Deep Ocean or Deep Freeze: How Animals Have Evolved to Survive

Animals are pretty remarkable – we can find them in virtually every environment on Earth, from the perpetually frozen hallows of Antarctica to the pressurized, pitch black depths of the ocean. How did the animal kingdom come to dominate such a diverse set of environmental conditions? We can begin to answer this question by looking at the process of how animals adapt to their surroundings.

Adaptation is the process that underpins evolution. Charles Darwin’s famous theory states that over long periods of time, species can adapt so well to their environment that they can eventually split and become a new species altogether. Darwin established that animals (and other living things) evolved into a new species through a process called natural selection, where those individuals who are best adapted to the habitat they live in are more likely to survive, and therefore are more likely to have offspring who will also carry those traits. Meanwhile, those individuals who failed to adapt as well would not have as many offspring, and those weak traits would eventually disappear.

Some of the most fascinating adaptations have occurred in our oceans. In two of the most extreme environments on Earth, Antarctica and the deep sea, environments where you would think life shouldn’t exist at all, animals have evolved over millions of years to cope with life in there.

Antarctica is colder, windier and drier than anywhere else in the world. In fact, it hasn’t rained in some places there for nearly 2 million years! And temperatures can reach as low as -22°F (-30°C) on land and 28°F (-2°C) in the sea. The only reason the water doesn’t freeze is because it is filled with salt.

Despite the extreme conditions in Antarctica’s waters, they are still teeming with life.  As it happens, most of the organisms that inhabit Antarctic waters have lived there for millions of years. All this time has allowed these creatures to adapt and become masters of their environment. For example, small planktonic copepods, a species of crustacean that lives in Antarctica’s waters, have 70% body fat, making them one of the fattest organisms in the world. This fat gives them an energy reserve that they can tap into when food becomes scarce across the unpredictable seasons of Antarctica.

You might be wondering, if the water surrounding Antarctica is so cold, why don’t these copepods and other fish freeze? Well, you can credit another adaptation: special anti-freeze proteins. These proteins circulate in the fish’s blood and bind to any ice crystals that begin to grow, preventing them from growing any further.

how animals adapt to extreme environments
Antarctic Sea Spider. Copyright Keith Martin-Smith

Some invertebrates in Antarctica express a different adaptation that which at first, is the most baffling. Animals such as this Pycnogonid or ‘Sea Spider’ are huge. In fact, some species are 1000 times heavier than sea spiders in temperate waters, and they can grow up to the size of a dinner plate. Imagine seeing one of those crawling across your bedroom floor!

Why do these spiders grow so big? Well, to really understand this, we need to look at the water itself. Firstly, cold water can hold more dissolved oxygen than warm water. This is because molecules move slower the colder they get, so dissolved oxygen gas can pack more tightly together in the water. Secondly, these animals’ metabolisms work much slower. Since it is so cold, they move around very little, and as a result, their cells don’t need as much oxygen to metabolize. So, what we have now is an animal whose body is rich in oxygen but has such a slow metabolic rate that their cells don’t need it. Rather than wasting all this oxygen, the animals will use it to create more and more new cells. And new cells mean more tissue and therefore, a bigger body.

However, scientists are starting to get concerned about how fast animals in Antarctica will be able to adapt to the increase in water temperature in the oceans. This research is becoming more and more of a hot topic in marine biology (no pun intended).

The Deep Sea is another extreme ocean environment where the animals here have had to adapt in extraordinary ways in order to survive. The Deep Sea is one of the least explored places on earth; we know more about the surface of Mars than the ocean floor, and scientists estimate that they still have thousands of species down there to discover.

The deepest part of the ocean is the Marianas Trench, which at 36,000 ft deep, could fit Mount Everest inside and still have a mile or so to go before it reaches the surface. So, what happens when the ocean is this deep? Well, the deep ocean contains a lot of water, and a lot of water means lots of pressure pushing down on you. If you dove five miles down, it would feel like someone stacked 50 airplanes on top of each other and then dropped them on your head! To avoid being crushed to death down there, most fish are built differently than those that live closer to the surface. Fish on the surface of the ocean have swim bladders that are filled with gas, which helps them float up and down. But fish in the deep sea don’t have those. Instead, these fishes’ swim bladders contain oil, which is denser, or they don’t have a swim bladder at all. These fishes’ bodies also contain a lot of water, and they have very thin skeletons, which helps them match the pressure of the water outside with the pressure inside of their bodies.

deep sea or deep freeze
Barreleye fish

The deep sea is not only under a lot of pressure, but it is also very dark. But animals such as this Barreleye fish have evolved excellent eyes for seeing in near-total darkness.

Eyes contain a type of light receptor called rods. Rods help eyes sense light. Humans have a good number of these, but Barreleye fish retinas are packed full of them, which makes their eyes more sensitive to light. Furthermore, their eyes have also adapted to give them another advantage: they can roll to the top of the fish’s head so the fish can see prey swimming above them. Once the fish spots a target, it rolls its eyes forward again and enters hunting mode.

The Deep Sea also has its own language. A variety of animals down, from plankton to cnidarians to fish, use bioluminescence as their main form of communication. These animals contain a light-producing organ in the body that contains a protein called luciferase. When the luciferase is oxidized, it emits light (fireflies carry the same protein). In the Deep Sea, bioluminescence’s main purpose is to either attract prey or confuse predators.

deep sea or deep freeze
Bioluminescent jellyfish

In some cases, animals mimic the bioluminescence of other animals to trick their prey. For example, sperm whales have white marks around their mouths that mimics the bioluminescence released by smaller fish. When the sperm whale’s prey, the Giant Squid, sees this, it thinks it has found food, so it swims over. Little does it know that it will soon cross paths with a whale and be eaten.

These are just a few of the amazing adaptations found in the Deep Sea and Antarctica. Less than 5% of the oceans have been explored, which means we can only imagine what other types of organisms we may find in the future, and what amazing adaptations that may have evolved to help them survive.

Ellis Moloney is a master’s student in applied aquatic biology at the University of Portsmouth in the United Kingdom. Ellis writes his own blog, The Science Rationale, and you can find him on Twitter @Ellis_Moloney.