The Pacific Sleeper Shark and the Greenland Shark are two of the ocean’s most enigmatic inhabitants, dwelling in frigid, deep waters. These large, long-lived sharks often spark curiosity due to their similar appearances and shared preference for extreme environments. However, distinct characteristics set them apart. This article explores the unique features and differences between the Pacific Sleeper Shark and the Greenland Shark, highlighting their adaptations to challenging underwater worlds.
Geographic Domains and Preferred Habitats
The Pacific Sleeper Shark primarily inhabits the North Pacific Ocean, with its range extending from Japan and Siberia, through the Bering Sea and Gulf of Alaska, and south along the west coast of North America to Baja California, Mexico. While it prefers deeper, colder waters along continental shelves and slopes, reaching depths of over 2,200 meters, it can occasionally be found in shallower coastal areas, particularly in higher latitudes where temperatures are colder. Some sightings have even been reported in the South Pacific and off Uruguay, suggesting a broader distribution than initially thought.
Conversely, the Greenland Shark is largely confined to the North Atlantic and Arctic Oceans. Its distribution includes waters around Greenland, Iceland, Canada, and Norway. This species exhibits an extreme preference for cold water, found between -2°C and 7°C, making it the only shark known to tolerate Arctic temperatures year-round. It commonly resides at depths between 450 to 800 meters, though it has been recorded as deep as 2,200 meters.
Physical Attributes and Size
Both the Pacific Sleeper Shark and the Greenland Shark possess robust, cylindrical bodies with short, rounded snouts and relatively small dorsal fins. Their skin is rough due to placoid scales, which are tooth-like structures that give the skin a bristly texture. Each species has two dorsal fins that are roughly equal in size, and neither possesses an anal fin.
The Greenland Shark attains greater lengths. Adults range from 2.4 to 4.3 meters (8 to 14 feet) in length, with the largest confirmed specimen measuring approximately 6.4 meters (21 feet) and weighing over 1,000 kilograms (2,200 pounds). Females tend to be larger than males.
The Pacific Sleeper Shark, while also a large species, reaches lengths of 3.7 to 4.3 meters (12 to 14 feet) for adult females, with an average adult length around 4.5 meters. However, there are reports and photographic estimates suggesting some individuals may reach up to 7 meters (23 feet) or even more.
In terms of coloration, the Pacific Sleeper Shark is a uniform grayish-brown or dark gray to black, often with blue-black fins. The Greenland Shark, conversely, displays a medium gray or brown hue, sometimes featuring small spots or blotches of lighter tones or darker transverse bands, occasionally with a purplish tinge. A unique characteristic of the Greenland Shark is the presence of a parasitic copepod, Ommatokoita elongata, which frequently attaches to its eyes, potentially impairing its vision.
Dietary Habits and Hunting Strategies
The Pacific Sleeper Shark consumes a wide array of marine organisms found both on the seafloor and throughout the water column. Its prey includes bottom-dwelling fish such as rockfish, flatfish, and Pacific salmon, as well as various invertebrates like octopuses, squids, and crabs. Larger individuals consume marine mammals such as harbor seals and Steller sea lions, and they also scavenge on whale carcasses.
This shark employs a suction-feeding mechanism to inhale prey. Its lower teeth are bladelike with oblique cusps, allowing it to slice up larger food items. Pacific Sleeper Sharks are stealth predators, capable of gliding through water with minimal movement and noise, which aids in ambushing prey. They may also perform vertical migrations, moving from deeper waters to shallower depths at night.
The Greenland Shark is an opportunistic apex predator with a varied diet, including fish such as Atlantic cod, Greenland halibut, wolffish, and skates. It preys on seals and has been found with remains of land animals like polar bears, reindeer, and horses in its stomach, likely from scavenging. Younger Greenland sharks consume more squid, while larger, older individuals shift their diet to include more fish and seals.
Despite its slow movement, less than 1.6 kilometers per hour (1 mile per hour), the Greenland Shark is an effective ambush predator. They sneak up on sleeping seals or other prey, using their excellent sense of smell to locate food in the dark, cold depths. When feeding on large carcasses, the shark uses a characteristic rolling motion of its jaw to cut off chunks of flesh.
Lifespan, Growth, and Adaptations
The Greenland Shark holds the distinction of being the longest-living vertebrate known to science. Studies utilizing radiocarbon dating of eye lens nuclei estimate their lifespan to be at least 272 years, with some individuals potentially reaching ages between 300 to 500 years. This extraordinary longevity is attributed to its extremely slow metabolism, a direct adaptation to its frigid deep-water environment.
The Pacific Sleeper Shark, while not as long-lived as its Greenland counterpart, still exhibits significant longevity, with estimated lifespans of at least 40 years. Both species share a common trait of deep-sea, long-lived animals: exceptionally slow growth rates. Greenland Sharks, for instance, grow at approximately 1 centimeter (0.4 inches) per year and do not reach sexual maturity until around 150 years of age.
To survive in their near-freezing habitats, both sharks possess unique physiological adaptations. Their tissues contain high concentrations of urea and trimethylamine N-oxide (TMAO), which act as natural antifreezes, preventing ice crystal formation and stabilizing proteins under immense pressure and cold. The liver oil of Pacific Sleeper Sharks contains low-density compounds like diacylglyceryl ethers and triacylglycerol, which remain fluid at low temperatures, providing buoyancy and energy reserves. This slow metabolism also contributes to energy conservation, allowing them to endure prolonged periods between meals.
Reproductive specifics for both species remain limited due to their deep-water habitats. Both are ovoviviparous, meaning their young develop from eggs that hatch internally, and live pups are born. Greenland sharks can have litters of up to 10 pups, while Pacific Sleeper Sharks may carry around 300 eggs.