Barreleye fish, or “spook” fish are a curious and uncommon species of deep ocean fish that basically move around like “ghosts of the sea”, lingering at the edge between surface waters lit by sunlight and the darker depths of the deep ocean. These fish are cunning predators that are able to easily avoid being caught, while feeding on zooplankton and small fish. Some members of their species also have distinctive features that are extremely unique, as well as quite shocking for anyone not accustomed to the seeming strangeness of deep sea creatures.
Barreleye fish are a curious type of deep ocean fish found primarily in areas of the Pacific, Indian and Atlantic oceans. They mostly prefer temperate waters, and their distinctive, barrel-shaped eyes are their most interesting feature, as well as one of their most potent tools for finding prey. Barreleyes can either be very slender, featuring elongated, “aerodynamic” shapes for slicing through the water more easily, or stout and somewhat larger. The large, curiously shaped telescopic eyes that constantly gaze upwards, as well as their sizable dome-shaped heads and toothless mouths are among their most distinctive features.
Barreleyes are able to detect their prey far more easily than most types of fish, due to their specific development and evolution. Unlike many types of deep water fish, barreleyes don’t go deeper than about 2,500 meters. In fact, they are more commonly found at a depth between 400 and 1,500 meters, and are known to inhabit places where the depth allows the light to just barely penetrate through the denseness of the water. This way, they can see their prey from below, while not being spotted as easily. The large number of rods in their eyes allow spook fish to spot even the slightest movements and the faintest light flickers coming from the above silhouettes of their prey. They can, therefore, accurately spot the location and predict the trajectory of even the smallest zooplankton and various pelagic crustaceans that they commonly feed on.
The curious evolution of spook fish have made them not only very efficient when stalking and detecting their prey, but also extremely well-versed in blending into the environment and avoiding predators. One of their main defensive strategies is based on counterillumination – the process of blending with the ambient light from above, so that any predator viewing them from below would find it difficult to detect them. This is precisely where the “ghost-like” qualities of these fish can truly be seen most clearly, as their reflective soles are able to complete the job of hiding them in the background quite well.
One distinctive barreleye species known as the Pacific Barreleye will really end up spooking you, due to a distinctive, ghostly feature that was enough to shock scientists when they first examined this weird fish: its head is completely transparent. The Pacific Barreleye can be found in North Pacific waters around depths of 400-600 meters, and its fluid-filled, transparent head mainly allows it to have a much wider angle of sight. A solitary fish, the Pacific Barreleye is also an opportunistic predator, managing to often steal small fish out of the tentacles of various siphonophores.