Where the presence of light ends, the aphotic zone starts. The aphotic zone is the area in a deep body of water beyond the reach of light. Many creatures live in this inky darkness, either as permanent residents or "commuters" that move to the surface at night. The specific depth the aphotic zone begins in varies depending on water clarity but is often 200 meters or deeper.
Giant tubeworms (Riftia pachyptila) live near hydrothermal vents in the ocean floor. These worms grow to 3 inches long. They come from an ecosystem that has no sunlight a mile below the surface. These bright-red, lipstick-shaped organisms do not have digestive systems. Instead, they rely on symbiotic bacteria living within their cells. These bacteria can metabolize toxic sulfur to provide their host with the nutrients they need to grow.
Giant and Colossal Squid
Both the giant squid (Architeuthis dux) and the colossal squid (Mesonychoteuthis hamiltoni) spend most of their lives in the deep ocean. These gargantuan invertebrates live on fish. In turn, these giants face predation from large, predatory whales like sperm whales. The elusive giant squid has been videotaped only once, while its slightly larger cousin the colossal squid is known only from dead and dying specimens.
More than 200 species of anglerfish dwell in various oceans. Some species live roughly a mile under the surface. They vary from a few inches to about 3 feet long. All anglerfish have a tiny lure, an extension of their dorsal fin they use to bring prey close. Some of the deep-sea species take it a step further: Their lures light up to attract prey in the dark. Some of the aphotic species have another unique adaptation: The smaller males will actually bite into their mates and dissolve into their bodies.
The goblin shark is a hideous, deep-sea version of shallow-water sharks. A large, shovel-like nose dominates the goblin shark's face, which has needle-like teeth and beady, dark eyes. Goblin sharks grow to 10 feet. They exist nearly worldwide but are rarely seen. Dead and dying specimens occasionally find their way to the surface or wind up in by-catch, but these sharks are not a common sight at the surface.