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Box jellyfish

jellyfish trio
Matteo Tarenghi

Jellies and comb jellies have recently reminded us that almost anything we think we know about evolution is apt to be overturned at a moments’ notice. Creation ’scientists’ must be rejoicing. Comb-jellies (like the one below) are not true jellyfish, as they lack stinging cells. They’re members of the group ctenophora. But even true jellyfish continue to muddy our simple, logical ideas about evolutionary succession.

comb jelly
Courtesy: National Science Foundation

See, according to their morphology, jellyfish are simple animals. They’ve no ‘front’ end, so they function perfectly well from any angle. They lack the central layer of embryonic tissue found in higher animals that develops into muscles, but they do have rudimentary eyes and nervous systems. In the traditional evolutionary tree, these features place them neatly between sponges and bilaterans (creatures with a front and back, like us). Later, when animals became bilateral, they were able to develop specific organs for different parts of the body, and this gave rise to the incredible increase in diversity known as the ‘Cambrian explosion’. But when things fit together that neatly, you know it’s too good to be true.

sea nettle
Christopher Chan

It turns out that jellyfish are more complex than was previously thought. They do in fact possess the genes that program for a front-to-back axis, they simply don’t utilize them. Either that, or these genes are being used to specialize their brains in some incredibly subtle way. This may mean that cnidarians (the group that includes jellyfish) are in fact descended from more complex, bilateral animals, and secondarily adopted their simpler shape! So while a common ancestor of cnidarians did plug the link between sponges and bilaterans (and there are ideas about what that animal may have been), the cnidarians themselves have continued to evolve until they became the jellyfish we know today.

Moon jellyfish
Kyle Tsui

For us humans, the most unsettling part is that these genes are the same as those present in all vertebrates. So some of the ‘advances’ usually attributed to vertebrate body form may in fact be much older…

Leucothea

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