Deep Sea: the Twilight Zone and Beyond
Pacific Sleeper Shark
Basically a huge dogfish, the Pacific Sleeper Shark (Somniosus pacificus) leads a mysterious, slow-motion life haunting continental slopes and shelves. In warm to cool temperate seas, it typically inhabits moderately to very deep water but, in polar regions, may occur in shallow coastal areas and even in the intertidal zone — in June 1958, an 11-foot (3.9-metre) female specimen was shot in Kachemak Bay, Alaska, after having been trapped by the receding tide.
Just the Facts:
Habitat: Intertidal, Rocky Reefs, Deep Sea, Polar Seas
Depth: surface to at least 6,560 ft (2000 m)
Distribution: Arctic, South Pacific, Temperate Eastern Pacific, Tropical Eastern Pacific, Argentinean, Southern African, Central South Indian, Southeast Australian/New Zealand, Japanese
Most records, however, are from water deeper than 780 feet (240 metres). In September 1989, a large female Pacific Sleeper — estimated to be 23 feet (7 metres) long — was filmed from the viewing ports of a submersible at a depth of 4,000 feet (1,220 metres) off Saruga Bay, Japan. Another large individual — estimated to be between 16 to 26 feet (5 to 8 metres) in length — was photographed by a robot camera at a depth of 6,300 feet (1,920 metres) off Oahu, Hawaii. Neptune only knows how much deeper Pacific Sleepers can go.
The Pacific Sleeper Shark exhibits some fascinating physiological adaptations to its harsh deep-sea habitat. Unlike a typical shallow-water shark, its liver oil contains no squalene — perhaps because in the chill depths (where temperatures can reach 39 degrees Fahrenheit or 4 degrees Celsius), this hydrocarbon would solidify into a relatively dense mass that would provide little buoyancy and could not be mobilized as an energy store in the event of prolonged poor feeding. Instead, the Pacific Sleeper’s liver oils consist mostly of low-density compounds called “DAGE” and “TAG” that — together — maintain their fluidity even at very low temperatures. Like many deep-sea sharks, the Pacific Sleeper stores relatively little urea in its tissues but astonishingly high concentrations of another nitrogenous waste product called “TMAO”. By a mechanism that is not yet fully understood, TMAO helps stabilize proteins — such as those that compose swimming muscles, digestive and reproductive hormones — against the crushing pressures and intense cold the Pacific Sleeper encounters at depth.
In its feeding structures, too, the Pacific Sleeper Shark is well adapted to its habitat and mode of life. Because food is relatively scarce in the deep-sea, the Pacific Sleeper is able to store food in its capacious stomach. For example, the stomach contents of a 12-foot (3.7-metre) female specimen from Trinidad, California, had a mass of 300 pounds (136 kilograms). Like those of other dogfishes, the jaws of the Pacific Sleeper are short and transverse, enabling it to inflict a powerful bite so that it can crush or wrench flesh from even the toughest food items. The upper jaw teeth are spike-like, while the lowers are broad with strongly oblique cusps and overlapping bases. Such a dental arrangement is well suited to grasping and sawing hunks from food items too large to be swallowed whole. Lastly, the Pacific Sleeper’s caudal fin is short and broad. Such a caudal fin shape is not suited to prolonged high-speed cruising (as is the caudal fin of the Shortfin Mako, Isurus oxyrinchus) but it is ideal for bursts of rapid acceleration — enabling the Pacific Sleeper Shark to conserve its energy between meals yet be able to capture prey in brief explosions of violence.
Diet of the Pacific Sleeper Shark has been most extensively studied in the Gulf of Alaska. Here, 73% of Pacific Sleeper stomachs examined contained remains (principally the horny, parrot-like beaks) of the Giant Pacific Octopus (Enteroctopus dofleini). Alaskan Pacific Sleepers also prey heavily on bottom-dwelling teleost fishes — including a variety of soles, flounders, pollocks and rockfishes — but also take fast swimming prey — such as squids (Loligo, Ommastrephes, and others), Pacific salmon (Oncorhynchus spp.), and Harbor Porpoise (Phocoena phocoena) — as well as crunchy bottom-dwelling invertebrates — such as shrimps, hermit crabs, and even marine snails (including large, heavily-shelled tritons of the genus Fusitriton).
Like that of the White Shark (Carcharodon carcharias), the diet of Pacific Sleeper Sharks seems to broaden as they increase in size. A 3.7-foot (1.1-metre) female specimen from off Trinidad, California, was found to have fed mostly on Pacific Giant Squid (Moroteuthis robustus). The sharks included in the Gulf of Alaska study cited above ranged in length from 6.5 to 10 feet (2 to 3 metres) and had fed mostly on flounder, pollock, and cephalopods. Larger Pacific Sleepers, between 11 and 14 feet (3.6 and 4.3 metres) in length consume not only teleosts and cephalopods, but also marine mammals. A 13.5-foot (4.2-metre) male specimen captured off Patagonia contained three, 3-foot (1-metre) long Patagonian Toothfish (Dissostichius eleginoides) and numerous squid beaks. A 12-foot (3.6-metre) female Pacific Sleeper caught off the coast of Chile had in its stomach a whole, 34-inch (87-centimetre) Southern Rightwhale Dolphin (Lissodelphis peronii), while the 11-foot (3.9-metre) female that was stranded and shot in Kachemak Bay, Alaska, contained three octopuses, complete and shattered Tanner Crabs (Chionectes bardi), fragments of Hairy Tritons (Fusitriton oreganensis), and portions of at least three Harbor Seals (Phoca vitulina).
Interestingly, although they often live near haul-outs of Steller’s Sea Lions (Eumetopias jubatus) and have the dentition to ‘process’ them, Pacific Sleeper Sharks in the Gulf of Alaska are not known to feed upon these large, calorie-rich pinnipeds. However, like any card-carrying shark, the Pacific Sleeper is ever ready to scavenge whenever the opportunity arises. And the annual 13,000-mile (21,000-kilometre) round-trip migration of the Grey Whale (Eschrichtius robustus) along the Pacific Coast of North America — from winter feeding grounds in the Gulf of Alaska to summer calving lagoons in the Sea of Cortez — provides ample opportunities, as many participants cannot endure the arduous journey.
According to one recent estimate, there may be a Grey Whale carcass on the seabed every 10 miles (16-kilometres) or so along this species’ migration route. However many failed Grey Whale migrants litter the bottom, each carcass represents a tremendous bounty of blubber and protein to any scavenger that can endure the demands imposed by the deep-sea environment, locate and process it. In addition to its deep-adapted physiology and powerful, scoop-like jaws, the Pacific Sleeper Shark has very large olfactory bulbs, granting it an exceptional sense of smell — the better to track down the irresistible ‘perfume’ of a decomposing whale.
In June 1998, a research team from the University of Hawaii became the first to film a Pacific Sleeper Shark feeding on a whale carcass. The carcass was that of a two-year-old, approximately 35-ton (31 tonne), adult Grey Whale that had become wedged under a pier and drowned near Santa Barbara, California. It was towed 75 miles (120 kilometres) out to sea and allowed to sink 5,700 feet (1,740 metres) to the ocean floor. Six weeks later, the whale carcass was visited by a research submersible for scientific study. To their delight, the researchers discovered a large (approximately 16-foot or 5-metre) Pacific Sleeper Shark feeding on the carcass. Video shot at the time clearly shows the shark peeling off great strips of fat-rich blubber, consuming it and gorging on the protein-rich muscle beneath. Alongside it, swarms of bluish-grey hagfishes writhed like a Gorgon’s hair. Unconcerned, the Pacific Sleeper seemed to be working the carcass methodically, taking full advantage of this rare ‘food fall’ from the sunlit world far above.
The bulk of the whale’s carcass will be gone in about a year. After the Pacific Sleeper and other sharks have removed all the flesh they can, hagfishes, rat-tailed fishes called “grenadiers”, crabs, and swarms of amphipods will strip it to the bone. For years afterward, the whale’s remains will play host to more than 150 species of worms, molluscs, crustaceans, and bacteria. That the whole process of reclaiming the energy locked within a dead whale’s tissues is often started by Pacific Sleeper Sharks makes these giant dogfishes play a role not unlike that of a deep-sea undertaker.