Humans get off easy when it comes to puberty. The vast majority of animal species, from frogs to ants, experience two or more different lives, with completely different body shapes, diets, and environments.
But the jellyfish has a life cycle so radical you can hardly call it a transformation. The blobby marine mushroom beachgoers know and fear represents just the tip of the biological iceberg. If you could ask the creature itself (and it could respond), it might tell you it considers its main form to be the humble “polyp”—a small rod on the ocean floor that pumps out stringy medusa, which we know as jellyfish.
How one animal with a fixed set of genes pulls off stints as both a semi-immortal blob factory and then a short-lived predator is a mystery, but biologists have taken one big step toward finding out. After years of painstaking work, a team of evolutionary biologists has released the genome of the moon jellyfish, Aurelia aurita, in a new paper in Science. Their results suggest that, counterintuitively, Aurelia lives its double life without resorting to much genetic trickery at all.
“The weirdest thing about jellyfish is how normal they are,” says David Gold, a paleobiologist at UC Davis and one of the paper’s authors. His work on the moon jellyfish represents the first genome from the group of “true jellyfish” to be published. (The genome of a different variety of jelly hit the online collection of bioRxiv over the summer, but has yet to be peer reviewed.)
Easy to raise and lacking a painful sting, the moon jellyfish has become a fixture in labs and aquariums, making its gene sequence a hotly desired commodity. “I’ve wanted the data for a really long time,” says Rebecca Helm, a jellyfish researcher at the University of South Carolina, Asheville. “I’ve taken David out to beers and been like, ‘Ok David, when is the genome coming out?'”
The team carried out the research in two parts. They first assembled chopped up bits of DNA into the proper order. About half of the strings of characters were so repetitive as to be nearly indistinguishable, like a jigsaw puzzle that’s half clouds—and has hundreds of millions of pieces. Once they felt reasonably sure the DNA sequence passed sanity checks like the right overall number of genes and the inclusion of animal “must-haves” like basic cell functioning, they moved on to sequencing the creature’s RNA.
RNA tells you what genes are awake and making proteins at a certain moment, so it was the perfect tool to figure out what was letting the stationary polyps shoot off all these swimming medusas—the most recognizable form of the jellyfish. By comparing the active genes at each of six life stages, they built up a picture of what the genome was doing in each phase. All told, the work took about seven years.
Gold expected that the key to the metamorphosis would lie in the genes found only in jellyfish, as opposed to those they share with their relatives the corals and sea anemones, which live only as polyps. When it came time to break free as medusas, those jellyfish-exclusives would kick into overdrive at that specific moment, he surmised.
But that’s not what the group found. Rather, throughout every stage of its lifecycle Aurelia draws relatively evenly both from its pool of special genes and from the ancestral genes found in corals and anemones too. Some exceptions aside, the creature appears to possess little in the way of novel genetic machinery to drive their spectacular transformation—a result that flies in the face of the common assumption that an animal must undergo many mutations to evolve new shapes and abilities.
The new genome suggests the group’s evolutionary history went one of two ways. Perhaps the simple polyp came first, and then environmental pressures caused one population to repurpose old genes to allow for transformation, picking up a swimming medusa stage and splitting off to become the modern jellyfish.
Or the opposite could have happened. Perhaps the common ancestor started with a multi-stage lifecycle of both polyps and medusas, and then some populations lost the medusa stage and became corals and anemones. The relative paucity of genetic tricks unique to today’s jellyfish means its evolution could have gone either way. “We have a tendency to think that the more complex creatures out there are more likely to be the more evolved organism,” Gold says. “We’ve seen time and again that’s not the case. Some animals become simpler over time.”
The maverick jellyfish has never been shy about breaking with biologists’ expectations. A number of ocean-going species have somehow ditched the polyp phase altogether, like caterpillars deciding to skip the pupa stage and fly straight into the skies. “There are over a million species of insect and none of them can do that,” says Helm. “If it happens once that’s already too many but it’s happened [in jellyfish] at least four times.”
Understanding how jellies reinvent themselves, over their own lifespans as well as in the evolutionary record, will speak to how transforming animals like sea urchins, eels, and beetles develop, maintain, and perhaps even lose their complex life cycles. This case emphasizes that natural selection can give animals new traits in more than one way: it can pressure them to develop new genes, or to update how they use genes they already have.
Helm likens genes to the instruments in an orchestra and an animal’s form to the song they play. Booming trombones make one stage, while chirping flutes produce another. To mix things up, you can either add new instruments, or, as the jellyfish does, repurpose old instruments for a drastically different sound.
The resulting research questions are just as diverse. Gold worked with a group of 10 researchers at seven different institutions. He wants to figure out what Aurelia, which evolved shortly before organisms with skeletons showed up, can reveal about the environment of the earliest animals. Neurobiologists hope to learn how an animal with no brain can sense and hunt plankton. And others are interested in how the polyp is nearly indestructible—you can basically blend it and the cells will find their way back together—while the medusa is so delicate.
That the jellyfish life cycle seems so alien to us just proves we view the tree of life through a narrow lens, Helm says, focusing on familiar animals with spines and brains. And each genome sequenced represents a step toward seeing our one-note species for the outlier it is. “We really know very little about how animals like jellyfish make their way through the world,” she says. “Adding these additional genomes means that we can really begin to develop a non-biased perspective about what it means to be an animal.”