This is a photograph of Macropinna microstoma, also called barreleyes. It has a very peculiar optical arrangement. When you first look at this photo, you may think the two small ovals above and behind its mouth are the eyes, and that it looks rather sad…wrong. Those are its nostrils. The eyes are actually the two strange fluorescent green objects that look like they are imbedded in its transparent, dome-like head.
Video frame-grab of Macropinna microstoma at a depth of 744 m, showing the intact, transparent shield that covers the top of the head. The green spheres are the eye lenses, each sitting atop a silvery tube. Visible on the right eye, just below the lens on the forward part of the tube, is the external expression of a retinal diverticulum. The pigmented patches above and behind the mouth are olfactory capsules. High-definition video frame grabs of Macropinna microstoma in situ are posted on the web at: http://www.mbari.org/midwater/macropinna.
It gets the name "barreleyes" because it's are cylindrical, rather than spherical; this is an adaptation for better light collection in the dim depths where it lives, using very large lenses but not building a giant spherical eye to compensate. It's ore like a telescope than a wide-angle camera. Here's what a single eye in a side view looks like — the lens (L) is what is glowing so greenly in the photos.
Chapman's (1942) mesial view of the left eye of Macropinna microstoma. Abbreviations: RS = rectus superior, L =lens, OS = obliquus superior, OI = obliquus inferior, RIN = rectus internus, RI = rectus inferior, RE = rectus externus, OP = optic nerve.
As if that weren't weird enough, the animal has a completely transparent skull cap, and the eyes swivel about within the skull to look out through that translucent cranium. In the two pictures below, the animal is first looking straight up through its head (the eyes are in the same orientation as in the diagram above), and in the right frame it has rotated the binocular-shaped eyes forward to look ahead.
Lateral views of the head of a living specimen of Macropinna microstoma, in a shipboard laboratory aquarium: (A) with the tubular eyes directed dorsally; (B) with the eyes directed rostrally. The apparent differences in lip pigmentation between (A) and (B) are because they were photographed at slightly different angles. (A) was shot from a more dorsal perspective and it shows the lenses of both eyes; the mouth is not sharply in focus. (B) shows only the right eye, with the lips in sharper focus.
Nature is always coming up with something stranger than we would imagine, and Macropinna is a perfect example. Apparently, the function of this arrangement is to give the animal a sensitive light detector for tracking its prey, bioluminescent jellyfish, and at the same time to shield the eyes from the stinging tentacles of the jelly while it's eating it.
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