June 16, 2005—This extremely rare type of crab pulled from the Chesapeake Bay last month does not know whether it is Arthur or Martha.

It was the blue claw that caught waterman David Johnson's eye when he pulled his crab pot from the waters off Gwynn's Island on May 21. Crabbers know that male crabs have blue claws and that they are uncommon in the mainstream of the U.S.'s largest estuary at this time of year.

But then Johnson saw the red claw and other characteristics of female crabs. How can a crab have both a female claw and a male claw? In his 25 years on the water Johnson had never seen anything like it. He called in the Virginia Institute of Marine Science (VIMS) for help.

VIMS crab expert Rom Lipcius scoured the literature and found a report of a crab that was caught near Smith's Island in 1979. It too was split down the middle—one half male the other half female. It's a phenomenon known as bilateral gynandromorphy, and it's been observed in butterflies, moths, and lobsters. Each half has all the characteristics of its gender, right down to divided reproductive organs. It's caused when the two halves develop separately due to a genetic error early in the process of cell division.

Little is known about the sexual development and breeding behavior of blue crabs, so scientists are studying the crab to see what they can learn from it. The Washington Post reported today that the crab, alive and well in a VIMS aquarium, has been named Springer. "The watermen thought it was strange enough to be on The Jerry Springer Show," the newspaper reported.

—David Braun