Common Name: Barreleye Fish
A.K.A.: Macropinna microstoma (and related species)
Vital Stats:
- Size: 15cm (6″) long
- Depth: 600-800m (2000′-2600′) below sea level
- Discovered: 1939
- First Photographed: 2008
Found: Subarctic and Temperate regions of the North Pacific
It Does What?!
As you have likely already noticed, fish don’t have necks. At least not in the sense that they are able to look upward. So for a bottom-dweller lurking about in the cold depths of the ocean, being able to see that tasty bit of food floating by above is something of a problem. Some species get around this issue by floating vertically in the water so their whole bodies are pointing upwards. Simple enough. But in the spirit of meeting every challenge with an impossibly bizarre solution, nature has also produced a fish with eyes directly on the top of its head. After all, why re-orient the entire fish when you can just shift a couple of parts?
That would be its nose.
But the strangeness of the Barreleye Fish goes a little further than that. These aren’t just normal fish eyes in an unusual location. This species’ main prey are jellyfish and their relatives, which frequently come equipped with stingers that could damage the eyes of most predators. So rather than a normal spherical eye perched on top of its head, Macropinna has a tubular structure with the lens buried deep within its head (the dark green areas in the images). Overlying the tubular eyes is a tough, fluid-filled, transparent shield which the fish can look through. That’s right, it looks through the top of its own head. This way, stings from jellyfish will never damage the delicate ocular tissue.
What’s more, the fish’s unique tubular eyes are supremely adapted for the dark depths of the ocean. They allow unusually accurate depth perception (due to a large overlap of the two visual fields) and enhanced light gathering compared the spheroid eyes. In an environment up to 2600 feet (800m) down, where little daylight penetrates and everything appears in monochrome, these adaptations enable the barreleye to distinguish even faint shadows and silhouettes moving above it, and to precisely gauge how far up they are.
Researchers had long been puzzled as to how the barreleye eats, since, with its eyes on top of its head, its visual field didn’t include the area around its mouth. The species has been known since 1939, but only as small mangled bodies caught up in deep-sea fishing nets (adults are only about six inches long). In each case, the transparent casing of the fish’s head had been destroyed by the nets and the rapid changes in pressure as the nets were pulled up, making its anatomy difficult to study. In 2008, however, scientists from the Monterey Bay Aquarium Research Institute sent remote operated vehicles with cameras down to try, for the very first time, to snap some photos of these oddballs in action. What they learned was that, when it spots prey, the barreleye can actually rotate its entire tubular eye downward, like moving the telescope in an observatory. This way, it can turn and look at its target straight on as it pursues. Most of the time, though, the fish was seen to use its large, flat fins to hold itself nearly motionless, looking up through its personal sunroof, just waiting for some unlucky jellyfish to float on by.
Says Who?
- Robison & Reisenbichler (2008) Copeia 4: 780-784.
- Monterey Bay Aquarium Research Institute
All images taken by the Monterey Bay Aquarium Research Institute (MBARI)