• Biodiversity Pacific Sleeper Shark
  • Pacific Sleeper Shark: Giant of the Deep

    Somniosus pacificus

    Imagine you’re out on a fishing charter boat, trolling for deep-water fish like halibut or rockfish. You catch something on the line and reel it in excitedly. It takes a long time coming up from so deep, and you can tell it’s a big one.

    You see a shape materializing under your boat. As it gets closer, it turns into a long, inky black blob—not quite the fish you were expecting.

    Could it be Cthulhu rising? Thankfully, not quite. It just might be a Pacific sleeper shark (Somniosus pacificus) that you aroused from its slow crawl in the depths of the Pacific.

    The Pacific sleeper shark is related to the famous Greenland shark (Somniosus microcephalus), but beyond that and a few other facts, very little is known about this mysterious shark.

    What does a Pacific sleeper shark look like?

    Pacific sleeper sharks aren’t your everyday silver-and-white, streamlined predator. They have a long, torpedo-shaped body with a wide, blunt head and a relatively small mouth. They can reach up to 22 feet in length, but most Pacific sleeper sharks don’t extend beyond the mid-teens.

    Unlike most sharks, they are the same color all over—a deep grayish-black that allows them to better blend in with the inky blackness where they prefer to live. They have a short, small dorsal fin and a long, sweeping caudal (tail) fin.

    Pacific sleeper sharks have very small eyes. It’s a good thing they rely on their sense of smell to find prey, because many Pacific sleeper sharks’ eyes are actually colonized by a special parasite, Ommatokoita elongata. These nightmare-inducing little beasts burrow into the cornea (outer layer) of a shark’s eye, where they feast on the tissue. Sharks with this parasite are nearly blind.

    Where do Pacific sleeper sharks live?

    Pacific sleeper sharks live in…you guessed it…the Pacific Ocean! In fact, they are the Pacific equivalent of the Greenland shark that lives in the Atlantic Ocean.

    People used to think that Pacific sleeper sharks were a far-north species because that’s the only place they would see them. In fact, they may even come right up to the surface of the water in the Arctic where the water is cold all throughout the entire water column.

    However, people have also discovered that this gigantic shark has a much more southerly range, meandering as far west as Taiwan and east to Mexico. At these latitudes, the Pacific sleeper shark prefers to stay far down where it’s colder, often a mile or more under the sea.

    Coping with the Deep, Salty Sea

    At these significant depths, the sharks have developed some special physiological coping mechanisms. More urea is able to permeate its tissues, allowing it to cope with the osmotic pressure from the increasingly-salty water. Without this adaptation, it would likely shrivel up and die. For a cool demonstration of what might happen to a shark if it’s not in osmotic balance, check out this egg osmosis experiment.

    Urea helps keep the shark’s tissues in osmotic balance with its saltier environment, but it comes with a drawback: it destabilizes proteins, which are the building blocks of the entire body. To counteract this effect, the shark produces another protein-stabilizing chemical: trimethylamine oxide (TMAO).

    The deeper the shark lives, the more urea it needs, thus the more TMAO it develops. People who eat the meat of animals with high concentrations of this chemical experience an interesting side effect: because it is toxic, it makes them stone-drunk. While there isn’t a widely-documented history of people eating Pacific sleeper shark meat, it has caused serious problems in people who eat Greenland shark meat without proper preparation.

    What do Pacific sleeper sharks eat?

    If the tiger shark is considered “the Ocean’s Garbage Can,” Pacific sleeper sharks are similarly the garbage disposals of the deep. They’ll eat anything that happens to float to the bottom of the ocean and cross their path.

    Surprisingly, they’re also very effective predators. Their dark skin acts like camouflage, and their bodies are especially designed for slow, easy cruising. It’s actually quite easy for them to sneak up on prey. Before their prey even knows they’re there, they’ve swung open their jaws to suck the prey in, much like a wobbegong does. Pacific sleeper sharks will eat all kinds of fish and cephalopods this way, even fast-moving top-swimming fish like salmon.

    What are the threats to Pacific sleeper shark conservation?

    The biggest threat to keeping these guys around is that we just don’t know much about them. Commercial fisherman have been finding more of them as bycatch in recent decades, however scientists aren’t sure if that really means their populations are increasing in size, or if fisherman are just getting better at catching them. The answer is unknown because no population studies exist on these interesting fish.

    It’s difficult to study animals that live such a reclusive lifestyle because it’s hard to reach them at such deep depths. Furthermore, people don’t eat or use these sharks, so there isn’t much economic incentive to learn more about them, as with school sharks or porbeagle sharks. But maybe you can become the shark biologist that shines a light on the mysteries of the Pacific sleeper shark and gets everyone else interested in them too!

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    Written by Lindsay VanSomeren

    Lindsay graduated with a master’s degree in wildlife biology and conservation from the University of Alaska Fairbanks. She also spent her time in Alaska racing sled dogs, and studying caribou and how well they are able to digest nutrients from their foods. Now, she enjoys sampling fine craft beers in Fort Collins, Colorado, knitting, and helping to inspire people to learn more about wildlife, nature, and science in general.

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