I’ve been thinking a lot about eyes lately. Mostly that’s because of my best friend, editor, business associate, and fellow Cuthbert Burbage enthusiast, Maygen. She’s having LASIK this week, which has led to a lot of conversations like:
So needless to say, eyes and vision and lasers were all on my mind when I spent a rainy weekend binge watching Blue Planet. The Deep Ocean episode caught my attention with its slew of bizarre creatures that inhabit the depths of the ocean where only a very small amount of sunlight trickles down from the surface. The animals there live in a sort of perpetual twilight and as a result, many have developed enormous eyes to capitalize on what little light there is. But that isn’t all. The dim lighting has done some serious tinkering with these animals’ visual systems--and it doesn’t just affect their eyes.
The mesopelagic zone covers waters from 200 to 1000m deep. At those depths, there’s just enough light to be able to distinguish night from day, but just barely. It’s an alien world to be sure. Even the light there is much different than the light on the surface. As it travels through water, the longer wavelengths of light, reds and oranges, are absorbed. Only slower wavelengths make it down to the mesopelagic. It is a blue and black world.
Bioluminescence for everyone! Clockwise from the top: the underside of a hatchetfish, warning display of a jelly, a lanternfish, and a dragon fish (Photo from Johnsen Lab at Duke)
But the sun isn’t the only source of light there. Evolution was feeling rather glow-happy when it created mesopelagic fish: over 95% of fish species in the twilight zone bioluminescence. And they are definitely not alone: there are light up squid, psychedelic jellyfish, flashy shrimp, and glow in the dark sharks. (I couldn’t work lasers into this post, but I made it to glow in the goddamn dark sharks, so I think that’s worth something.)
Let’s talk about those sharks. The cookie cutter shark (Isistius brasilensis), like many mesopelagic creatures, is covered in light-emitting organs called photophores--more on those in a bit. Well, it’s covered except for a distinct patch under its chin. (Glow in the dark shark with a soul patch!) From below, the patch looks like a fish. Would be predators swim up from the depths thinking they’ve got a nice meal in sight only to become prey themselves. Sneaky!
Lots of fish and other carnivorous creatures spend their time lurking in the darker depths, watching for the silhouettes of potential meals swimming above. As you may imagine, this has led to lots of prey organisms evolving methods to mask their silhouettes. Some have become razor thin. (The better to disappear from sight, my dear.) Others have gone as transparent as possible--which is probably more impressive than you’re giving them credit for. Building transparent tissues is incredibly difficult--it involves a lot of very accurate crystal alignment and a whole host of other logistics--but it’s not impossible. Just think about the lenses and corneas of your eyes, you silly human.
But, not content with just being transparent and therefore badass, the Solmissus incisa jellyfish, like many of its brethren, has gone so far as to absorb sea water into its tissues. The less difference in density from its flesh to the surrounding water, the less light will refract off of it and thus give it away to predators.
Imagine if every time you ate something, everyone could see your meal inside you. Mostly I think you'd be shocked at the sheer amount of ice cream and doughnuts I consume. (Image from mbari.org)
Photophores play a big role in the silhouette camouflaging game as well. It isn’t just our cookie cutter friend glowing by himself down there. Many mesopelagic animals have photophores lining their undersides. They use sensors on their topsides to detect the intensity and scattering of sunlight and then mimic it with their photophores.
The two types of light--the down-welling light from the surface and the light generated by bioluminescence--have driven the evolution of some pretty funky eyes. Generally speaking, as one goes deeper in the ocean, the eyes of the creatures get bigger and bigger, until they just aren’t necessary anymore. Since the mesopelagic is the last depth that has sunlight, it represents the peak in eye-size shenanigans. Huge eyes work like drag nets to haul in every photon of light possible.
Yellow lenses, like on the larger eye of the cock-eyed squid, are fairly common as well. They have the disadvantage of filtering out nearly all the blue light (which, if you recall is most of the available light), but they also have one huge advantage. Photophores mimic down-welling light, which means their light is blue. But they don’t mimic it perfectly, so the light has a greenish tinge that becomes easily detectable when the rest of the ambient blue light is filtered out. Find the photophore, find a meal.
Who me? I'm just chilling, you know, using my yellow lenses to filter out ambient light and hone in on bioluminescence.The usual. No biggie.
Then there are the tube eyes. Oh god, the tube eyes. Where our eyes are spheres, some mesopelagic creatures have eyes that are, well, tubes. Tubular eyes aren’t very good at precision vision, but they are great at collecting and focusing small amounts of light--think of a telescope. These eyes are favored by ‘lie in wait’ predators like the telescope fish (Gigantura chuni).
Tubular eyes are great at collecting light, but they leave creatures with a very narrow field of vision. To counter that, many tube-eyed creatures have developed ‘accessory retinas’ on the sides of their tubes. These extra retinas can’t necessarily form images--vision the way you and I think about it--but they can detect light and movement, alerting the fish where to direct its better equipped tubular eyes. I’d advise buying your accessory retinas now, once the press gets a hold of this it’s really going to take off.
I found this image on an ebay site selling build your own telescope fish 3D model kits. Tragically, they were sold out when I got there. But it makes me happy that there are at least four people in the world who had a need for a 3D telescope fish model.
There are lots of tube-eyed creatures, but I’m going to go ahead and give the award for “Best Dressed/ Most Evolutionary Effort” to Macropinna microstoma, aka the barreleye fish. Known since 1939, a specimen had never been found intact, much less alive and swimming until 2009. Their corpses had always washed up covered in a goopy slime where their heads should have been, but the 2009 video showed something else. That goopy slime was its head. The fish has a translucent skull.
I'll just leave this here... (Thanks for photo MBARI)
But wait, there’s more. The dark spots on the front of its mouth are not eyes, they’re nostrils. Its tubular eyes are actually the green orbs perched on top of its head. Those eyes, by the way, can swivel to the front of its head so that it can see not only the jellyfish it eats, but also the tiny creatures it snatches away from other jellyfishes’ tentacles. To quote Stephen Colbert, “that’s the craziest f#?king thing I’ve ever heard.”
