Jellyfish have lasted 500 million years, evolving a complex life cycle. But until now, how they transform from polyp, their juvenile form, to medusa, the tentacled, translucent, gelatinous, adult form was a mystery.
For the first time, scientists have sequenced and published the full genome for a jellyfish, the moon jelly Aurelia (Aurelia aurita), and find that the metamorphosis appears to be carried out by recycling existing genes.
“These findings provide further evidence that evolution doesn’t necessarily make the genetic code more complex,” says David Gold, from the California Institute of Technology, US, lead researcher on the study.
“Jellyfish can build a big, complex life history using many of the same genes found in simpler animals.”
The researchers suggest two hypotheses that could explain this aspect of jellyfish evolution. One option is that medusozoans, the clade they belong to, uniquely developed the ability to go from polyp to medusa.
“This interpretation, if correct, suggests that animals can transition into radically different ecological niches (in this case, transitioning from benthic to pelagic carnivores) without major innovations in gene content,” the researchers write in a paper published in the journal Nature Ecology and Evolution.
They say this hypothesis suggests that genetic innovations might be less important to early evolution than previously thought.
The second, more controversial, hypothesis is that the last common ancestor of the cnidarian phylum, which includes anthozoans – corals and sea anemones – as well as jellyfish, had a medusa life stage, but that at some point the former two groups stopped metamorphosing.
“Our results can’t distinguish between these two scenarios,” explains Gold, but adds that if the second hypothesis is correct, “swimming, carnivorous animals may be even older than we think”.
Both hypotheses are reasonable, Gold and his colleagues say, because of the unique characteristics of the species.
“Despite the current popularity of the ‘polyp-first’ scenario, it is worth reiterating that neither the polyp nor medusa life stage is found outside of cnidarians,” the authors write.
“It is therefore equally parsimonious for the first cnidarians to have had a biphasic life cycle that was lost in anthozoans, or for the medusa phase to have originated in medusozoans.”
Related reading: Digging deep into jellyfish genomes