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If you consider a "profession" to be something one does in exchange for money, then technically I’ve been a professional marine biologist since I was fourteen years old. I’ve never had any other job. And in these past thirty years I’ve learned about many mind-bogglingly incredible sea animals. Fish that migrate thousands of miles to spawn, Disney-inspired octopods, fossilized teeth of fifty-foot long predatory sharks. Although I’m still just as awed by the natural world around us as that kid back in 1980, there just isn’t very much left that truly makes my jaw drop.

The Pacific barreleye made my jaw drop.

With large fins to help it maneuver in open water, Macropinna microstoma - it’s only about six inches long - wouldn’t seem all that remarkable. Until you look at the transparent head. Yes, this fish's head is completely see-through. When I first received the email notice from National Geographic last night, I thought this had to be a joke.

I couldn’t believe I’d never heard of this fish before, and wanted to use the term "newly discovered" in this diary. But the fish has been known since the late 1930’s. The problem was all previously caught specimens were badly deformed by the time they were pulled up from the 2,000-foot depths it inhabits, so I guess nobody recognized the uniqueness until now since the transparent head dome is destroyed by the pressure differential. The photos here, as well as the video at the end, were taken by the Monterey Bay Aquarium Research Institute, who were kind enough to send me their high resolution images for this essay. Although they were shot back in 2004, these photos weren’t released until this week. MBARI has been doing some cutting edge marine research in the past few years, and they’ve outdone themselves with this species.

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In addition to the diaphanous head, Macropinna has tubular-shaped eyes that can actually rotate in their sockets, and these two weird features compliment each other to increase the fish’s visual abilities. The eyes are normally facing upwards, but if it needs to check if something is sneaking up from behind, the fish barrel-rolls its eyeballs and looks right through its own skull. When feeding, and we’ll get to the feeding behavior in a bit, it can follow the food right into the mouth by rolling its eyes downward, following the prey by looking through its own snout.

There seems to be a little bit of confusion about what organs we are seeing here, so I’ll try to clear this up a bit. The white glowing things are not the eyes. These are olfactory organs used to sense food. You can clearly see the two large, green structures higher up on the forehead. These are optical lenses used to filter light entering the eyes, and the barrel-like eyes are the two dark structures just beneath them.

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Although we still have much to learn about this animal, we do know a little bit about its food preference, which is also kind of bizarre. Barreleyes depend on jellyfish-like animals called siphonophores for their sustenance. But they don’t eat them, they steal their food. Like jellyfish, siphonophores capture drifting plankton by trapping them in a net of poisonous tentacles, collecting the tiny prey into small clumps to be swallowed en masse. Before this plankton ball can be eaten, the barreleye will snatch it away and swallow it for itself. Barreleyes are plankton eaters but are not filter feeders. Filter feeders eat plankton by filtering the tiny plants and animals from the water. I don’t know why I find this so amazing, but trust me; a non-filter feeding plankton eater (say that three times fast) is highly unusual. Because siphonophores can grow to thirty feet in length, one siphonophore may keep a six-inch barreleye busy for quite some time. I’m assuming they are immune to cnidarian stings, but I haven’t seen this documented.

Like many types of deep-sea jelly-like animals, siphonophores are bioluminescent, and this may be why this fish has such large eye filters (the green organs mentioned above). Living in total darkness, these filters may allow the fish to capture the faint light produced by a living siphonophore, like the one pictured below.

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As promised, here is the video MBARI took of this fish.

For regular readers, I’ll get to my krill essay, which won the MLS poll a few weeks ago, as soon as I stop getting distracted by shiny objects.

Other diaries in this series can be found here.

Originally posted to Mark H on Thu Feb 26, 2009 at 03:13 PM PST.

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