East Coast - What Good Is An Eye Halfway Around Your Head?
Over the next two videos we’ll be taking look at two unique fish you can find on the east coast of Canada. They both live in the sand. And they’re both gateways to wild stories of evolution. Today we investigate the humble flatfish - and through them, solve a mystery that drove Charles Darwin crazy.
EPISODE NOTES
This one probably wins the prize for least actual footage to longest actual episode. For a few days in Nova Scotia I was on the hunt for the platonic tide pool with no luck before finally hitting the jackpot (thank you Martinique Beach). That started a frenetic afternoon of capturing as much of it and its denizens as I could. A probably unsurprising thing about these videos is, when you have committed to the expense of going out to the east coast for two weeks, you need to try to make something out of everything you encounter. The amazing thing - the thing I love - is that I have never had an instance of having captured some critter at random, and then failing to find a really interesting story to tell about them. Ditto: flatfish. I caught this flounder in the tide pool and had just enough time to fire off maybe 6 shots before he dashed off, never to be seen again. But dammit, I was gonna turn those six shots into something. Voila.
TRANSCRIPT
Today we’re starting part one of a weirdly unintentional two-parter. Two fish. Two evolutionary mysteries. United, somehow… by sand.
Just beneath the churning waves of the atlantic ocean, this seemingly empty water is host to an invisible menagerie.
Plankton! Innumerable tiny species drifting on ocean currents. Each individual, I love this, is called a ‘plankter’.
Among them are larvae who will develop into countless species of fish.
But if you look close, some of those larvae… are not like the others. As their neighbours start to inhabit familiar forms these guys… get freaky.
Specifically: hairs in their inner ear, which fish use to stay oriented in the water, shift so that swimming on their side feels ‘normal’ to them.
One side of their body turns pale and colourless while the other can grow chromatophores, letting them change their colours.
One of their eyes starts to move up and over their heads. They lose their swim bladder, the thing that lets fish stay effortlessly afloat.
And with that, they drift down to the sandy bottom to start their adult lives… as flatfish.
They might remind you of skates or rays but I wanna be clear on how different they are. Skates and rays gradually flattened out from what was a more vertical, shark-like body in a way that… makes sense.
But flatfish just… they just flipped over. Their overall body plan is still the same as their vertical relatives in a lot of ways. They just… have both eyes on one side of their heads.
And dammit, it worked! There are more than 800 species of flatfish in the world, 39 in Canada. Their ability to both hide from predators and be ambush predators themselves has made them really successful! But also: a giant evolutionary headache. All the way to Charles Darwin.
Flatfish drove him crazy. One of his biggest critics, George Jackson Mivart, a catholic and zoologist, in that order, used them as a cudgel against Darwin. Intellectually, not literally:
“…the accidental occurrence of such a spontaneous transformation is hardly conceivable. But if this is not so, if the transit was gradual, how such transit of one eye a minute fraction of the journey towards the other side of the head could benefit the individual is indeed far from clear.”
In other words, from the folks who brought you ‘what good is half an eye?’ comes ‘what good is an eye halfway around your head?’ How do you square evolution’s slow, incremental progress with: the flatfish.
Good question. For a long time, we had no good answers. Flatfish seemed to appear in the fossil record about 60 million years ago with no transitional fossils - no smoking gun with that hypothetical ‘eye halfway around their head’. That was where we were stuck until 2008.
That’s when researcher Matt Friedman took a closer look at some overlooked fossils using CT scans, to discover that we’d had the missing link all along. A fish with one of its eyes halfway in transit from one side of its head to the other.
And the irony is, these fossils had been unearthed more than 200 years ago, meaning while this question was giving Darwin migraines, the evidence was just sitting there gathering dust in a museum. If only Darwin’d had a CT scanner.
In any case, big win for the theory of evolution. But the question remains: how on earth could this be advantageous enough to be selected for?
Well, we’ve got another piece of information to work with. Flatfish seem to have appeared around 60 million years ago. Can you think of anything else that happened around 60 million years ago that might have opened up avenues for evolution? Something that for example, I dunno, maybe wiped out half of all life on earth?
Nature abhors a vacuum. And the aftermath of the Chicxulub Impactor, the asteroid that wiped out the dinosaurs, left a lot of vacuums. Including, the theory goes, bottom-dwelling ambush predators. A niche just begging to be filled.
And we have some evidence of this today: non-flat fish will sometimes exhibit similar isolated behaviours, like hiding temporarily from predators on the bottom, digging into sand or lying in wait for prey.
So let’s say flatfish ancestors really leaned into this behaviour and made it their whole deal. At this point they’ve still got an eye on each side of their head, so they’d likely prop themselves up on their bottom-facing fin. And it’s really from this pose that their evolution starts to make sense.
because any individual with their bottom eye a little closer to the top of their head gets a wider field of view and less need to prop themselves up. That evolutionary pressure can slowly add up to a complete transit of the eye - and fast. Another study in 2016 showed through DNA analysis that it only took about 3 million years - light speed in evolutionary terms.
The flatfish experiment clearly proved successful. But maybe the only way it got the chance was thanks to the suddenly wide-open playing field in the wake of the Chicxulub Impactor. In that world - and maybe only in that world - could a fish decide to flop over on the ocean floor and today represent 800 species worldwide.