As part of the “Abyss” group, we might wonder what things look like down there, in the deep ocean. As you probably already experienced, diving in water comes with (uncomfortable) changes of temperature and pressure. And that’s only a few meters! The conditions keep changing going deeper in the water column (more than freezing toes!). In oceans, the abyssal waters represent the part lying between 2000 m and 6000 m under sea level. At these depths, the temperature is constant around 0-4°C, the pressure is up to 200-600 atmospheres, and there is no light. And light is not only useful to see around but it is also the energy for photosynthesis and hence life sustenance at the surface of the Earth. Yet, although very poorly known, these depths allow life to exist. And what comes out of discoveries is sometimes very interesting or unexpected!
Life has to adapt to these difficult conditions of low temperature, high pressure, absence of light and scarcity of nutrients. The result is not exactly what we are used to, evolution sometimes leads to cool physical and morphological features! Let’s have a look at some inhabitants of the abysses.
Beyond 100 m in the dark cold water, plants disappear, life in the deep sea is 100% animal, likely because photosynthesis is impossible. With disappearance of light at depth, numerous species evolved to be blind or, conversely, grew big, globular eyes in the attempt to catch any remaining light like our very cute friend in Figure 1.
In the darkness, the animals have commonly developed the ability to create their own light through chemical reactions; it is called bioluminescence. Creatures use this process for many different purposes: to light their way, attract their preys, or even seduce a partner. A well-known example of this feature is the Humpback Anglerfish (Figure 2), who developed bioluminescence at the end of a “fishing pole” on his forehead. These lures allow the Humpback to stay motionless in the water, and to wait for a curious fish to come by and to serve as a meal or a mate.
Another striking characteristic of some species you might find in the abysses is the size of their mouth. Our friend the Anglerfish can swallow organisms twice its size! Another even more striking example is the Gulper Eel, known as the Pelican Eel or Umbrella Mouth, see for yourself (Figure 3); no wonder where it got its nickname, its mouth is almost its whole body! But why? Well, this is due to the Eel’s diet. Since it is not equipped of large teeth, it is believed to mainly eat small crustaceous. A mouth this big can be used as a net to capture a large amount of shrimps and small animals while swimming. The Pelican Eel is also known to be bioluminescent at the end of the tail, probably to attract a large group of preys and regroup them closer to its mouth.
The deep sea dragonfish (Figure 4) is another deep sea bioluminescent fish but this guy decided that two different colours would bring more fun to this dark world! The two different light sources, called photophores, are found at the end of its barbel and right under its eyes (on the “cheek”). They produce the common blue light, as well as red light. Just like the Anglerfish and the Pelican Eel, it is a way to attract its potential meal (or meet some friends). Once dinner is close enough, the dragonfish can attack, and having a look at its teeth, I would say the poor prey doesn’t get away easily… Some specimens can be pretty scary and science fiction-looking! (Figure 5)
From cool to scary, deep sea life and evolution brought some unexpected features that we are not used to. Living in a sunny warm environment makes it quite difficult to imagine how it would be possible to live in a way less welcoming place! But these few examples of deep sea organisms show how life adapts and conquers all the available and even extreme space on Earth where nutrients can be found.