Identical micro-animals live in two isolated deep-sea environments. How is that possible?

The researchers traveled on the research vessel Polarstern to South Sandwich Trench where they collected sediment samples.
Photo Credit: ©Anni Glud/SDU

Halalaimus is a microscopic nematode genus commonly found in sediment on the seafloor. It lives 1–5 cm below the sediment surface and grazes on bacteria or organic materials in the sediment. 

It does so in the Aleutian Trench as well, which lies in the northern Pacific Ocean, near the Bering Sea. We now know this because PhD Yick Hang Kwan from Danish Center for Hadal Research at the Department of Biology has isolated its eDNA in sediment samples collected from the depths of the Aleutian Trench. 

“But we also found its eDNA in sediment samples from the South Sandwich Trench, which lies 17,000 km away in the South Atlantic. And that inevitably makes you ask: How is it possible that the same nematode genus exists in such extremely isolated deep-sea environments so far apart, when it has a very limited ability to move – and when the trenches are up to eight kilometers deep?” Kwan asks rhetorically. 

Sediment cores have been retrieved from the deep.
Photo Credit: ©Anni Glud/SDU

The Meiofauna Paradox  

Meiofauna is a collective term for microscopic animals, many of which remain undescribed by science. Despite their tiny size and extremely limited mobility, many meiofaunal nematodes have astonishingly wide distributions and occur in every ecosystem on Earth. How can organisms that cannot move on their own end up in such vastly different environments and colonize most habitable places on Earth. This question lies at the core of the meiofauna paradox. 

Can barely move on their own 

At first, the researchers found that the nematode communities in the two trenches were different. 

"That is what we expected, so far so good, but then we realized that three identical genera live in these different communities in different trenches. I think of it this way: Everywhere is different, but something is everywhere", comments head of HADAL and co-author of the study, Ronnie N. Glud. 

The fact that Halalaimus appears in two such remote locations illustrates the meiofauna paradox: that the same genera and species of tiny sediment-dwelling animals can occur thousands of kilometers apart and in vastly different environments. But when these animals are so small and essentially immobile, how can they be separated by entire ocean basins? 

“It’s remarkable,” says Kwan. “How has a nematode like Halalaimus managed to colonize both the Aleutian Trench and the South Sandwich Trench when it can barely move beyond the tiny colony in which it lives in the sediment?” 

Desmoscolex is one of the nematodes found in both South Sandwich and Aleutian Trench.
Photo Credit: ©Aleksandr Novikov

Distinctly different habitats 

In their study, Kwan and his colleagues examined sediment samples from nine different sites in the two deep-sea trenches and found eDNA from 58 genera of nematodes. Three of these genera were present in both trenches. So, it is not only Halalaimus that lives on the bottom of both the Aleutian Trench and the South Sandwich Trench: 

Desmoscolex and Chromadorita were also found in both places. Like Halalaimus, they are so small that they must be measured in microns and cannot be seen with the naked eye. All three genera are colorless, nearly translucent, and extremely abundant in the sediment samples – often numbering in the hundreds or thousands in just 10 grams of sediment. 

An interesting observation was that the three identical genera of nematodes did not live in identical habitats in the two trenches: Instead, they lived in distinctly different habitats in the two trenches. This means that the sediments were different and contained different concentrations of carbon and nitrogen and thus offered different concentrations of food for them. 

Head of research, Ronnie N. Glud, is working with sediment samples in the cold room, which mimics the in situ temperature.
Photo Credit: ©Anni Glud/SDU

Most of them probably die trying 

"The nematodes living there belong to the same genus but have adapted to very different habitats in the ocean trenches. That was surprising for us," said Kwan. “Because you would not normally expect the same animal to live in such different environments”. 

How the three nematodes reached both trenches and now thrive in such different habitats there, remains unknown. Kwan thinks that resilience has a lot to with it, though: 

“When they disperse and try to adapt to new environments, most of them are likely to die trying, we think. Probably only the most resilient manage to establish new communities in new environments”. 

As for how the dispersal happens, the researchers find it unlikely that they crawled out of one deep-sea trench and moved across the seafloor to the next, as they can hardly move on their own. Instead, it seems more plausible that connected colonies living near a trench slid down its slope and ended up in the trench interior. Earthquakes occur regularly and can send large amounts of sediment – including nematodes – cascading into the depths. 

And now time to collect more from the deep.
Photo Credit: ©Anni Glud/SDU

One little colony at a time 

As for how they have spread to other parts of the world, Kwan suggests they may spread by forming new colonies very close to the parent colony. Over many generations, this slow local expansion could allow it to occupy new areas of the seafloor, eventually leading to its presence in nearly all ecosystems on Earth. 

“With our study, we’ve shown that the meiofauna paradox extends into the deepest ocean trenches. That suggests that some form of connection exists between the trenches, even though this doesn’t seem immediately possible. We’re very excited about that”, he concludes. 

Published in journal: Progress in Oceanography

Title: Trenches apart, yet worm to worm: inter- and intra-trench comparisons reveal divergent and convergent dynamics in hadal nematode biodiversity

Authors: Yick Hang Kwan, Sofie Derycke, Blandine Trouche, Frank Wenzhöfer, Mathias Middelboe, Angelika Brandt, Mauricio Shimabukuro, Ronnie N. Glud, and Daniela Zeppilli

Source/Credit: University of Southern Denmark | Birgitte Svennevig

Reference Number: mb121625_01

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