Tried your scotopic vision lately?

There’s a word I’m particularly enjoying right now because it has a susurrus sound and because it’s a new notion for me. 

I came across it a couple of days ago while reading Sentient (which by the way, so far, is a fantastic book. Each chapter focuses on a different animal and could be read alone). Anyway, the term comes up in relation to the spookfish which, incidentally, has excellent scotopic vision, a type of vision that apparently even us humans have. 

If, like me, you are unfamiliar with the term, I’ll break it down. Basically, it refers to an eye’s ability to see under low-light conditions. If you have scotopic vision, well you have some of that ability. 

As you can imagine, the spookfish is a deep sea fish (think mesopelagic deep, down to 1,000 meters) and though this vision comes hand in hand with a poorer ability to discriminate between colors and lower visual acuity, those with scotopic vision are highly sensitive to light and are capable of seeing in starlight-level darkness. Incidentally spookfish are really also quite cool because they’ve got mirror-based eyes. I won’t go into it now but it’s super rare. So rare in fact that only two vertebrates on our planet are known to have a retinal mirror in their eye rather than a lens. 

What struck me as most surprising is that though humans aren’t great at seeing in the dark, after some time—say 20 to 30 minutes—we too can tap into our scotopic powers as we rely solely on our rod photoreceptors cells in our retinas, to make sense of the dark. That marker of time is when rods are supposed to approach near-max sensitivity. 

Though uncorroborated, it’s also apparently why pirates wore an eye patch—so that they could quickly go between the bright above deck and the dark below deck level and still see. It’s also apparently what my boyfriend does when he goes to the bathroom at night. He closes one eye against the light, does his thing, turns off the light, then uses the dark-adapted eye to navigate his way back to bed. 

Of course, I had to test this for myself and so, for the past few nights I’ve been lying awake paying attention to my vision improving without lights. Thankfully, the blackout blinds in our bedroom allow for experiments like this. The results truly are stunning. 

If you head outside to try it at night, I’ve got another factoid you might appreciate: the fovea in your eye (the sort of central area) has no rod photoreceptor cells. This means that the center of your gaze is actually quite poor at night. And so, if you want to see better in near darkness levels of illumination, try averting your vision and looking at the thing you want to see out of the side of your eye. Star gazers will often use this trick! 

And this is what I love so much about learning. Though I’m only at the start of embedding myself in the world of animal behavior, already I’m enraptured. It doesn’t even matter that my first textbook focuses solely on dogs, it too is fascinating. 

All I can say is I’m relieved I’ve switched to a contract deal with my employer (such a hard decision given I had a fairly good position) as it buys me the mental space to start filling my head with new information. It feels like a return to meaning and somehow that makes me both immensely happy and somewhat sad. Why have I taken this long to return to academia? Learning is definitely one of my love languages.

Final aside: I feel so damn sorry for all the ratfish that Pacific Northwest divers blind when diving those darker waters. I still recall the first time I saw the fish. I was bowled over by its enormous eyes. Turns out that like the spookfish (it’s actually part of the same group of deep-sea cartilaginous fish called chimaeras) ratfish eyes are highly adapted to low-light conditions, and they lack the ability to regulate the amount of light that enters them. This means that a diver shining a light in their eye can actually blind them and who knows for how long. If only our dive agencies were required to teach marine behavior etiquette specific to their own areas. And I mean REQUIRED.