We don’t make this stuff up. Researchers who wanted to know more about why jellyfish can move around but corals and sea anemones are anchored got some insights into that, via genome mapping—but also some other ones that they had not been expecting. From ScienceDaily:
“We expected that the genome organization in the two jellyfish would be more similar to each other than to the genomes of sea anemones or corals,” said Khalturin. Surprisingly, the gene order in the moon jelly genome resembled anthozoans [anemones] much more closely than fire jellyfish. In contrast, the genetic composition of the two jellyfish hardly overlapped; their genomes differ as drastically as humans do from sea urchins.
The results suggest that the giant box jellyfish genome must have been vigorously reshuffled at some point in its evolution. The dearth of similarities between moon and giant box jellies convinced the researchers that there is no universal region within jellyfish genomes responsible for orchestrating the medusa stage formation [when they can move around].
So there is no Darwin switch. Different jellyfish converge on moving around via different sets of genes.
Now, about why corals and anemones don’t move around?
Remarkably, they found that coral and anemones contain about two-thirds of the genes active in the moon jellyfish’s medusa stage. But moon jellyfish have a special genetic toolkit: an elite arsenal of genes that activate during their medusa stage but are absent in anthozoans. Devoid of a jellyfish stage, corals and anemones lack the genes to grow certain organs and tissues, such as eyes and specialized swimming muscles. The researchers found that water and fire jellyfish share about 100 of these species-specific genes that only switch on in their jellyfish stages. A large proportion of these genes code for transcription factors, proteins that fine tune which genes are expressed, when and in what quantities.Paper. (paywall) – Konstantin Khalturin, Chuya Shinzato, Maria Khalturina, Mayuko Hamada, Manabu Fujie, Ryo Koyanagi, Miyuki Kanda, Hiroki Goto, Friederike Anton-Erxleben, Masaya Toyokawa, Sho Toshino, Noriyuki Satoh. Medusozoan genomes inform the evolution of the jellyfish body plan. Nature Ecology & Evolution, 2019; DOI: 10.1038/s41559-019-0853-y More.
Did the jellyfish evolve this elite toolkit or did the anemones and corals lose it?
See, this is what genome mapping does to Darwinian evolution.
See also: Evolution appears to converge on goals—but in Darwinian terms, is that possible?
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