SCIENCE

Crazy Crab Migrations

June 9, 2010

I've been slowly watching the 11 episodes of the BBC/Discovery series Life over the last few weeks, and I've been amazed by much of it. While many of the segments focus on small groups of animals, and sometimes just one---like the bouncing pebble toad---huge migrations and gatherings of creatures abound. Some are well known, like flamingos in the Great Rift Valley or monarch butterflies in Mexico, but crabs? The Life series turned me on to two and then I remembered a Smithsonian story that added a third to the list. Looking some more, I came up with a list of six:

Red Crab, Christmas Island, Indian Ocean: These crabs, about 4 1/2 inches wide, are colored bright red or, sometimes, orange. One of 14 species of land crabs on the island, the red crab numbers around 120 million. Starting when they are 4 or 5 years old and sexually mature, the crabs participate in a great migration to the shore. At the beginning of the wet season, usually in October or November, streams of crabs crawl from the forest to the sea, clambering down cliff faces and creeping through towns. After the crabs mate, the females retreat to burrows. Over the next 12 to 13 days, they'll develop tens of thousands of eggs that they release into the ocean, in perfect synchronicity, at the turn of the high tide during the Moon's last quarter. (There's a similar migration on Cuba.)

Horseshoe Crab, Delaware Bay: Horseshoe crabs are technically not crabs at all (they're more closely related to spiders than to crustaceans), but their name puts them on this list. The largest gathering of horseshoe crabs occurs each May on the shores of the Delaware Bay when they come together to spawn. All those yummy eggs draw red knot birds, who stop there on their migration from Tierra del Fuego on the southern tip of South America to Canada. The tiny birds each gobble as many as 25,000 eggs per day.

Blue Crab, Chesapeake Bay: Blue crabs are a familiar site to anyone in the Mid-Atlantic--they're Maryland's state crustacean and very tasty. After the crabs mate in late summer or early fall, the females begin a long journey to the mouth of the bay where, in November or December, they release their eggs. After the larvae develop into juvenile crabs that can walk and swim, they use tidal currents to migrate back up the bay through the shallow, grassy areas.

Majid Spider Crab, Tasmania, Australia: Most of the year, these crabs live at depths of up to 2,625 feet. But in late fall and winter, they come to shallow waters to mate. A female can't mate until she divests herself of her old skeleton, but until her new outfit hardens up, she's vulnerable to predators like manta rays. (The Life documentary had some great footage of a ray stalking the crab party.)

Red King Crab, Alaska: These are the crabs that those big, yummy, orange legs come from. Between the ages of 2 and 4, the juvenile crabs gather in pods of thousands, relying on their numbers for protection. In late winter, adults migrate up to 100 miles to shallow water to mate. Once the females hatch their young in spring and molt their exoskeleton, they make the long journey back to deep waters.

Soldier Crab, Caribbean islands: These hermit crabs live throughout the Caribbean and are also popular pets (they can live 30 to 40 years in captivity). They make their home on land, but around August, they travel to the shore---gathering en masse in places known as "soldier washes"---where they leave their shells and enter the water to spawn. When they're done, they return to shore, crawl into a shell left behind by a neighbor and make their way back home.