East Coast - The Chameleon Of The Maritimes
Today we’re diving into the concept of convergent evolution by diving into another east coast tide pool. Here we’ll find the diminutive sand lance, which shares a bizarrely specific set of evolutionary adaptations with a creature that otherwise could not be more different. How’d they end up this way? Watch to find out!
EPISODE NOTES
Years and years ago, a family member ill-advisedly bought me a chameleon. I think they based the decision entirely on my having seen a man at a bus station take a tiny chameleon out of a little terrarium and have it crawl all over him while he waited for his bus. Older and wiser, I now realize that man was probably a very bad chameleon owner and that chameleon was probably stressed out of its mind.
But regardless: I was given a chameleon on the strength of that encounter. That, and I was in the aftermath of a bad breakup and maybe they thought I needed the companionship. On that count especially, a chameleon was a poor choice. I learned some tough lessons about them very quickly. Chameleons are not personable. They are ornery, fearful, sometimes aggressive animals. It’s possible, through brute force acclimatization, to eventually make them pliable enough to be handled without much fuss. But the road to get there means putting them through a totally unacceptable amount of stress and hardship all so you can have them crawl around your fingers without biting you.
I, obviously, refused to put my chameleon through that stress. At which point the calculus of caring for this animal felt silly to me. Some people are made for this kind of pet. They should be thought of more like tropical fish, to be admired and cared for from afar and never directly interacted with. But that’s not my kind of pet, especially when chameleons spend most of their days motionless under a heat lamp, without even the hypnotic back and forth of a fish. And on top of that, keeping them hydrated (through misting the cage), properly aerated and fed is a huge headache. You feed them live crickets (which means you now also need to care for a lot of crickets) but to be sure they’re getting all the nutrients they need you need to shake the crickets around in a coating of calcium before feeding.
I think the breaking point with my chameleon was when I was doing the twice-daily feeding routine. As I shook the dozen crickets around to coat them in calcium, the top of the container came loose. In a split second I’d flung 12 crickets out into my bedroom where they immediately scurried into every nook and cranny they could find, impossible to round up again. Something clicked. This was not for me.
So, what to do with an unwanted chameleon? We eventually found a reputable chameleon fanatic who worked with some local exotic pet rescue outfits and had his bona fides. Better yet, he wasn’t just offering to find a home ‘somewhere’ for my chameleon, he was offering to take him himself and basically put him to work as a stud - he was also a breeder. Dare I say, not a bad place for him to end up.
I’d say I had him for a couple months at most? So first, this is a warning to anyone in that all-too-common position of seeing an exotic pet like a chameleon and thinking no further than ‘that would be cool’. They are a lot of hard work - more than you think. They are not to be handled or used as an accessory. If you’re not deeply committed to caring for them properly, don’t for one second think this is a casual animal you can keep healthy and happy just by refilling food and water occasionally. In a word: don’t.
But second, as you’ve probably figured out by now: he’s the (primary) chameleon in this video! While I still had him I shot some material of him hunting with a then brand-new macro lens I barely knew how to use. I overexposed. I was so wide open on the macro lens at f2.8 that the focal plane was like a millimetre. The footage sat on an aging hard drive for more than ten years until I started editing this video and thought: wait a minute… don’t I actually have some bespoke chameleon footage somewhere?! And tada.
TRANSCRIPT
So there’s this odd phenomenon in biology called ‘convergent evolution’.
That’s when two species, despite being separated by millions of years or lots of branches on an evolutionary tree, independently develop similar characteristics.
Think: bats and birds both evolving similar wings. Sharks and dolphins evolving toward the same streamlined swimming shape. Or… dolphins and bats both winding up with echolocation.
And you can find one of the most unlikely examples of convergent evolution on the shoreline of Canada’s east coast.
In shallow waters with sandy bottoms there are these schools of tiny, elongated, darting fish.
Watch them for long enough and you’ll actually see them start to dart in and out of the sand.
That’s where they get their name: these are sand lances.
They’re widespread, popular bait fish. Also quite a chameleonic species. And I mean that literally. Sand lances have insanely specific convergent evolution… with these guys.
And it converges on their eyes. Just like a chameleon, sand lances are able to move each of their eyes independently.
And that’s already pretty wild - but not the end of the story. After all, puffer fish and sea horses do the independently moving eye thing as well. The connection between sand lances and chameleons goes much deeper.
To understand how, we first have to look at how both of them hunt.
Chameleons, famously, shoot out an extending tongue with extra-sticky saliva to snatch insects.
Sand lances, meanwhile, lurk in the sand, then dart out to snatch tiny copepods out of the water.
They’re very similar motions at equally blazing speeds. A chameleon’s tongue and a sand lance strike both take around a hundred milliseconds. That’s about as long as this flash.
That speed means that both animals need to be super accurate at a distance. And that brings us back to their eyes.
See, generally, predators have both eyes closer to the front of their heads, while prey animals have their eyes closer to the sides.
Why? Because eyes in front allows for overlap of the visual fields and binocular vision, which lets your brain quickly judge distance from the difference between what each eye is seeing.
The sacrifice being: your field of view is narrowed. For a prey animal it matters more to have nearly 360 degree vision, to watch out for predators, than to have better depth perception.
But hold on. We have the sand lance and the chameleon both doing ridiculously accurate distance judgment at ridiculous speeds, with independently moving eyes on the sides of their heads. So how do they do it?
For one, they’re able to manually adjust the focal plane of their eyes - letting them compare various ‘layers’ in the environment to judge relative distance.
But they likely also make use of an even more remarkable and subtle technique. In brief, the unique shape and layout of their eyes means that just rotating them has a parallax effect.
See, objects at different distances move different amounts when we shift our perspective. That’s ‘parallax’. Us binocular animals need to move side to side to make this happen, just moving our eyes doesn’t do it.
But chameleon and sand lance eyes have a wider separation between their nodal point and their axis of rotation than most animals. And what that means is by just rotating their eye, they actually get a different perspective on what they’re looking at.
Maybe a helpful analogue is closing one of your eyes and then the other, and seeing that objects shift a little bit relative to their distance. Sand lances can achieve that, and use it judge distance, just by rotating one eye.
That gives them another huge advantage. Because they can do that distance calculation with just the tiny twitch of one eye, they can be amazing ambush predators, keeping almost perfectly still until the moment they strike.
There’s even more minutiae to the shared construction of chameleon and sand lance eyes - in fact, ten specific parallels, like the relatively large focal length and therefore magnification of the image in each eye, or your classic deep… convexiclivate… fovea in the retina. But I guess the big question is… does it all work? Does this unique, convergent evolution between these two very different animals demonstrably benefit their lightning-quick hunting technique?
Well, as it turns out that is one last thing they have in common. Under normal conditions, once a sand lance has committed to a strike - or a chameleon has committed to launching its tongue - as far as we can tell, they basically never miss. So, how’s that for validation?
Two animals that almost couldn’t be further apart. One buried in the sand underwater, one high in the branches of a tree. But a specialized hunting style brought them together in a completely unexpected - and shockingly effective - way.
Convergent evolution at its finest.