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Monday, March 19, 2012

Overlooked Species Theatre presents: Pacific Sleeper Shark

One of the great benefits of living so near the ocean is that every once in a while, something really cool washes ashore. Such was the case last week when a resident of Malcolm island found a dead Pacific sleeper shark on her doorstep.

Well, by the time she found it, it actually looked a bit more like this:

(I must warn you that that the pictures don't get any less graphic from here on in)

I had the opportunity to take some measurements and samples for a fisheries scientist who studies sharks. Having never dissected a shark, I soon found it was unlike anything I had ever seen up close before.

It was about nine feet long, though I later found that sleepers can grow to over 14 feet. It also had incredibly thick muscles along the spine for propelling the shark forward with powerful strokes of its tail.

Unlike most groups of fish, sharks have no swim bladders and must swim continuously to keep from sinking. Their skeleton is made entirely of cartilage, which is much lighter than bone. But this is not the only advantage the shark possesses for staying afloat.

The first thing I noticed when I cut open the abdominal cavity, was the shark’s massive, double-lobed liver. This organ can account for up to one quarter of the total weight of some sharks. The liver not only stores energy in the form of fat reserves, but also secretes oils that make the shark more buoyant as they are much less dense than water.

Although their livers help them to maintain buoyancy, they have also caused many species of shark to be extensively hunted. This is mainly because most sharks’ livers contain large amounts of squalene, an oily substance that is used in cosmetics and vaccines. Squalene is even thought to protect people from some forms of cancer when it is a part of their diet or taken as a supplement.

However, sleeper sharks live at much deeper depths than other sharks (as deep as 9,000 feet) where temperatures can get as low as 4°C. At these temperatures, squalene becomes solid and provides little buoyancy, so the livers of sleeper sharks contain entirely different substances.

The stomach of this particular shark was empty, but those of other sleepers have been found to contain everything from small invertebrates to ground-fish to giant Pacific octopus. Their diets continue to expand as the sharks grow. One of the most unusual sources of food for sleepers is the many carcasses of migrating grey whales that end up in the cold depths of the North-East Pacific (as seen in the video below). Sleepers have an exceptional sense of smell, which allows them to detect the decomposing whales from miles away.

If you're curious to know more about the eel-like fish crawling over the whale carcass in the video, they are called hag-fish and you can read more about them here.

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