Strange forms of marine life thrive in the deep waters of the ocean. These creatures have been forced to support complex adaptations to cope with lack of food, darkness and high pressure in the abyssal zone. An example is the gulper. It features a huge mouth that opens wide to swallow food much larger than itself. Another is the flashlight fish which utilized a light organ to attract mates and prey. A third form of bizarre fish is the black sea dragon. It utilizes a lighted lure to attract other fishes.
The Deep World
The upper 300 feet of the world’s oceans are blessed with life-giving sunlight. But this rich, familiar layer of the ocean is just the thin cover of an enormously deep basin that averages two-and-a-half miles in depth and accounts for 97.5 percent of the oceans’ capacity. In fact, the deep ocean is the largest living space on earth.
It is not, however, the most hospitable living space on earth. At 300 feet, there is barely enough sunlight for a plant to photosynthesize. At 3,000 feet, sunlight is absent, and fish swim in utter darkness. At such great depths, the water temperature is generally a constant 39 degrees Fahrenheit. To cope with the darkness, lack of food and high pressure of the abyss, life in the deep sea has been forced to evolve complex and bizarre adaptations.
Stars of the Abyss
In the darkness of water 1,000 meters deep, fishes with elaborate light sources are relatively abundant. These light sources may attract prey, signal mates, render the fish invisible or confound predators. The pattern of light organs for a given fish distinguishes the species and may be used as recognition signs. For example, the black dragonfish uses muscles to pull a screen of black pigment cells down to hide the light organ below its eyes, and also has a row of light cells along its body. These fish recognize one another by displaying individualized arrays of lights.
And there are other uses for these organs. Deep-sea anglerfish use “fishing poles” with luminous lures to attract other fish. The rod is an extension of a basal bone, at the tip of which hangs a bulbous light lure. If the winking lure attracts a gullible fish, squid or prawn, the prey is drawn toward the anglerfish’s large, powerful jaws and long, pointed teeth. Viperfish have light organs within their mouths that lure prey right into a waiting stomach.
The Eyes Have It
Very faint levels of light may filter down to depths of 1,000 meters, and many species in this twilight zone have developed tubular eyes that look upward in an attempt to catch all available light. It’s been estimated that these fishes can see 15 to 30 times better in dim light than humans can. Even so, much of the vision consists of spotting the silhouettes of other creatures. However, these prey have evolved various mechanisms to avoid casting a silhouette. Against this background of weak light, fish may merge into the downwelling light by being transparent, by reflecting light to match the background, or by having a very low reflectance. Dark colors conceal well, but shades of red work just as well in the depths, where the water has filtered out all red light. Many shrimp and an entire family of fish, the whalefishes, are colored a startling deep red. The hatchetfish possesses light organs all along the sides of its body, which emit blue-green light at the exact intensity of the light downwelling from the surface, and thereby avoids casting a shadow.
Feasts of Flesh
Life in the deep ocean depends on the productivity of surface waters for food. Falls of large dead animals such as sharks and whales may provide a rare bonanza for scavenging fishes, which can gather to a corpse with astonishing speed. Remotely operated camera systems and, more recently, manned submersibles have documented feeding frenzies of hagfish (eel-like, primitive fishes which burrow into a fish, eating it from the inside out), isopods (large crustaceans resembling a pillbug), and even large sixgill sharks at baited sites, thousands of meters down.
Because of the lack of food in the deep, animals have developed feeding strategies that capitalize on rare good fortune. Should a fish encounter a potential meal, it will likely eat it, regardless of its size. Large female deep-sea anglerfish have been recorded taking prey two or three times their own length, and one family of fish is well-known for an enormously distensible stomach that is frequently found filled with prey the same size as the predator. Many fishes have developed huge fangs, hinged mouths and tremendously extensible stomachs to accommodate large prey.
The cookie-cutter shark has the bizarre ability to throw its jaws completely out of its body, much like the creature in the Alien movies. Most dolphins, whales and large fishes that make deep dives have semicircular scars from encounters with these foot-long fish, which bite into their prey and twist away with a chunk of flesh, hence the name “cookie-cutter.” The fish down here don’t get to eat very often, so they take full advantage of every chance they get.