What lies at the bottom of the ocean? For most, it’s a dark, mysterious place filled with creatures we know little about. But sometimes, discoveries shed light on those mysteries. A new study focused on an unusual deep-sea creature that loves to eat sargassum did just that.

Bathyopsurus nybelini, an isopod that lives about 3.7 miles (6,100 meters) below the ocean surface, has a unique diet: it feeds on Sargassum algae that sink from the ocean surface. The study, led by hadal ecologist Johanna Weston at the Woods Hole Oceanographic Institution (WHOI), shows how much the surface of our oceans is connected to the deep sea depths.

Sargassum in the depths of the ocean

In the summer of 2022, a research team embarked on an adventure aboard the manned submarine Alvin to explore the Puerto Rico Trench and Central Cayman Spread in the Caribbean Sea. The journey became a reality thanks to Alvin’s recent upgrades, which include better depth-of-field capabilities and high-resolution imaging.

While exploring the depths of the ocean, the team spotted something unexpected: an isopod swimming upside down and detaching itself from the seabed, clutching a sheet of sargassum seaweed as long as its own body. At these depths, ranging from 5,001 to 6,284 meters, Alvin photographed 32 individual isopods and collected two samples for further study.

According to Weston, “It was very interesting to watch this beautiful animal actively interacting with the sargassum in the deep ocean. This isopod is very rare; only a few specimens were collected during the pioneering deep-sea Swedish expedition in 1948.”

An unexpected food source

Sargassum is a type of brown algae that normally floats on the ocean surface, forming dense mats that stretch for miles. It thrives in the sunlit environment by photosynthesis like other plants. But when a sargassum dies or separates from its floating mats, it begins its slow journey into the dark abyss.

This is where Bathyopsurus nybelini comes in. This deep-sea isopod appears to have been patiently waiting, adapting to find and feast on this submerged food source.

“This isopod shows that an animal in the dark, high-pressure environment of the seafloor has evolved multiple adaptations to feed on algae growing in a sunlit ecosystem,” explains Mackenzie Herringer, a deep-sea physiologist at San Francisco State University and lead author of the study. “It’s an unusual food, but it provides essential nutrition.”

Isopods and deep-sea sargassum

Why is this discovery so important? First, Bathyopsurus nybelini has evolved unique adaptations to survive in an environment where most creatures have difficulty finding food. It uses a specialized swimming method, moving upside down and backwards with its broad paddle-like legs to lift sargassum leaves from the seabed. This may be a clever strategy to avoid predators by lifting food into the water column.

But the devices don’t stop there. This isopod has a serrated mouthpart that’s ideal for tearing into and swallowing tough, fibrous sargassum seaweed. Its gut is home to bacteria that help break down the algae’s complex polysaccharides—molecules that are notoriously difficult to digest.

“Life everywhere, even at the deepest depths of the sea, is inextricably dependent on the microorganisms that surround it,” says Logan Peeples, an aquatic microbiology ecologist at the Flathead Lake Biological Station.

The web of life from the surface to the seabed

The presence of Sargassum at such great depths has major implications for our understanding of ocean ecology. This research highlights the close connection between the surface ocean and the deep sea.

As the Sargasso Sea sinks, it plays a role in cycling and storing carbon, which in turn influences global climate change. We are already seeing significant environmental and economic impacts on coastal communities due to changes in sargassum abundance in the tropical Atlantic and Caribbean.

Researchers want to learn how much sargassum reaches the seafloor and how its amount varies with the season and long-term changes in the environment. Understanding these dynamics could help us predict how deep-sea ocean communities will respond to changing environmental conditions.

Why is this important?

The discovery of the sargassum-feeding Bathyopsurus nybelini also highlights the importance of advanced technology in deep-sea exploration.

The Alvin submarine that made this observation possible was recently certified to dive to a depth of 6,500 meters. This achievement opens new doors for marine research and highlights the importance of developing new tools and methods to explore the most remote areas of the ocean.

As Anna Michel, chief scientist at the National Deep Sea Diving Center, put it, “The discovery described in this paper was made possible by the new capabilities of deep sea diving, which is very exciting for the Alvin team.”

Deep water ecosystems and human impact

Why should we care about an isopod munching on algae on the ocean floor? It all depends on how interconnected our planet’s ecosystems are. As human activities change ocean conditions, from climate change to pollution, understanding how surface processes relate to deep-ocean ecosystems is critical.

Every discovery like that of Bathyopsurus nybelini reminds us that even the most extreme environments on Earth are part of a vast, interconnected web of life. The more we learn, the better we can protect these ecosystems and manage the consequences of our actions.

Sargassum’s impact on deep-sea regions of the ocean

This research is just the beginning. Scientists plan to continue investigating how sargassum and other surface materials affect life in the deep sea. More research is needed to understand seasonal and long-term changes in sargassum abundance and its impact on deep-sea food webs.

As research continues, new discoveries will undoubtedly change our understanding of these hidden ecosystems. “We’re excited to share his amazing story of adaptation and this important reminder that habitats and organisms on our planet are deeply and intricately connected,” Weston said.

The ocean is vast and still holds many interesting things. What other secrets lie hidden in the deep, dark waters? Only time, curiosity, and further exploration will reveal.