Biology Genetics

Deep-sea bug genetics 101

Piertney, S. B., Wenzel, M., & Jamieson, A. J. (2023). Large effective population size masks population genetic structure in Hirondellea amphipods within the deepest marine ecosystem, the Mariana Trench. Molecular Ecology, 32(9), 2206-2218.

The deep ocean is vast, dark, and home to a wealth of unexplored biodiversity. The deepest parts of the ocean form the so-called “hadal zone,” which occurs at depths below 6000 meters. Most of the seafloor is not that deep—the ocean’s average depth is about 4000 meters—but deep-sea trenches can reach all the way down to 11,000 meters, at the bottom of the famous Marianna Trench. Trenches are like reverse mountains: narrow, isolated depths carved into the shallow seafloor. And just like mountains, ocean trenches are home to unique ecosystems and animals.

Trench ecosystems can be very different from shallower areas because animals need special adaptations to survive the extreme pressure. Species can become specialized and isolated from one

An example of a deep-sea amphipod, Alicella gigantea, measuring several inches in length. Image source: Wikimedia Commons

another. For example, many species of amphipods—shrimplike crustaceans—are only found in a single trench. We still lack a clear picture of biodiversity in the hadal zone simply because it’s very, very hard to study the deep sea… but luckily, new robot submersibles are making this job a little easier. Scientists are using these remote-control subs to get a detailed glimpse into amphipod populations within the Mariana trench, the deepest habitat on Earth.

DNA tells a story, even 10,000 feet below

Scientists can learn how closely-related animals are by studying their DNA. Over time, generations pass and mutations occur. If two populations are completely isolated from one another, their DNA will become different and they may even eventually become different species. However, if members of the populations sometime mate with one another—essentially mixing DNA—that will offset those changes and increase the similarity between their genomes.

Map showing the location of the Mariana Trench. Image source: Wikimedia Commons

Using robot submersibles, the researchers sampled amphipods from different locations in the Marianna trench, spanning 50 kilometers of distance and from 8000 – 10,000 meters depth. After examining their DNA, the scientists found that distant populations were less genetically related, and that deeper populations were distinct from shallower ones.

Importantly, this shows that the trench amphipods are not just one big, interbreeding population. This may be related their lifestyle, as these amphipods do not swim long distances and brood their young internally. Basically, they don’t get around much.

A clearer picture of biodiversity in the deep ocean

This study is one of the first to describe deep-sea populations over short distances. Not only are trenches separate from one another, but populations can diverge within a single trench. The deep sea is not just one big, interconnected ecosystem, but genetic isolation can occur on very small spatial scales.

Companies are currently opening the deep sea up to mining, which risks destroying these unique ecosystems even before we’ve had the chance to describe them. Studies like this demonstrate the uniqueness and value of deep-sea habitats, and can be used to plan conservation efforts in the future.

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