The “Can­de­labra” black smoker at a wa­ter depth of 3,300 meters in the Log­atchev Hy­dro­thermal Field on the Mid-At­lantic Ridge (Image Credit: MARUM − Zentrum für Marine Umweltwissenschaften, Universität Bremen, CC BY 4.0, Image Cropped)

The deep sea is an unimaginably large and dark environment, and humanity’s attempt to learn about it is comically clumsy. Sampling the animals in the deep sea is often done “blindly”, by dragging nets along the ocean floor or through the water column, or bringing up cores of deep-sea sediment. The most sophisticated, precise and least destructive method is using underwater robots that have arms that can be controlled remotely to sample specific animals in real time, though naturally, this is also the most expensive.

These sampling efforts are comparable to sampling a rainforest with a helicopter. At night. With a map that a kindergartener drew. How long would it take to get a reliable record of all the different species of bird, beetle, monkey and flower found in the rainforest? How long to find a male and female of every species?

When humans first started sampling the ocean and describing its species, they believed that no life could possibly exist past 550 meters. It took until the late 19^th century for scientists to notice that there was indeed a wide abundance of life in the ocean’s depths, especially on the ocean floor, that was yet to be described.

Then, a century later, in the late 1970s, geologists discovered a never-before seen ecosystem in the deep sea: hydrothermal vents.

They had noticed that at mid-ocean spreading centers, where tectonic plates are moving apart, temperature anomalies in the water column were common. These temperature anomalies were often accompanied by unexplainable events, like dead benthic fishes floating to the surface. Scientists hypothesized that volcanic activity was responsible for many of these anomalies.

To explain what was happening, geologists set out on an expedition with a camera sled and a manned submersible, making a discovery that would change our perception of life on Earth forever. What they saw was unbelievable: vents expelling black smoke, with bizarre animals living in close association with the vents. While the expedition did not include any biologists, the geologists on board still managed to collect biological samples from the vent site using their manned submersible Alvin. The most famous species described from this expedition is a giant tube worm, Riftia pachyptila. They can get up to three metres long and live on the vent itself, stretching out their characteristically red plumes, filtering the water for essential nutrients. 

Before hydrothermal vents were discovered, scientists believed that all life on Earth was dependent on the sun to fuel primary production. On land, primary production is mostly carried out by plants, while in the sunlit layers of the ocean it is carried out by algae. Primary producers form the base of the food web, for example algae are eaten by small crustaceans, which are then eaten by small fishes, and those are consumed by predators at the top of the food web. At the ocean floor, small particles, such as fecal pellets, fish scales or other degraded organic material are deposited, fuelling life in these environments. Amazingly, hydrothermal vents are the only ecosystems known to us where life is independent of sunlight. The primary producers here are chemosynthethic bacteria, which often live in symbiosis with other vent-associated animals. These bacteria produce all the necessary organic material needed to sustain life in such extreme environments.

Related Content: How dead whales form unique ecosystems

In contrast to the animals found associated with the deep-sea sediment that makes up the rest of the sea floor, the animals living at hydrothermal vents have to withstand enormous challenges. Superheated water, a flurry of chemicals mixed in the water, and high competition for the best spot on the vent requires organisms to be highly adapted to these conditions. There is also the constant threat of a volcanic eruption that could cause the extinction of the entire ecosystem at any moment. In fact, the first discovered vent site was later found to have been covered in lava entirely, with only a few small vent sites remaining in the area. 

Since the 1970s, scientists have found hydrothermal vents all along mid-Ocean ridges, and have described the life there in detail. However, due to the ever-changing nature of these vent sites, new sites are discovered every year, and we have discovered that veering off-course and exploring the environments farther from the ridges can also yield interesting discoveries. The pressure to explore these sites in more detail grows every year, as deep-sea development like mining for oil or precious minerals means we run the risk of destroying ecosystems before we even know they exist.

---

Eva Paulus is an evolutionary marine biologist who just started her PhD at the University of Konstanz. Follow her on Twitter @Deep_Sea_Dirndl, or read more of her writing at her Ecology for the Masses profile.