The Pacific barreleye – Macropinna microstoma – has been one of my favorite fish ever since I first learned about it.
They’re small fish, usually around 15 centimeters long, with small, somewhat pointed mouths like all fish in the barreleye family. Their fins are large relative to their bodies and allow for very precise and gentle movement, but they can also wiggle their tails for faster and more forceful swimming.
Of course, their most recognizable physical trait is the tubular eyes they’re named for. The Pacific barreleye has sensitive eyes to help it see in the dark water. Since most light comes from directly above, these eyes face upward, to spot the silhouettes of prey above it. Unlike most species with tubular eyes, the Pacific barreleye can rotate its eyes forward, so that when it swims upwards to investigate something it can still clearly see where it’s heading. Its eyes are bright green, and it’s possible that this coloring filters out some sunlight from the surface while letting through light from bioluminescence, letting it see animals that use their bioluminescence to camouflage themselves. The eyes are also completely encased inside its transparent head.
The Pacific barreleye lives within the twilight zone in the open ocean, usually in the range of 600 to 800 meters deep. It mostly eats zooplankton, including some crustaceans and jellies. It’s speculated that it may also steal food off the tentacles of larger cnidarians, and that the shield covering its eyes may be to protect them from getting stung while it does. Although it hasn’t been observed, it’s most likely that they are pelagic spawners that release their eggs into open water and don’t care for their young.
In over 5,600 dives, MBARI has only encountered the barreleye fish around nine times. We’re only able to learn about them through a few observations and speculation based on similar species, but the little we know so far is really unique and really interesting.
References:
Monterey Bay Aquarium
Current Biology
(Originally written for an assignment in my marine biology class from ASU!)