On both land and sea, the sun is the primary source of energy in most ecosystems. Plants or algae use sunlight to create food through photosynthesis. Animals eat the plants and algae, and then get eaten themselves by predators higher up the food chain.

Without the sun providing a base amount of energy input, most ecosystems would swiftly collapse. Even so, there are some animals that live beyond reach of the sun’s rays.

The deepest parts of the ocean are over 36,000 feet below sea level. Light can only penetrate 3,300 feet through seawater, and the range where there’s enough light for photosynthesis to take place is even more restricted.

The sunlight zone extends 650 feet. Below that depth, photosynthetic activity quickly ceases. Photosynthesis is integral to most communities, but somehow the animals living deep underwater have found a way to survive.

How can life exist in the deep ocean without getting energy from the sun? They’ve found other sources of food, and evolved some very special features to help them make the most of it.

Energy Sources:

Marine Snow

They might not have the sun shining, but that doesn’t mean that deep sea communities don’t get anything from above. Almost all of the resource input in the deep sea comes from overhead. All sorts of debris drifts down, including many edible particles. There are sinking bodies of dead fish and other sea creatures, fecal matter, decaying microbes, and plants. Sometimes even chunks of wood will find their way down.

Rather than sunlight, this “marine snow” is the primary source of energy for many deep sea communities. This has created a very scavenger-heavy food web. Sea cucumbers, brittle stars, rattail fish, and numerous other crustaceans, snails, and other invertebrates making a living off marine snow.

Marine snow is not a tremendously energy-rich food source, so deep sea life is much sparser than that in shallow waters. However, there are rare events that bring feasting to even the darkest of seafloors: Whale falls.

Whale Falls

Bacterial mats coating gray whale bones at 5,500 ft below sea level.

Whales are the largest animals that have ever lived. The heart alone of the gargantuan blue whale weighs 400 pounds. They can eat almost 8,000 pounds of krill in a single day and their bodies represent immense stores of biologic energy, accumulated over the course of decades, or sometimes even centuries. When they die, that energy suddenly becomes available to the wider ecological community.

The death of a whale is a huge event wherever it happens, but it has the most impact when the whale remains sink to the deep ocean floor. A single whale corpse can have over 2 tons of carbon. For the nutrient-poor deep sea, this is a true bonanza of nutrients.

If a whale dies in shallow water, it doesn’t take long for its body to get broken down. The upper levels of the ocean are packed with hungry creatures waiting for an easy meal. What sharks and other scavengers don’t devour gets swiftly decomposed by bacteria. Whale remains that sink thousands of feet under the water find much less hospitable climes. Freezing cold temperatures dramatically slow bacterial decomposition and there are a lot fewer mouths to feed. Nevertheless, the scavengers do come. The ecosystems that appear around whale falls are completely unique, and utterly fascinating.

The decomposition of a whale fall in the deep sea has three recognizable stages. In the first stage, mobile scavengers descend upon the carcass. Hagfish and sleeper sharks feast on the soft meat and blubber. Giant isopods, bristleworms, prawns, shrimp, lobsters, and sea cucumbers also have a seat at the table. It can take up to two years for scavengers to pick the bones clean and the next stage to begin.

Stage two is the enrichment opportunist stage. At this point, what nutrients remain are locked in the bones and enriching surrounding sediment. Polychaete worms, crustaceans, and snails colonize the skeleton and nearby seafloor. It can take another two years until they’ve used up what nutrients they can access and this stage is complete.

The final part of the decomposition of a whale fall is the sulfophilic stage, named for the sulfophilic bacteria that are the major players during this time. Rather than using oxygen, these bacteria break down sulfate and release hydrogen sulfide for their metabolic processes. They feed off lipids in the bones, and as whale bones are very lipid-rich, it can take 50 – 100 years for the bones to be depleted. Dense bacterial mats grow over the bones, and mussels, limpets, clams and sea snails make meals of the bacteria. Whale skeletons are the dense metropolises of the deep sea. As many as 30,000 organisms have been found colonizing a single pile of bones.

Some dead whales are smaller or less complete than others and all proceed through the decomposition stages at their own pace. Partial remains may progress more quickly, or skip stages altogether. Big or small, whale falls are hugely important in ecosystems where free lunches are few and far between. There are species of Osedax worms that are specialized to feed on mammal bones. Without occasional whale falls, these creepy worms might go extinct.

Chemosynthesis

A colony of tube worms living near the Galapagos Islands.

It’s hard to find an environment more extreme than near a hydrothermal vent. Hydrothermal vents are places on the ocean floor where superheated mineral-rich water rises from rifts in the Earth’s crust. It isn’t a very hospitable habitat, but some animals still thrive there. Giant tube worms reaching lengths of 6 feet cluster around the vents. The ability of these worms to withstand intense heat and pressure is unusual and other aspects of their biology make them even stranger. Unlike almost every animal on the planet, these tube worms have no digestive tract.

A special arrangement with symbiotic bacteria means these worms have no need to consume outside food. The bacteria live within the worms’ bodies. The bacteria get hydrogen sulfide, carbon dioxide, and oxygen from the blood of the worm. The bacteria use these components to produce sugar, which the tube worm feeds off of. Both sides benefit and the tube worm is able to enjoy a life in a place that would instantly kill most other creatures.

There are other species with similar relationships to symbiotic bacteria, like mussels, snails, shrimp, and giant clams.

Adaptations and Behavior

There are many other adaptations that help deep sea creatures make the most out of their environment. Some of these adaptations are not in form, but behavior. Fish and other animals that live at moderate depths will swim up to the surface at night to feed in the food-rich upper ocean. They swim back down again before the sun rises, avoiding daytime predators. Throughout their daily migration they experience massive changes in temperature and pressure. The mechanisms for their ability to tolerate these intense changes are still poorly understood.

For animals that live at greater depths, it isn’t possible to return to the surface to eat every night. They modify their behaviors in other ways. Deep sea animals are much more sluggish than their shallow-water counterparts. When it’s hard to get food it becomes important to conserve as much energy as you can. Rather than swimming around to actively hunt, many deep sea predators wait in place until they can ambush their prey.

Some attract prey with light, like the anglerfish that uses attractive glowing lures. Many also have huge mouths and expandable stomachs that can engulf large prey all in one gulp. An extreme example of this is the black swallower fish which regularly swallows fish larger than itself and swims around with a grotesquely distended belly. Another common feature in deep sea fish is long inward-pointing fangs, designed to trap and puncture prey.

Muscles are another area where deep sea fish have made significant changes. The muscles of a fast-paced fish like a tuna are 20% protein. They can swim swiftly but require a lot of energy to survive. Something like a deep sea viperfish or blacksmelt has gelatinous muscle that contains only 5 – 8% protein. They require less energy to maintain their muscles but are much weaker swimmers as a result.

An Emerging Danger

A plastic microfiber in a sea pen polyp. Image by Michelle Taylor/University of Bristol

There’s something else that the deep sea animals are eating, and unlike rancid whale meat, it isn’t good.

Just a few months ago in the autumn of 2016, Dr. Michelle Taylor of Oxford University and her team of researchers published chilling new findings on deep sea life: They’re ingesting plastic. Apparently human-generated trash has made it over a mile below the surface, and deep sea animals are eating it.

Hermit crabs, squat lobsters, sea cucumbers, and other animals were found to have eaten plastic microfibers. These microfibers are found in a similar range of sizes as marine snow debris, leading to a tragic case of mistaken food identity. The plastic could have come from fishing nets, but also could be byproducts of washing clothing with synthetic fibers, something that just about all of us partake in. This is the first time microfibers have been found at such depth and the ominous discovery calls into question our environmental and consumer habits.

Admittedly, we have made steps forward in addressing oceanic microplastics. President Obama signed into law a measure outlawing the use of plastic microbeads in rinse-off cosmetics. However, we still have a long way to go. Plastic microbeads are still allowed in detergents, sandblasting, and cosmetics that are left on the skin. It isn’t always obvious which products contain microbeads, making it difficult for environmentally-savvy consumers to avoid ecologically harmful products.

Even if we did outlaw microbeads all together, what is to be done about the microfibers from clothing? It is a difficult question with no clear answer. However, if we want to preserve the health of both shallow and deep marine communities, it is one that we must address.

- Kate Dzikiewicz, Paul Griswold Howes Fellow