Welcome to You Ask Andy

George Yee, age 11, of Ottawa, Ontario, Canada, for his question:

What is a spider crab?

Imagine a crabby creature with skinny legs long enough to help him to stand more than four feet tall. If he chooses, he can extend his two clawed arms to measure 12 feet from tip to tip. Centered in his five pairs of arms and legs is a flattish body about the size of a dinner plate. No, he is not an alien monster from outer space. He is the genuine earthling Macrocheira kaempferi, alias the giant spider crab.

Several species of spider crab live in the oceans, though most of them are rarely seen. As qualified crabs, they have eight skinny walking legs and one pair of leggy arms equipped with fearsome pincers. Their bodies are sheathed in crusty material, with bands of pliable leathery material to make it possible to bend the joints. All crabs wear their eyes on stalks. But the remarkable spider crabs are able to pull down their eye stalks to safety inside their shells.

The various spider crabs are remarkable in other ways. The giant of the family, who can extend his arms 12 feet wide, is the largest of all the crustacean creatures. He looks like a ferocious monster. But the big sissy lives a very lazy, peaceable life in the deep sea water off Japan. He is a scavenger who prefers to dine on dead meat, perhaps because catching live animals is too bothersome.

All his relatives look more like spiders than crabs, and some are very tiny fellows. One of them is the bashful little kelp crab, who lives a secretive life in the seaweed jungles off the Pacific coast of North America. He dangles on the long tangles of trailing kelp and, of all things, he changes color to match the scenery. He wears green to match green seaweeds and changes to brown when surrounded by brown seaweeds. Naturally, the two inch kelp crab is rarely noticed.

Other spider crabs go to even more trouble to conceal themselves. They are called decorator crabs because they disguise themselves by adding bits of the scenery to their shells. One species coaxes ribbons of red seaweed to grow on top of his shell. It is easy to mistake him for a waterweed. What's more, when pausing to rest, he cunningly rocks to and fro. This adds a touch of extra realism by making his seaweeds appear to wave gently with the flowing current.

Other species disguise themselves with sponges, sea anemones or other marine animals. They pick up their selected decorations and use sticky saliva to attach them to their crusty backs. Some have hairy tufts that hook onto decorative items for disguise purposes.

A human researcher tested these talented creatures by placing a moss covered spider crab in a tank full of sponges. The clever fellow immediately removed his mossy decor and added a few small sponges to his crusty back. The bashful creature seemed happy when he blended with the scenery. But he was moved to an aquarium which had only bare sand. There the poor crab scuttled frantically around, obviously seeking some sort of clothing to cover his nakedness.

 

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