Nearly 900 meters down in the northern South China Sea, in water so cold and dark it barely qualifies as a habitat at all, giant isopods crawl across the seafloor mud. They look like enormous, armored pill bugs, some stretching close to a foot long. Food rarely comes their way. When it does, in the form of a fish carcass or other scraps drifting down from above, these animals gorge themselves, packing their stomachs to bursting. Then they simply wait. Some isopods in this group are known to go more than five years without eating again.
This extraordinary endurance seems surprising for an animal of this size. Big bodies usually need more energy to keep cells running, tissues repaired, and organs functioning. Yet these deep-sea isopods have both worked out, over millions of years of evolution, how to grow unusually large and how to survive unusually long stretches without food. A team of researchers, led by a marine biologist, Jianbo Yuan at Chinese Academy of Sciences, set out to understand how the two traits fit together, and their findings, published in the journal Cell, point to an unexpected source of the animals’ survival trick: a small piece of genetic material that the isopods appear to have picked up from bacteria, long ago, and then put to use in a very particular way.
Isopods with different traits
The researchers focused on two related species of giant isopod in the genus Bathynomus. One, Bathynomus jamesi, lives around 898 meters down and grows to a supergiant size, with an average body length of about 232 millimeters, or roughly nine inches. The other, Bathynomus doederleini, lives in shallower water, around 300 meters down, and tops out at less than half that length. For comparison, the team also studied a much smaller relative, Cleantiella isopus, which lives in the intertidal zone near the shoreline and grows to only about an inch long. The deeper an isopod lives, the researchers found, the bigger its body tends to be, a pattern long observed in ocean life and known as the “bigger-deeper” trend.
When the researchers dissected these animals, they found that the deep-dwelling B. jamesi carries an enormous stomach that can take up about two-thirds of its entire body cavity, and in the animals they examined, it was crammed full of a “finely ground, extensively digested mud-like mixture,” the researchers write in the paper. The shallower B. doederleini, in contrast, had a much smaller stomach, sometimes containing only fish bone fragments, and occasionally nothing at all. The researchers concluded that B. doederleini eats more often, in a moderately food-scarce environment, while B. jamesi has adopted “a strategy of episodic hyperphagia coupled with highly efficient nutrient utilization to cope with extreme food scarcity in its deeper, more nutrient-limited environment,” say the researchers.
Yuan et al.
The team also measured several markers of metabolic activity, including enzymes involved in energy production and the buildup of reactive oxygen species, a byproduct of burning fuel in cells. Across the board, both deep-sea isopods showed a noticeably slower metabolism than their shallow-water relative, and B. jamesi‘s metabolism was even more suppressed than B. doederleini‘s. This matches what scientists already knew about life in the deep sea: animals living below about 800 meters tend to have reduced metabolic rates compared with those closer to the surface, likely because conditions down there — cold, dark, and stable — change very little no matter how much deeper you go.
Why prominent body size?
But growing to a large body size demands energy, and the researchers describe this directly as “the energy paradox in the deep-sea isopods: how do these apparently energy-efficient organisms sustain their enormous size, given the sporadic food availability in the deep sea?” To find out how the isopods manage this feat, Yuan and his colleagues turned to the animals’ genomes, sequencing the DNA of B. doederleini for the first time and comparing it against the previously sequenced genome of B. jamesi.
The genomic traces revealed a gene called ND1 that did not appear to belong there at all. Its sequence closely resembled a gene from bacteria involved in a cellular energy-production system called the oxidative phosphorylation pathway, the same pathway mitochondria use to generate usable energy in animal cells. Yet this particular version “shares sequence homology with NADH dehydrogenase within the OXPHOS system, yet it encodes a short peptide (∼88 amino acids) of unknown function,” says the paper. In other words, it looked bacterial, but its job was unclear.
Genes do not usually move between totally unrelated species; they are passed down from parent to offspring. But every so often, in a process called horizontal gene transfer, an organism ends up with a gene that came from an entirely different branch of the tree of life, sometimes from bacteria living inside or around it. Through genetic testing, including designing PCR experiments that specifically targeted the borders between the foreign gene and the isopod’s own DNA, the researchers confirmed that ND1 really had been swapped into the isopod genome from a microbe, probably a bacterial symbiont, and that this happened before B. jamesi and B. doederleini split into separate species around 16 million years ago.
Most genes acquired this way tend to fade out over time or stay quietly inactive, in part because scientists have observed that genes with unusually high activity levels tend to resist being successfully transferred between species in the first place. But the ND1 got copied multiple times within the isopod genome, and one of those copies became highly expressed. This single copy “displayed the highest expression level among the 23,221 gene repertoire of B. jamesi, which exceeded the expression of host paralogous genes by over 24-fold,” says the researchers. “While post-transfer duplication and high expression are rarely documented in HTGs, ND1 has uniquely overcome both barriers.”
The researchers also found a likely explanation for how ND1 got switched on so strongly: a chemical tagging system called histone acetylation, which helps loosen up DNA so genes can be read and copied more easily. They found that the isopods’ stomach tissue, especially in B. jamesi, had far more of this chemical tagging activity than in B. doederleini, concentrated right around the ND1 gene’s control regions. Notably, this same kind of tagging showed up more strongly in the stomach bacteria of B. jamesi as well, suggesting that the isopod and its resident microbes evolved this energy-regulating trick together.
ND1 at work in Zebrafish
To prove it actually helps the isopods survive starvation is another, and since these animals can’t easily be raised or manipulated in a lab, the researchers inserted a copy of the isopod’s ND1 gene into zebrafish, roundworms, and human cells, then tested how each fared under starvation.
The results depended heavily on temperature. At a typical warm temperature, zebrafish carrying the isopod gene actually did worse under starvation. According to the study, these fish showed “accelerated catabolism during prolonged food deprivation, accompanied by significantly elevated mortality (23% increase…) and greater body weight loss (18% increase…)” compared with control fish. The gene appeared to rev up their metabolism, burning through their reserves faster.
But when the fish were instead kept at a colder temperature meant to mimic the chilly conditions of the deep sea, the pattern flipped. Cold naturally slows an animal’s metabolism, and in fish carrying the ND1 gene, that slowdown went even further, pushing several markers of energy use and cell stress even lower than in cold-acclimated control fish. The payoff was longer survival: “survival in ND1-knockin zebrafish was extended by 37% longer than controls” under starvation. Similar effects turned up in gene-modified roundworms, which burned through their fat reserves more slowly in the cold, and in human cells grown in the lab, which showed reduced energy-production activity when starved at a lower temperature. Summarizing the pattern across all three systems, the study authors describe it as “an energy trade-off mechanism” — the same gene that speeds up metabolism in ordinary conditions helps suppress it further once the body is already running cold and lean, which is exactly the situation these isopods live in permanently.
Additionally, B. jamesi‘s stomach, in contrast to its shallower relative’s, is unusually rich in a group of microbes called Chlamydiae. Chlamydiae are better known as agents of disease in humans and other animals. These particular Chlamydiae are missing certain enzymes they would normally need to run their own energy cycle efficiently, and the researchers suspect the isopod’s ultra-active ND1 gene may help compensate for that gap, in exchange for the bacteria’s role in helping the isopod store fat.
“Chlamydiae are typically recognized as pathogenic microorganisms in humans and many other animals, whereas they persist as a tight associate within the stomach of B. jamesi,” and the researchers describe the overall arrangement as “a win-win cooperation model between the host and Chlamydiae: the host acquires a bacterial gene to promote metabolic and redox robustness during starvation, while Chlamydiae gain metabolic support from the host, ensuring their persistence in this unique niche.”
Journal Reference: Cell. DOI: 10.1016/j.cell.2026.05.012
