The barreleye fish,
L. Macropinna microstoma
, was first described in 1939 and has baffled physiologists for decades.
The barrelfish belongs to the Opisthoproctidae family, and they are sometimes referred to as spook fish. The fascinating aspect of these fishes is the tubular eyes. The fish has tubular eyes which are excellent at collecting light. This is an important function if you spend your life at depths up to 2500m, well below the photic zone. The tubular eyes are found in many bathypelagic species, and perform marvelously when it comes to vision in the darkest of environments.
The puzzling part was that the eyes appeared to be fixed in place, giving the fish a very narrow field of vision directly above it’s head, tunnelvision. How could the fish see what it is eating (they do not have large mouths like other deep sea adapted fish do).
A new paper from Bruce Robison and Kim Reisenbichler solved the issue. Using a remotely operated vehicle (ROV), they studied the fish at depths ranging from 600-800 metres. They discovered a prviously unknown fact regarding these fish, the tubular eyes exist behind a transparent, fluid filled dome. Previous illustrations looked something like this:
The new images from the ROV revelaed the new structure, which likely would not survive the trip to the surface:
So, the answer to the puzzle? The eyes can rotate. So they can look up for silouettes of potential prey items, and look forward to grab the food item.
Robinson and Reisenbichler used this data and the habitat information they have to formulate a hypothesis on how the animal makes it’s living. They postulate that the barreleyes, who share their habitat niche with siphonophore jellyfish like this:
are using their directional focussing of the tubular eyes to maneuvre between the tenticles that trail behind these jellies, picking off the captured organisms.
Here’s a video of the barrelfish:
YouTube – Macropinna microstoma: A deep-sea fish with a transparent head and tubular eyes