National Geographic : 1971 Feb
increased to an inch or more. Then they begin to exhibit one of the strangest behavior pat terns known among animals. Two- and three-year-old crabs collect in aggregations called pods-vaguely spherical clumps as big as basketballs or even umbrel las. They stand on each other's backs, com monly facing outward, waving their antennae at the world (pages 270-71). "Once I swam down to a huge crowd of crabs that had been spotted from the air," Guy said. "They stretched a quarter of a mile along the beach, out to a depth of 30 feet, and appeared to be pods lined up end to end. "On another occasion I followed one pod for 45 days, watching the same crabs every day. They moved from water a foot deep out 264 to 60 feet before I lost them. The pod was beachball-size and included more than 500 two-year-old crabs, both males and females." Pods may contain 3,000 individuals. They disband to change locations or to feed, then reassemble. Apparently the behavior serves for group protection, as fish schooling is be lieved to do; lone crabs would be picked off easily by such predators as halibut, sculpins, other crabs, or octopuses. Crabs May Live a Quarter Century After three or four years of shallow-water existence, king crabs move offshore. They live in water 120 to 240 feet deep in summer, migrating to 600 feet or more in winter. "Up to about seven years, the age of a crab can be determined accurately from its size," Dr. Hoopes said. "They grow fairly regularly Blue tint shows continental shelf 0 1000 STATUTEMILES DRAWNBY ISKANDARBADAY GEOGRAPHICARTDIVISION © NATIONALGEOGRAPHICSOCIETY Realm of the king crab, shown in brown, Munching daintily, a stretches across the northernmost Pacific Ocean. 2- to 3-year-old king crab Japanese, Soviet, and U. S. fishermen exploit grasps a brittle star with this cold-water bounty. Americans use only handlike claws in a ma traps. Russians and Japanese capture the crabs rine research laboratory mostly with tangle nets (below). Concrete at Auke Bay. Biologists weights and glass floats hold the 8-foot-high, tag and release the ani 150-foot-long nets upright, linked together in mals to determine their rows 5 to 10 miles long and 1,500 yards apart, growth rates and migra Some fields extend over 160 square miles, tory habits.