Living jellyfish =/= earliest stage in metazoan evolution…

So, admittedly, I wasn’t interested enough in Bielecki et al. (2013) to read the whole thing. But if the abstract is an accurate reflection of their reasoning, then “WTF” is an accurate reflection of my reaction.

The reason I went to have a look at this shiny new PLoS paper is that it was titled “Fixational Eye Movements in the Earliest Stage of Metazoan Evolution”. Anything to do with early metazoan evolution automatically interests me, plus my immediate reaction was to ask how the hell they discovered any kind of eye movement in the earliest animals (which have been, you know, dead for like 600 million years).

Turns out they didn’t. Turns out all they found was that the rhythmic contraction of a box jelly‘s bell keeps the image in its eyes changing so they don’t go blind from photoreceptor fatigue. We accomplish the same effect by constantly moving our eyes (though apparently that’s more for the brain getting bored than photoreceptors burning out?), but the jellies supposedly don’t have the same level of nervous and muscular control over their eyeballs.

Yes, box jellies have frickin’ amazing eyes, complete with lenses. In fact, they have 4 sets of 6 eyes, two of the six being proper camera eyes and the other four much simpler. They can use their eyes to navigate around obstacles and stuff. They are pretty cool creatures. Here’s a box jelly eye cluster (rhopalium) in its full glory from the UCMP:

(Was that just a little bit unsettling? :D)

But these complex, image-forming eyes are an innovation of box jellies. No other cnidarian – in fact, no other animal outside the Bilateria – has them. So complex eyes and good vision are examples of convergent (or should I say parallel?*) evolution, not inheritance from a common ancestor. Conversely, bilaterians don’t have bells like jellyfish, so anything they do to move their eyes has to be an independent invention from the get-go.

So, while box jellies are awesome and it’s always cool to learn more about them, I’m not sure what profound insight about animal evolution we are supposed to find here. That animals with eyes have ways of avoiding visual fatigue? Well, duh. Of course they would, it’s really useful. But I’m not even sure the pulsation of a jellyfish should be regarded as a vision-enhancing adaptation, never mind an adaptation with any relation to what we do. To me it seems like the default way a jelly moves just happens to be good at keeping its eyes entertained. Evolution doesn’t have to do anything special about it.

Of course, the whole thing is soaked with that grandmother of evolutionary misconceptions, exemplified by this quote from the introduction:

Cnidarians were the first of the extant metazoan phyla to develop a nervous system which is therefore considered close to the evolutionary origin of all nervous systems [9].

Nooooooo, for the love of hungry anomalocaridids, don’t do this to me.

Cnidarians and bilaterians shared a common ancestor with a nervous system. Never mind “phyla” – phyla are arbitrary lines humans drew around the branches of the phylogenetic tree. Our ancestors and theirs had nervous systems for the exact same length of time. Neither of us was “first”. Life is a tree, not a goddamned ladder.

Well, at least we got to look at some disembodied jellyfish eyes. Yay!

*goes away to growl quietly*


*The difference being that parallel evolution is convergenceĀ  from a common starting point. While complex eyes are clearly later inventions, the common ancestor of cnidarians and bilaterians might well have possessed simple eyespots of some sort, providing said common starting point. But we’re getting pedantic here.



Bielecki J et al. (2013) Fixational eye movements in the earliest stage of metazoan evolution. PLoS ONE 8:e66442


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