The hairy green crab is homely, its dark body covered in coarse brown hair. (Pesky, too, when alive -- the claws are tied snugly to the body with string.) The meat is sweet and silky, yet firm, and the roe is rich and unctuous. In Shanghai, many restaurants serve hairy crabs -- which the Chinese have adored since the sixth century -- and other green crabs. But Xin Guang, a plain three-story place that opened in 1991, is said to be the first to specialize in crabs; it serves some 800 pounds a day. The meat is served sautéed with scallions and sliced asparagus; brightened with roe on a bed of silken tofu, drizzled with black vinegar; and in a palate-cleanser of chicken broth with tiny crab won tons.