“No Tag” keeps company with another female, “Left Tag.” These sort-of names start to make sense when you see the identification tag on the latter’s left pectoral fin. The third shark, who is significantly smaller, needs no identification designation other than “the male.” Even when fully grown, male sharks are about a foot shorter than the 4-foot females.
Michelle is looking forward to the sharks breeding, especially the one born at Shedd, which would be a milestone in sustainable husbandry. But, she says, “The male might be a little immature yet. He’s practicing, though, chasing the females and biting their fins.” Such is bonnethead love.
Bonnetheads are native to subtropical waters in the western Atlantic, including throughout the Caribbean, as well as in the eastern Pacific from southern California to Ecuador. Like other members of the hammerhead family, their eyes and nostrils are at the outer edges of the head, giving them extra wide fields of vision and smell.
But the stretched-out head also provides plenty of real estate for other sensory organs, most notably the ampullae of Lorenzini. Sharks, rays and a few other “primitive” fish groups have these electroreceptors, which are visible as dark spots or pores on the face. As they swim, bonnetheads sweep the head from side to side, scanning. Michelle compares it to someone using a metal detector. But instead of locating coins and jewelry, the ampullae of Lorenzini detect minute fluctuations in electrical fields.
Every animal produces an electrical field through muscle contractions, such as heartbeats. Experiments have shown that sharks are so finely tuned that they can detect the electrical fields of paralyzed animals, revealing how they locate prey buried in the seafloor.
With their shovel-like faces, bonnetheads burrow into mud, sand, even under coral heads, to get crustaceans, shrimp, mussels, snails, octopuses and small fishes. Blue crabs are a favorite. Bonnetheads grab prey with their small, sharp front teeth, then crunch hard shells with their flat back teeth. Strong digestive chemicals do the rest.
Unlike the big hammerheads, which have a reputation for ferocity, the bonnetheads are timid and harmless to people. Still, they are not hand-fed like the other fishes in the Caribbean Reef.
At a feeding station on top of the exhibit, volunteer Pam McPherson plops a small weight on a rope into the water. “That’s their indicator that it’s time to feed,” she says. Sharks and other fishes have another sensory system, lateral lines that run along the sides of the head and body, which pick up small movements and vibrations. When the sharks circle the feeding station, Pam uses 32-inch grabber-style tongs to dispense pieces of fish—herring heads this time. Michelle has scored the necks of the fish to make the food easier to grip with the tongs, prompting Pam to say, “Look at you, like a sushi chef!”
The sharks eat well. In fact, the real trick to feeding them is keeping green sea turtle Nickel from grabbing the food instead. “Nickel knows the indicator, too,” Michelle says, grinning. Pam adds, “We like to wait until Nickel is having a nap before we start.”
The International Union for the Conservation of Nature (IUCN) lists 64 species of sharks as endangered, with some of them facing extinction. Bonnethead populations, however, are numerous and healthy throughout the species’ range.
Posted by Karen Furnweger, web editor