Aptly named for their lethargic and sluggish movements, Pacific sleeper sharks (Somniosus pacificus) are gray in color, with a blunted nose and small dorsal fins, and a large torpedo-shaped body. Their mouth and teeth are not your ‘Jaws’ type of ferocious-looking, voracious shredding and feeding machine. Instead, they have very small hooks on their upper jaw, and very small but razor-shark teeth on their lower jaw. They are thought to be adept suction feeders that ingest smaller prey whole but can cut large chunks cookie-cutter-style out of larger prey. In their eyes, they frequently carry an ectoparasitic copepod: Ommatokoita elongata, a crustacean that is almost unrecognizable as such. This parasite blocks and scars the cornea of the shark eye, prompting some to suggest that these sharks do not have good vision. Like most sharks, they are however likely to have an acute sense of smell and can detect even the most minute variations in electrical fields to help them locate prey.
Pacific sleeper sharks that are caught in Gulf of Alaska fisheries or for research typically range from 1.5-3.5 meters in total length. It is thought they may potentially grow to 6 or 7m (23 ft!), but reports of sharks at these sizes are not officially confirmed.
When do Pacific sleeper sharks reach sexual maturity? Where do they give birth? How many pups are in a litter (they are thought to give birth to live young)? Do juvenile Pacific sleeper sharks eat different prey and utilize space differently than sexually mature adults?
These are all great questions we are seeking answers to! However, so far these questions remain largely unanswered. What we do know: it is likely that females do not reach sexual maturity, or the age at which they mate and give birth, until they are at least 3.5m long. If we assume similar age patterns to the Greenland shark, this means that Pacific sleeper sharks may not reach sexual maturity until they’re over 150 years old!
Check out these cool informational animated video clips on sleeper sharks, by ‘Facts in Motion’.
Pacific Sleeper sharks have been found on continental slopes and shelves in the North Pacific, the Bering, Chukchi, Okhotsk, and Beaufort Seas, the Gulf of Alaska, down through British Columbia, Washington, Oregon, and through Baja California. While there are reports of sleeper sharks off the coast of Chile and other regions in the south Pacific, these may be sightings or catches of their relative, the southern sleeper shark (Somniosus antarcticus) – the third large species of this family. The two Pacific species share many morphological similarities and are easily mistaken for one another where their ranges overlap but where one species’ range ends and another begins is poorly understood. Similarly, in the Arctic, it is thought that Pacific sleeper sharks and Greenland sharks likely overlap in their ranges and may even hybridize.
Sleeper sharks are generally considered to be cold-water sharks, seeming to prefer water temperatures between 2 and 7 degrees Celsius (35-45 F). Effectively, this means that in lower latitudes they are deep-sea sharks, but at higher latitudes, they find cold waters at much shallower depths. Data from depth recording tags have revealed that they may range through the water column from 2,000m deep to just a few meters below the surface! Several researchers have noticed the tendency for larger sharks to be found at deeper depths than smaller sharks, and this pattern may be driven by temperature and preference for cooler waters.
In the Gulf of Alaska where this project takes place, we work in coastal fjords—or deep narrow bays formed by glaciers–that mimic the temperatures and depths of habitats that are typically only found much farther offshore.
Sharks are increasingly being recognized as playing important roles in ecosystems as predators. From looking at the contents in the stomachs of sharks caught in fisheries, we see that young/smaller Pacific sleeper sharks feed upon a mix of squid, cod, pollock, rockfish, salmon, and flounders. However, there is evidence that as sharks get larger, their diet includes less squid, and more fish and marine mammals. In fact, data from telemetry studies on juvenile Steller sea lions suggests Pacific sleeper sharks may be an important predator of these pinnipeds:
In cold blood: evidence of Pacific sleeper shark (Somniosus pacificus) predation on Steller sea lions (Eumetopias jubatus) in the Gulf of Alaska. Horning M, Mellish JE (2014) Fishery Bulletin 112:297-310