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The bodies of dozens of pale brown Tanner crabs were strewn over the blue-mussel-lined beach at the North Douglas Boat Launch last week.

"It's like something straight out of a horror movie," said Debrah Clements, a North Douglas resident who saw them there.

But these aren't corpses from some mass death. The shells are empty and their owners were just doing what young male Tanner crabs do starting this time of year: molting.

That's the process whereby crabs cast off their shells and begin growing new ones.

"It's very easy and not at all surprising to mistake a cast-off shell of a crab for actually being a crab," said shellfish biologist Gretchen Bishop with the state Department of Fish and Game. "They will even leave behind the outer coating of the eyeballs and mouth parts."

Bishop said the shells are most likely blown in by wind and water from deeper areas. Males that usually spend their days between 50 and 70 fathoms (or 300 to 420 feet) have been found in shallower areas during molting time.

Juneau divers have documented mass molting in the Fritz Cove area since the 1970s. No other species is known to use the same area year after year for molting, according to a 1999 paper by Fish and Game biologist Bob Stone.

"It's quite a fancy process the crab goes through," Bishop said.

A crab preparing to molt pulls the minerals out of its shell into its bloodstream. It takes in water, which makes it swell in size. The swelling splits the crab down its back, right down the line where a human would to clean it. Then the crab backs out of its shell - carefully. Exiting takes between 10 minutes and a half-hour, Bishop said.

Then the crab can build a bigger shell.

Tanners tend to hang out in groups segregated by size and gender. That may be a good strategy for younger males particularly when molting, Bishop said. It's called predator saturation: If everybody's molting at the same place and time, predators won't be able to pick them all off.

A few soft, red crab remnants were left on the beach Friday along with all the shells.

Mortality is higher during molting, Bishop said. Without their usual armor, crabs are susceptible to getting chomped by various other species and their own kind, for crabs are highly cannibalistic.

• Contact reporter Kate Goldenat 523-2276 or e-mail kate.golden@juneauempire.com.