Species: Astyanax mexicanus
Range: Flooded caves of northeast Mexico
Weird Feature(s): Navigation via pressure reading, lack of pigment, lack of eyes
How do you find your way through complete darkness? If you’re like most of us, you do it by knocking over everything in the room before finally finding a light switch. If you’re like the Mexican blind cave fish, you do it by creating a force field that allows you to sense your environment so well that you don’t even need eyes.
Scientists have long wondered how the Mexican blind cave fish is able to move so nimbly through its pitch black environment. And they knew it wasn’t super sensitive eyes – they don’t have any. Roi Holzman of Tel Aviv University took a closer look at the behavior of these fish and eventually noticed that they sucked water into their mouths and expelled it out their gills unusually often. To figure out if this might be connected to their navigational strategy, Holzman and his team did something that’s really mean to do to blind things: they rearranged all its stuff, moving all of the objects in the fish’s enclosure to see how it would get its bearings with the new layout. Instead of stumbling around and stubbing its fins on everything, the fish started to suck in water four times more often and moved about slowly, seemingly trying to get a sense of its new interior design. The closer it got to a large object the faster and faster it sucked – sucking up to 6x the normal rate when 2 cm away from something.
A little background on fish anatomy is necessary to understand what seems to be happening here. All fish have what’s called a lateral line running down the sides of their bodies. This line is a super touchy sensory system than can detect even the faintest disturbances and pressure gradient changes in the surrounding water. It is thought that by sucking water into its mouth, the fish create a hydrodynamic velocity field. Even stationary objects surrounding the fish will alter this field and the fish can “read” these alterations as it gets closer or father away from something. The process is a little bit like echolocation, especially in the fact that the sucking motions become more frequent as the fish gets closer to an object.
Cod are one species of fish for whom the lateral line is immediately visible. (Wikimedia)
“We don’t know of any other fish species that do this, but there probably are some,” Holzman said. “It simply adapted tools it already had at its disposal, so it’s likely other fish have evolved it independently.”
Oddly enough, these strange fish are readily available at pet shops. You can pick a couple up yourself and them enjoy them in the pitch dark or your home. You won’t be able to see them, but you can rest assured they’re finding their way around just fine.