As twilight fades to black, we enter the ocean’s midnight (bathypelagic) zone, which begins at a staggering 1,000 metres deep. This is the third of the five ocean zones after the sunlight (epipelagic) and twilight (mesopelagic) zones.
- What are the different ocean zones?
- What the deep-sea robots saw when they went to the darkest depths of our oceans
Although we don’t know much about the midnight zone (because it’s so difficult for researchers to study – they have to use special remotely operated vehicles, also known as ROVs), it is the world’s largest habitat. 70% of all the seawater on Earth is found in the bathypelagic zone.
Here, none of the sun’s rays can penetrate the water. Like in the twilight zone, this means no photosynthesis can occur. This makes it incredibly dark – but not entirely, there are some twinkling sparkles of bioluminescence.
It’s also very cold. Typically, the waters linger around the 4°C mark. The crushing pressure can be anywhere between 100 to 400 atmospheres, which would kill a human instantaneously.
This inhospitable environment is a challenging place to live so the organisms that call it home must develop ingenious ways to survive. Many create their own light (bioluminescence) to aid their search for food or mates – anglerfish, which can also be found in the twilight zone, are a good example of this.
Others use bioluminescence to protect themselves from predation – either using it as an invisibility cloak of sorts, to make them harder to see, or as a decoy to distract predators.
Lots of deep-sea animals, such as owlfish, have enormous eyes – all the better to see bioluminescence in the dark, my dear. Whalefish, on the other hand, have pinprick eyes because they don’t rely on sight: they have special pores across their body that help them sense the world around them.
Like many other deep-sea animals, being scarlet helps the fish blend into the midnight sea around it. Red light has longer wavelengths which are absorbed by the water at this depth, so bright red colouration appears black (without the lights of a research vessel) so they blend seamlessly into the inky waters.
Other animals have developed gigantic mouths to give them the best chance of chomping down on prey – food is hard to come by here.
The gulper eel (found in both the twilight and midnight zones) is one of these. This bizarre party-balloon-like animal is also known as the pelican eel because of its huge, extendable jaw which resembles a pelican’s bill.
Being able to gobble down enormous prey in that huge mouth isn’t its only talent. This is one of the many deep-sea creatures that can create bioluminescence. In the case of the pelican eel, it can create flashes of light from the tip of its tail, which could be a clever ploy to tempt prey closer.
Although it seems unlikely, some air-breathing mammals even visit these staggering depths zone looking for squid to devour. Sperm whales can dive to around 2,000 metres deep while Cuvier’s beaked whales have been recorded at nearly 3,000 metres: the deepest recorded dive by a mammal.
At 4,000 metres deep, the midnight zone gives way to the abyssopelagic zone, also known – rather ominously – as the abyss.
Top image: the mouth of gulper eel opens. Credit: BBC Natural History/Getty Images
