We might think of the ocean as being one vast mass of water but, as you drop down, the environment changes. Scientists have divided it into layers to help them categorise the differences between different depths. There are five of these ocean zones.
The first layer – the epipelagic or sunlight zone – is the 200 metres closest to the surface. This is the zone which still receives light from the sun. Below this, there are so few rays penetrating the water that photosynthesis can’t take place.
Once you dive below 200 metres, you’ve reached the twilight (mesopelagic) zone, sometimes also called the midwater zone. Only one percent of sunlight can penetrate these waters and this dusky dimness is what lent the twilight zone its name.
The lack of light also means that there are no plants because photosynthesis isn’t possible. This profoundly changes the habitat because the animals that live here can’t graze on plants so they need to find another way to eat. Some get around this conundrum by undertaking a great migration every day.
Many predators in the sunlight zone hunt by sight so hanging out for all to see is a big risk. So, lots of different types of plankton and other creatures spent their daytimes in the darkness of the twilight zone.
At night, these organisms float up to the epipelagic zone where they can feast under cover of darkness, minimising their chances of becoming prey themselves. This is the world’s largest migration by biomass: 10 billion metric tons of animals join this daily dance to the surface.
Other animals wait for showers of food to fall from above in the form of marine snow. These particles of organic matter, faeces and detritus fall from the surface to the deep. Vampire squids only eat marine snow and have special sticky tentacles to trap it as it floats by.
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This cold, dark world might feel alien to us but it’s brimming with life. There’s everything from the very small (microscopic bacteria and tiny plankton) to the gargantuan (although they come up to breathe, sperm whales and some beaked whales can be found swimming through these waters trying to find food).
The species found in the twilight zone could easily star in a science fiction movie. There’s pigbutt worms, which float through the darkness like disembodied buttocks, bloody-belly comb jellies which have burning red stomachs to disguise their last bioluminescent snack and goblin sharks whose unfortunate-looking faces hold their secret to catching prey.
Many of them display bioluminescence to help them survive in this gloomy habitat. The light they produce has several purposes. Some, like anglerfish, use bioluminescence to lure in tasty prey.
Others, such as the dwarf lanternshark, camouflage themselves by lighting up their bellies so animals looking up from below can’t see them against the surface. This is called counterillumination.
And some, like silky jellies can light up their tentacles and detach them from their bodies to distract a hungry predator, giving them enough time to get away.
The twilight zone is an important part of the ocean. The organisms that live there support the food web and also play a part in transporting carbon down to the deep sea where it can be locked away.
But, although this ecosystem feels far away from our lives, we are affecting it. Researchers have warned that climate change could cause up to a 40 percent reduction in life in this mysterious ocean zone by the end of the century.
Below 1,000 metres, the twilight zone ends and the midnight zone begins.
Top image: a comb jellyfish in the Red Sea, Egypt. Credit: Reinhard Dirscherl/ullstein bild via Getty Images
