Accounting for some 330 million cubic miles, the pelagic zone -- the offshore waters of the ocean -- is the world’s most extensive habitat. Although vast reaches of it are, compared to the lively richness of coastal realms, relatively barren, the open ocean plays host to a vast array of wildlife.
Among the top pelagic predators are large, open-ocean sharks, including various members of the requiem-shark family. The oceanic whitetip shark typifies the ecology of these magnificent predators: A stocky, pugnacious species that may reach 4 meters (13 feet), the oceanic whitetip opportunistically feeds on a wide range of prey, from offal to jellyfish to seabirds. Certain mackerel sharks are also notable oceanic species. The toothy mako sharks, for example, are powerful, swift fish capable of reaching speeds of 74 kilometers per hour (46 mph) in pursuit of active prey such as tuna, billfish and dolphins. Deep-water species include the wild-looking goblin shark, characterized by needle-like teeth and a prominent, horn-like snout.
A wheeling diversity of bony fish track the great currents of the open ocean, from small planktivorous species such as anchovies to high-level predators such as tuna and swordfish. Among them, too, is the most massive of all bony fish, the ocean sunfish. Some of these species undergo remarkable seasonal migrations encompassing huge distances. For example, Atlantic blue marlin -- biggest of the billfish -- have been documented traveling better than 14,500 kilometers (9,010 miles) between the Atlantic and Indian oceans. In the deep, black bathypelagic zone, anglerfish attract prey with bioluminescent lures. Pelagic freshwater fish include Africa’s Nile perch and North America’s lake trout.
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Many cetaceans -- whales and dolphins -- forage and travel in the open ocean. Baleen whales conduct long-distance migrations between feeding and breeding waters, some of which, such as the epic treks of North Pacific humpback whales between Japan or Hawaii and the North American west coast, include extensive time far out to sea. Among the several distinct types of orcas are little-known “offshore” killer whales that seem to prey heavily on sharks. Some marine mammals -- notably the sperm whale, beaked whales and elephant seals -- are capable of diving to great depths well beyond 1,000 meters (620 feet).
Several species of sea turtles roam the oceanic highways. The farthest ranging seems to be the leatherback turtle, which is also by far the biggest; these huge jellyfish-eaters trek across the Pacific basin between Indonesian nesting beaches and feeding grounds off the mouth of the Columbia River in the Pacific Northwest, as well as between the Caribbean and Newfoundland. Most sea snakes are fairly coastal in distribution and habit, but one species, the pelagic or yellow-bellied sea snake, readily swims the open ocean.
Efficient long-distance flyers, many seabirds roam the pelagic zone thousands of kilometers from land, mainly feeding on small fish and squid. Some spend most of their time on the wing, landing only to nest. The sooty tern and wandering albatross -- the latter one of the largest of the world’s flying birds -- are among the most famed pelagic travelers. Some open-ocean seabirds may associate with underwater hunters such as tuna, as they all target the same food-fish.
A plethora of squid species help compose the pelagic food web, functioning both as active hunters in their own right and as prey for numerous fish, birds and marine mammals. Among the most formidable is the Humboldt squid, which may weigh 50 kilograms (110 lbs.). Numerous jellyfish also passively ride pelagic currents, including the notorious Portuguese man o’ war, which also can sail by virtue of a specialized, wind-catching structure called a pneumatophore.