University of Western Australia
A tiny crustacean measuring just three centimetres in length is helping scientists unlock one of the most perplexing questions of the oceans’ hadal zone–how isolated are the animals that live in our deepest ocean trenches?
A new paper Barriers to gene flow in the deepest ocean ecosystem: Evidence from global population genomics of a cosmopolitan amphipod published in Science Advances by an international team of researchers discovered the answer is, probably extremely isolated.
The study was carried out during the 2018-2019 Five Deeps Expedition, the first manned descent with the submersible DSV Limiting Factor to the deepest point of each of the world’s five oceans, and focused on the amphipod Bathycallisoma schellenbergi which was unexpectedly found in the traps of nearly every trench of hadal depth (between six and 11 kms).
Professor Alan Jamieson, founder of the Minderoo-UWA Deep Sea Research Centre, chief scientist on the Five Deeps Expedition and co-author on the paper, said the ‘cosmopolitan amphipod’ became the unofficial mascot for the expedition because it turned up at each destination.
“It was strange because traditional thought was that these trenches are separate from each other and operate as isolated ‘islands’ with their own distinct fauna,” Professor Jamieson said.
“It would be like the same species of butterfly being found on the slopes of the Rockies, Andes, Himalayas and Alps.
“We finally had a global specimen collection to test questions that have been around since the 1950s. This led us to question this paradox and wonder if maybe we were looking at multiple but very similar-looking species.”
What looked like the same amphipod was found across 12 populations in the Pacific, Atlantic, Indian and Southern Oceans and researchers were able to test whether it was the same species.
The study’s first author, Postdoctoral Scholar at the Woods Hole Oceanographic Institution Johanna Weston, said scientists used a short section of DNA to test if all the populations were the same species.
“Remarkably, we found the amphipod was Bathycallisoma schellenbergi for all the trenches except one, the Atacama Trench,” Dr Weston said.
“The amphipods at the Atacama Trench are likely a new and undescribed species that is very closely related.”
Researchers then used a technique called RAD-seq to find single base differences in DNA fragments across the whole genome to test whether interbreeding was occurring between populations.
“We found that overall populations were not genetically mixed between trenches, indicating they were highly restricted to the trench they were collected from,” Dr Weston said.
“We did find evidence of limited interbreeding between two closely located and connected trenches, the Kermadec and Tonga trenches, which are separated by just 1000km, so surmised some amphipods could have swum across what are relatively shallow depths.”
Professor Jamieson said the study advanced the field of hadal science, particularly at the intersection of evolutionary and geologic history, providing evidence to show that each of the hadal areas acted like an island-like habitat with populations on separate evolutionary trajectories.