Macropinna

The Pacific barreleye fish^[1] (Macropinna) is a genus of ray-finned fish belonging to Opisthoproctidae, the barreleye family. It contains one species, M. microstoma. It is recognized for a highly unusual transparent, fluid-filled shield on its head, through which the lenses of its eyes can be seen. It was originally believed that the tubular eyes of this fish were fixed in place, offering only tunnel vision above its head. However, in 2008, scientists discovered that its eyes were able to rotate both up and forward with its transparent dome.^[2][3]This feature allows for M. microstoma to observe prey while keeping its eyes protected from stinging tentacles of jellyfish-like organisms it feeds on.^[4]

Macropinna
Illustration
Scientific classification
Kingdom:	Animalia
Phylum:	Chordata
Class:	Actinopterygii
Order:	Argentiniformes
Family:	Opisthoproctidae
Genus:	Macropinna W. M. Chapman, 1939
Species:	M. microstoma
Binomial name
Macropinna microstoma W. M. Chapman, 1939

M. microstoma, a small fish of about 15 cm, has a tiny mouth and most of its body is covered in large scales.^[5] The fish normally hangs nearly motionless in the water, at a depth of about 600 metres (2,000 ft) to 800 metres (2,600 ft) in the mesopelagic zone, using its large fins for stability and with its eyes directed upward. In the low light conditions it is assumed the fish detects prey by its silhouette. This is because the eyes have no lens in the diverticulum, so images would most likely occur through reflection. Due to the eyes facing upwards, light is able to enter the eyes and through the illumination, silhouettes are formed.^[6] MBARI researchers Bruce Robison and Kim Reisenbichler observed that when prey such as small fish and jellyfish are spotted, the eyes rotate like binoculars, facing forward as it turns its body from a horizontal to a vertical position to feed. Robison speculates that M. microstoma steals food from siphonophores.^[7]

Discovery and Evolutionary History

Macropinna microstoma was discovered in 1939 by marine biologist W. M. Chapman, in deep temperate waters off of the Pacific, Indian, and Atlantic oceans. It is restricted to deep oceanic water by its light-sensitive tubular eyes. The eyes are capped with bright green lenses and surrounded by a fluid-filled shield. The presence of this fish was mostly unknown until 2004, when researchers at Monterey Bay Aquarium Research Institute photographed M. microstoma off the coast of the north pacific.^[8]

Fossil evidence has also shown to be especially important when discovering M. microstoma history. The genus M. microstoma has existed for millions of years that can be traced back to the Miocene epoch, with similar tubular-eye features preserved in a fossil species. Fossil evidence suggests that the barreleye's distinct features are not recent adaptations but are ancient traits precise for deep-sea life.^[9]

Another important aspect of the M. microstoma is that they represent an extreme example of deep-sea fish traits such as their eyes. Over evolutionary time, these traits have allowed M. microstoma to acquire an ecological niche, where the precise vision and protection from stinging prey are essential for species survival.^[10]

Environment

M. microstoma is a deep-sea fish, living around 1015 meters (about 3330.05 ft) below the water's surface off of California's coast and in the North Pacific Ocean. Because the barreleye's habitat is deep water, the species has enhanced sensitivity to faint light and bioluminescence. The evolution of enhanced light sensitivity is provided through small protrusions in their eyes called ocular diverticulum. With the use of their unique eyes and eyesight, the barreleye is able to utilize upward ocular viewing allowing the fish to detect silhouettes of prey that are upwards and towards the light.^[11]

M. microstoma can get as long as 6 inches (15 cm). It has a large, dome-shaped, transparent head. This protects its sensitive eyes from the nematocysts (stinging cells) of the siphonophores, from which it is believed to steal food. Through the dome, the entire inner part of the head can be seen, that is, their eyes, brain and all the nerve endings that make up their head. At first glance it seems that its eyes are at the front of their head in the form of two black holes; these are its olfactory organs. Marine biologists used to believe that the barreleye's eyes were fixed in its head, which would only allow it to look upward.

M. microstoma's large, flat fins allow it to remain nearly motionless in the water, and to maneuver very precisely. 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 jellyfish or other animals directly overhead. Recent studies have suggested that the transparent dome of Macropinna microstoma may help reduce light distortion, allowing the fish to better detect prey and bioluminescent signals in the deep sea.^[12]

Diet

M. microstoma shares its deep-sea environment with many different types of cnidarians. Some of the most common are siphonophores (which can reach length of 30 feet). The barreleye has a strong digestive system and it usually feeds on cnidarians and small drifting animals. The small marine creatures trapped on the tentacles of the cnidarians are also targeted by the barreleye fish.

Reproduction

M. microstoma is thought to be a pelagic spawner (the eggs are coated with a layer of oil that allows them to float on the closest surface of the sea until they hatch). The female lays eggs in water and male releases sperms in water. After hatching, the larvae begin to descend to the depths as they grow, feeding on zooplankton and other small particles of organic material.^[13] It is believed that there is no sexual dimorphism between the male and the female and that they do not care for their young.

M. microstoma has been known to science since 1939, but is not known to have been photographed alive until 2004. Old drawings do not show the transparent dome, as it is usually destroyed when brought up from the depths. It is broadly distributed across the northern Pacific Ocean, from the Bering Sea to Japan and Baja California.