Swimming crustacean eats unlikely food source in ocean depths

What do deep-sea crustaceans eat? A new study, “A deep-sea isopod that consumes sargassum that sinks from the ocean surface,” published in the journal Proceedings of the Royal Society B: Biological Scienceshighlights a remarkable isopod species named Bathyopsurus nybelini, a deep-sea isopod that consumes sargassum flowing from the ocean surface.

Thanks to the Alvin submersible, scientists encountered this isopod swimming at a depth of 6 km, with oar-like legs as long as your fingers, and eating an unexpected food source: sargassum.

Seaweed forests sinking into the ocean bring food to the ocean depths

Surprisingly, these isopods carry something more common to the ocean surface: large chunks of sargassum seaweed. On the surface, the sargassum seaweed grows through photosynthesis, forming floating forests of seaweed.

In this new study, researchers from Woods Hole Oceanographic Institution (WHOI), the University of Montana, SUNY Geneseo, Willamette University and the University of Rhode Island demonstrate that even when this algae sinks, its story is not over. The isopod waits, specially adapted to find and feed on this submerged nutrient source. These findings about a deep-sea animal that relies on food sinking from waters miles above underscore how closely connected the surface ocean is to the deep ocean.

Unveiling the connection with the ocean depths

In the summer of 2022, an interdisciplinary team of researchers and engineers embarked on the human-occupied submersible Alvin to the Puerto Rico Trench and Cayman Islands spill center in the Caribbean Sea. Alvin had recently undergone a refit, including increased diving capabilities.

At 6,100 meters, Alvin’s enhanced 4K imaging system captured an isopod swimming upside down and away from the seafloor, carrying a sargassum frond as long as its body. During this expedition, Alvin filmed 32 individual isopods at depths of 5,001 to 6,284 meters and collected two samples for study at the surface.

Johanna Weston, co-lead author of the study and a hadal ecologist at WHOI, explained: “It was exciting to see this magnificent animal actively interacting with Sargassum, deep in the ocean. This isopod is so rarely seen; only a handful of specimens were collected during the groundbreaking 1948 Swedish deep-sea expedition, which proved that life could survive in the deepest half of the ocean.”

“The last photo of one of them was taken in 2011. Using Alvin and its newly upgraded capabilities to capture video and collect samples gives us a better understanding of what makes this isopod so special.”

Specialized adaptations allow this isopod to feed on sunken algae

The team combined morphological analyses, CT scans, DNA sequencing and microbiological studies to show that this isopod is physiologically and behaviourally adapted to use this submerged resource. This integrated process of observation and analysis has helped uncover this important link in the ocean food web, a significant contribution to deep-sea ecology.

Mackenzie Gerringer, co-lead author and deep-sea physiologist at SUNY Geneseo, says, “Deep-sea ecosystems appear to be harsh environments, but the animals that live in these habitats are well adapted to meet these conditions.

“This isopod shows that an animal living in a dark, high-pressure environment on the seafloor has developed multiple adaptations to feed on algae that grow in a sunlit ecosystem. We are excited to share its incredible story of adaptation and this important reminder that our planet’s habitats and organisms are deeply and intimately connected.”

One of its unique adaptations is its specialized swimming. This isopod moves upside down and backwards using large paddles, allowing it to carry sargassum fronds above the seafloor. This unique locomotion may be an evolutionary strategy to avoid predation by lifting its food source into the water column. The isopod also has serrated, chewing mouthparts, ideal for tearing apart and consuming tough sargassum, and has bacteria in its gut to aid digestion.

Seaweeds like Sargassum are difficult for many animals to digest because their cell walls are made of tough, complex molecules called polysaccharides. The isopod’s gut microbiome has genes that break down these tough compounds. As in the human gut, the microbiome provides important carbon and nitrogen nutrients to these hosts. As lead author Logan Peoples, an aquatic microbial ecologist at the Flathead Lake Biological Station, explained, “Life anywhere, even in the deepest depths of the sea, is inexorably linked to the microorganisms around it.”

An ocean: surface processes impact deep waters

The abundance and distribution of Sargassum in the tropical Atlantic and Caribbean Sea appear to be changing, with large blooms having ecological and economic impacts on coastal communities in the region. With these changes, much remains to be understood about the abundance and use of Sargassum at great depths. The presence of Sargassum at such depths has important implications for carbon cycling and storage.

Further studies will need to assess how much and where Sargassum reaches the seafloor, how its immersion changes seasonally and over the long term, and its relative importance in the broader deep-sea food web. Understanding the ecological impact of altered Sargassum deposits will be essential for predicting the responses of deep-sea communities to changing environmental conditions.

A window open onto our ocean depths

The discovery of Sargassum-feeding isopods enriches our understanding of deep-sea biodiversity. Advanced technologies such as the Alvin submersible and other integrative tools provide invaluable opportunities to observe and sample these key ecosystems.

Anna Michel, chief scientist at the National Deep Submergence Facility and co-author of the study, explains: “In 2022, Alvin was certified to dive to 6,500 meters. The discovery described in this paper was possible because of its new deeper diving capabilities, which is very exciting for the Alvin team.”

As human activities continue to affect ocean conditions, from pollution to climate change, understanding the links between surface processes and deep-ocean ecosystems will be essential to developing strategies to mitigate these impacts.