Watermen say that female blue crabs "paint their fingernails," meaning the tips of their claws turn bright red as they age. The male crabs, on the other hand, have sky-blue claws -- a sign as masculine as a mustache in the world of crustaceans.
So when Robbie Watson dumped out a crab trap and found a specimen with one red claw and one blue one, the discovery stopped him.
"I set it aside for a while," Watson said. "I really wasn't sure what to think."
As his boat, the Wharf Rat, moved on to other crab pots, Watson, 42, studied the crab. Underneath, its shell should have had a design looking roughly like the U.S. Capitol dome if it were female, or a Washington Monument pattern if it were male.
Instead, Watson found a wavy arrow, which seemed to be a combination of both sexes.
"It was unreal," Watson said. "I've never seen anything like that, and I've worked the water all my life.''
Scientists said the crab, caught May 21 near Gwynns Island in the lower bay, is an extremely rare creature called a "bilateral gynandromorph" -- that is, split between two genders -- with its right side female and its left side male.
The last time such a crab was caught in the Chesapeake region was about 1980, scientists said. Since then, watermen have hauled in millions of crabs annually without noticing another.
On the day they caught it, Watson and boat captain David Johnson had been crabbing since about 5:30 a.m., dumping females into one basket and males -- who are often bigger and sell for twice as much -- in another.
"What basket should we put it in?" Watson asked the captain.
"I think we're going to put it in one up front," Johnson, 50, recalled telling him.
They covered it with a wet rag to keep it alive.
Now, the crab lives in an aquarium in the reception area at the Virginia Institute of Marine Science in Gloucester Point, Va. The creature is eating voraciously and showing no signs of health problems.
Kimberly Reece, a scientist at the institute, said the crab's condition probably resulted from what she called a "chromosomal mishap" shortly after it was conceived. As the cells began to divide, at one point a sex chromosome was lost or changed -- and as a result, the two halves developed according to different genetic blueprints, she said.
That means the crab could help scientists understand a crab's genetics and early development, which remain mysterious despite the creature's ubiquity around the Chesapeake.
"At what point in its development is its gender fixed?" asked Romuald N. Lipcius, a crab expert at the institute.
There are other questions: Can the crab reproduce? Can it mate with itself? Mating season has come and gone for this year, Lipcius said.
"It's possible that it already mated with itself," he said.
Before turning over the crab to the scientists, Johnson and other watermen conducted their own experiment into its sex life, with bewildering results. They dropped a female crab, which was just about ready to mate, into its tank.
First, the half-and-half crab cradled the female under his legs, as a male crab would do in preparation for mating.
Then, the crab seemed to lose interest in the female and let her go, Johnson said.
Then a day later . . .
"He ate half of her," Johnson said.
"The first day, the male side was coming out, the next day, it was the female side," said Lipcius, noting that in the wild, female crabs will often eat other competing females after they have shed their shells and become vulnerable.
Lipcius said that after the crab dies, it will be mounted and put on display at the institute next to another crustacean celebrity, a nearly 11-inch-wide crab that is the largest caught in the bay region.
But, he said, Johnson will always have the right to take it home to his family in Deltaville, in Virginia's Middlesex County.
Watson said the crab already has a name, Springer. The watermen thought it was strange enough to be on "The Jerry Springer Show."
"Our next goal is, I want to catch a mermaid," Watson said. "Hey, man, you never know."