If there's one thing Alan Henningsen has learned in 23 years of swimming with sharks, it's this: Never take your eyes off them.
"They're wild animals, and they're predators and like any wild animal, if you put it in a confined space, it's not going to like it," says the senior aquarist and research associate at the National Aquarium.
He should know. He has been bitten twice. And his hard-earned experience is highly valued at this time of year as he and other staffers get into the water with the aquarium's 11 large sharks to help with their annual medical checkups.
Scientists hope the sharks will be looking and feeling good when the aquarium's new exhibit, "Shark Quest," opens to the public March 15.
The world's 400 species of sharks have varied diets, habits and growth patterns. Some species can live to be 30. Some reach 45 feet in length and others weigh up to 2,000 pounds.
But occasionally, they get sick. That's why checkups are mandatory for the aquarium's marquee predators.
Each year, aquarium staffers drop door-sized screens into the Open Oceans exhibit tank and use the screens to steer each shark into a circular pool. There, they coax the shark onto a partially submerged stretcher and wrap it in cloth. Finally, the staff uses a block-and-tackle to lift the stretcher into a coffin-shaped tank where the shark meets the doctor.
Henningsen is a key player: He holds the shark's head while it is being snared, poked and prodded.
"He's the best there is at this kind of thing," said Alison Davidson, curator of fishes, who has also volunteered for the hazardous duty.
In addition to being careful, Henningsen says, work has taught him that sharks are individuals. "They can be very different from each other," he said.
That much was evident during examinations of three sand tiger sharks this week. All three 8-foot males, plucked off Ocean City and Delaware in recent years, looked the same and averaged 185 pounds.
But that's where the similarities ended. While the first shark accepted confinement and probing without a struggle, the other two thrashed like reluctant schoolboys.
"It's hard to ever tell how they're going to react," Henningsen said afterward.
Shark examinations take about 20 minutes. Veterinarians weigh and measure each shark, then take blood from a vein under the dorsal fin, stomach fluids from rectal cavities and a scrape of skin tissue from any cuts. They also shine flashlights into gills and the shark's steely dark eyes, looking for parasites and other signs of sickness.
The shark's blood and skin samples go through the same tests that hospitals perform on human tissue samples, including a cholesterol screening, said Dr. Brent Whitaker, the aquarium's director of animal health.
The results are entered in each shark's medical history. Scientists share the information with other aquariums, which conduct similar shark checkups and use the data to diagnose ailments.
A few years ago, Whitaker said, examinations confirmed that white spots covering the aquarium's lemon sharks were caused by a parasite known as a Trematode and could be treated with occasional fresh-water baths.
"The science of it comes in identifying any health problems and coming up with a treatment regimen," Whitaker said.
The good news is that sharks usually get better on their own. "These animals have amazing healing powers," he said.
Although twice bitten, Henningsen said sharks don't attack humans indiscriminately. The aquarium's new exhibit is designed to dispel such myths, he said.
"The chances of being attacked by a shark are very slim," said Henningsen, who has gone swimming with his subjects hundreds of times.
He said his shark bites occurred long before he came to the aquarium 13 years ago. Neither required stitches. Both were his fault, he said.
He once made the mistake of taking his eyes off a shark to talk to someone and was bitten on the left index finger.
The other bite, on the right hand, occurred when he was trying to take a hook out of another shark.