Macropinna microstoma the Barrel-Eyed Fish

Macropinna microstoma is a species of fish that shows that the evolutionary process really does not care as long as you have a survivable species. While most of its body is normal, its head looks like it should have been part of a submersible. You see, the Barrel-Eye, as it is commonly known, has decided that having eyes on the outside of its head was just some passing trend. Instead, it prefers to keep them inside the confines of its transparent head for safe keeping.

This fish laughs in the face of conformity.

Bizarre little critter, isn't it? Now that image needs a bit of explaining as the normal features one would expect just aren't right on this thing. Those two things up front that look like a cartoonists representation of eyes are in fact its nares. Nares are the olfactory structures of fish. These sensing organs are usually not quite this noticeable on your average fish, but the Barrel-Eye has a particularly developed set. The fact that they are facing forward on a transparent dome only makes them more conspicuous.

The real eyes are the two bubble looking structures inside the head. Those bubbles are actually massive lenses used by the fish to focus the dim light better. They get their name for the fact that the entire eye is cylindrical like a barrel. They tend to stay mostly motionless in the water column (which explains the large fins) with its eyes facing upward hoping to spot the silhouette of passing prey items. Also of note is the fact of the green pigment in the eyes. It is hypothesized that this is designed to filter out the last bits of natural sunlight that reaches the fish (as green light penetrates the deepest) so that it can focus on the bio-luminescence of its intended meal. The eyes get even stranger when you realize they can rotate to face forward.

Never has the simple turning of the eyes looked so strange.

So why does this fish have its eyes encased in that fluid filled head of its? The best guess comes from the prey items of this fish. Stomach contents show that it preys on sea jellies frequently. Having its eyes enclosed would allow them to be out of the reach of the cnidocytes (stinging cells) on the jellies tentacles. Another hypothesis is that the Barrel-Eye actually steals prey caught by siphonophores. Siphonophores being a colonial species of Cnidarian (the phylum all sea jellies, anemones, corals and hydras belong to) that looks like a long rope with tentacles along the length in clusters for catching prey.

The siphonophore Praya dubia, which may be one of the longest organisms with a length recoded to over 40 meters (131 feet)

Siphonophores are one of the more efficient hunters of the deep oceans, so feeding off of their catches would be a great strategy for a fish to develop. Even better, most siphonophores use bio-luminescence to attract prey, the green tinted lenses mentioned earlier would be especially helpful in picking this out.

The Barrel eye has been noted to live at the depths of 600 meters (2,000 ft) to 800 meters (2,600 ft), so unless you have access to a deep water submersible, there is little chance of encountering this critter on your next dive. Once again proving that the deep ocean gets to have all the crazy creatures.