Like a creative flourish from God's paintbrush, they are a dash of colour on the high seas, bringing both beauty and death wherever they go. Largely ignored by science for decades – outside of the Far East they're not commonly eaten, and so of little commercial interest – these poorly-understood creatures have recently been in the news for all the wrong reasons. Unexplained swarms of these enigmatic invertebrates have been causing trouble in Hawaii, Spain and Northern Ireland. Scientists have begun asking questions about jellyfish, and the answers may just undermine what we think we know about the origins of diversity on earth...
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.
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.
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.
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...
This comb jelly is a new species found recently off the coast of Tasmania. Copyright Martin George, QVMAG. Used with Permission.
However these findings are interpreted, we can no longer accept that cnidarians are an evolutionary relic. They are in fact highly evolved to take advantage of their habitat and the 'higher' animals within it, as their ability to kill all kinds of vertebrates (including humans) demonstrates. Soft-bodied animals don't leave fossils easily, and their exact phylogeny is always controversial. The terrifying truth about jellyfish is that they mess up our established ideas about evolution, and show us how much we have left to learn.
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