Researchers at the Monterey Bay Aquarium Research Institute recently solved the half-century-old mystery about the tubular eyes of the barreleye fish (Macropinna microstoma) which help collect light.and because of their tunnel shape can fix on one object directly above the fish’s head. Bruce Robison and Kim Reisenbichler’s research suggests that their unusual eyes can rotate within a transparent shield that covers the fish’s head. This allows the barreleye to peer up at potential prey or focus forward to see what it is eating. Their large, flat fins allow them to remain nearly motionless in the water, and to maneuver very precisely (much like MBARI’s ROVs). the two spots above the fish’s mouth are are olfactory organs called nares, which are analogous to human nostrils. Their small mouths suggest that they can be very precise and selective in capturing small prey. On the other hand, their digestive systems are very large, which suggests that they can eat a variety of small drifting animals as well as jellies. In fact, the stomachs of the two net-caught fish contained fragments of jellies.
These deep-sea fish in the family Opisthoproctidae “barreleyes” because their eyes are tubular in shape and usually live at a depth in the ocean where sunlight from the surface fades to complete blackness. They use their ultra-sensitive tubular eyes to search for the faint silhouettes of prey overhead.
When the barreleye is facing downward, its eyes are still looking straight up. The barreleye in the video is about 140 mm (six inches) long.
Robison and Reisenbichler used video from MBARI’s remotely operated vehicles (ROVs) to study barreleyes in the deep waters just offshore of Central California. At depths of 600 to 800 meters (2,000 to 2,600 feet) below the surface, the ROV cameras typically showed these fish hanging motionless in the water, their eyes glowing a vivid green in the ROV’s bright lights. The ROV video shows that this fish’s eyes are surrounded by a transparent, fluid-filled shield that covers the top of the fish’s head.
This face-on view of a barreleye shows its transparent shield lit up by the lights of MBARI’s remotely operated vehicle Tiburon. The two spots above the fish’s mouth are are olfactory organs called nares, which are like the human nostrils.
Most of the time, the fish hangs motionless in the water, with its body in a horizontal position and its eyes looking upward. The green pigments in its eyes may filter out sunlight coming directly from the sea surface, helping the barreleye spot the bioluminescent glow of jellies or other animals directly overhead. When it spots prey (such as a drifting jelly), the fish rotates its eyes forward and swims upward, in feeding mode.
Barreleyes deep-sea environment is inhabitated by many different types of jellies. Some of the most common are siphonophores (colonial jellies) in the genus Apolemia. These siphonophores grow to over 10 meters (33 feet) long. Like living drift nets, they trail thousands of stinging tentacles, which capture copepods and other small animals. The researchers speculate that barreleyes may maneuver carefully among the siphonophore’s tentacles, picking off the captured organisms. The fish’s eyes would rotate to help the fish keep its “eyes on the prize,” while its transparent shield would protect the fish’s eyes from the siphonophore’s stinging cells
Video of Macropinna microstoma narrated by Bruce Robison:www.youtube.com/RM9o4VnfHJU
Excerpts courtesy of MBARI.org 2009/barreleye
Image courtesy of the Spanish.Xinhuanet.com spanish.xinhuanetbarreleyeimage