, flashing lights or by creating decoys to divert attention (see the green bomber worms do this). Some animals take it a step further and illuminate their predator (known as a “burglar alarm”), making the hunter the targeted and hunted. Bioluminescence can also be social signals to communicate with others.

Firefly squid My favourite animals of the deep that use bioluminescence…

  • Firefly squids (Watasenia scintillans)- I find these creatures, sometimes called the sparkling enope squid, absolutely fascinating. Their light show puts all others to shame, flashing to lure small fish to a quick death by tentacle.


  • Ninja Lanternshark (Etmopterus benchleyi)- Eight specimens of the Ninja Lanternshark have been collected in the eastern Pacific Ocean to date, an expedition led by scientist Victoria Vásquez of Moss Landing Marine Laboratory. Like other lanternsharks, this small shark has light-emitting organs called photophores along their bellies that help them camouflage when they feed in shallower water.


  • Velvet belly Lanternshark (Etmopterus spinax) – These sharks make the list not because they use counter-illumination but because they also have photophores along their spines on the front edges of their two dorsal fins. In fact, the spines transmit the light from the glowing photophores, almost like light sabers!


  • Jellyfish (numerous species) – According to Britannica, more than half of all jellyfish species produce some kind of bioluminescence. This is often to deter or confuse predators, and is breath-taking to see against an inky black background.



Brittlestar Credit: Chester Zoo

No sunlight means it is mighty cold in the deep. Yet, water here never freezes (seawater freezes at -1.8°C). In some places, oxygen-rich water becomes so cool that it sinks to the bottom of the sea; this is called a thermohaline current! However, there are also oxygen-poor habitats, called oxygen minimum zones. Here, many feed on decaying food particles. Scientists are still investigating how animals survive under such harsh conditions.

My favourite animals of the deep that use the extreme environments to their advantage are…

  • Brittle stars (class Ophiuroidea) – These animals feed on decaying matter, detritus, planktons, oysters, and more. Like other star fish, they wrap themselves around whatever they want to eat, their stomach grabs at the food, digests it and later they excrete remains through their mouth (talk about an “ick” factor).


  • Japanese spider crab (Macrocheira kaempferi) – If you Google these animals, they can seem a bit terrifying but despite their “fearsome” look, they are apparently quite well-mannered. You can find them making deep sea vents and holes their homes, and they are the largest known species of crab. Japanese spider crabs may live to be 100 years old, and they feed on animal carcasses and shellfish. Yummy!


  • Giant isopods (genus Bathynomus) – Found in the deep, cold waters of the Pacific, Atlantic and Indian Oceans, they can be observed as deep as 2,000 metres. Related to the small pillbugs, they are mainly scavengers but have been known to hunt live prey. Having been around when Pangea was still a thing, they can go a few years without eating. If anything is going to survive an apocalypse, its these guys and I welcome our Giant Isopod Lords.



Hydrostatic pressure is one of the most important environmental factors affecting deep sea life. Pressure increases 1 atmosphere (atm) for each 10 meters (m) in depth. The deep sea varies in depth from 200 m to that whopping almost 11,000 (kilometres) in the Challenger Deep, meaning pressure ranges from 20 atm to more than 1,100 atm. High pressure can cause air pockets to be crushed (e.g. the air in fish swim bladders), but it does not compress water that much. This pressure allows for many animals to have almost gelatinous muscles. This can be seen in one my favourite animals that is held together by this pressure, the blobfish!

  • Blobfish (Psychrolutes marcidus) – The blobfish is a deep sea fish in the Psychrolutida family, and holds the title of “The World’s Ugliest Animal.” To be fair, these animals live anywhere between 2,000 and 4,000 feet deep and pressure can be up to 120 times higher than it is here! Because of this, blobfish (like some other animals that live that deep) tend to not have defined skeleton or much muscles. Meaning that when you take them away from the pressure that literally keeps their shape, then they become… well, a blob.


Do you have a favourite deep-sea animal that isn’t featured here? Tell us in the comments below!

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1 comment

  • Nyleve

    Very comprehensive article, even Mr. Blobfish said present, thank you.

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