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Survival in the deep sea is inherently challenging: darkness is abundant, temperatures are near freezing, and food is hard to come by. Yet instead of withering away in the harsh conditions, many deep-sea animals, from massive spider crabs to giant squids, have adapted by growing supersized, dwarfing their shallow-water or terrestrial relatives. Why these animals get so big has interested scientists for more than a century. Now asking a slightly different question – how do they get so big? – scientists are getting closer to the answer.
A team of researchers recently sequenced the genome of the giant isopod Bathynomus jamesi – a first for a deep-sea crustacean. With round, segmented bodies, giant isopods look like a roller—except they can grow as long and heavy as a Chihuahua. The team behind the work, led by Jianbo Yuan, a geneticist at the Chinese Academy of Sciences in Beijing, hopes that the details hidden in the animal’s genetic code will help us better understand what goes on behind the scenes, from a genetic perspective, with the deep-sea gigantism.
Analysis of the genes of Bathynomus jamesi suggests how this giant isopod evolved the key adaptations that allowed it to thrive in the depths. Photo by Jianbo Yuan and Xiaojun Zhang
Giant isopods, or bathynomyids, are the large cousins of the armored crustaceans found scurrying around under fallen logs. While the smallest isopods measure less than half a centimeter, bathynomids can grow 80 times longer. The niches that isopods occupy are just as diverse: there are more than 10,000 known species, and they are found everywhere from the bottom of the ocean to caves to mountaintops. This physiological and ecological diversity makes the isopod family tree the perfect place to look for clues about what drives the adaptive steps below.
Among the most interesting questions, Yuan says, is whether today’s deep-sea giants simply descended from large ancestors — animals like anomalocarids, large arthropod predators that existed about 50 million years ago — or rather evolved under the pressure of life in the deep sea . In the case of giant isopods, their genome points to the latter explanation.
Like their bodies, batinome genomes are incredibly large. The researchers discovered that B. jamesi has a large number of so-called jumping genes, transposable elements that can move from one place in the isopod’s genetic code to another. Jumping genes are associated with high mutation rates, something the researchers say could make the isopod better equipped to deal with environmental stress.
Having a large number of genes is something that B. jamesi shares with other deep-sea invertebrates. The fact that invertebrates—organisms generally considered less complex than vertebrates—have evolved some of the most complex, adaptive genetic codes has baffled scientists since genome sequencing began.
In addition to revealing the size of its genome, the scientists’ delving into the biology and genetics of B. jamesi also offered possible explanations for a number of key adaptations the animals use to thrive in the deep.
The stomach of B. jamesi, for example, can expand to take up two-thirds of its body. This ensures that when it can find food, it can eat as much as possible. Yuan and the team also found changes in B. jamesi genes related to thyroid function and insulin, which likely enhance the isopod’s ability to grow and absorb nutrients. In addition, they found a setting that slows down the breakdown of fat. Keeping extra junk in the trunk allows giant isopods to go years without eating.
Alexis Weinig, a deep-sea biologist and geneticist at the Leetown Research Laboratory in West Virginia who was not involved in the study, says she likes that Yuan and his team are trying to better understand deep-sea isopods through their genes. Living in the deep sea, isopods are hard to find and harder to study in the field. “I think getting into the underlying genetics is going to be a big player in understanding the root causes of gigantism,” she says.
Weinnig hopes the discovery will remind people that beyond their potential to help make sense of a scientific problem, deep-sea species deserve the spotlight.
“We lose track of how amazing it is that these animals live on our planet,” says Weinig. “They have to be resourceful everywhere…with reproduction, with metabolic processing. Everything must be used so that nothing is discarded.’
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