Saltwater fisherman know the best way to attract sharks is with chum, whose strong odor is a magnet to a shark's keen sense of smell.
People who watch Shark Week know sharks detect and capture prey using a suite of sensory cues, including vision, electrical impulses and vibrations.
For the first time, a study shows how shark senses work together to guide the predator to the prey and how sharks hunt when deprived of one or more senses — the study, by researchers from Mote Marine Laboratory, the University of South Florida and Boston University, was published Tuesday on the peer-reviewed journal PLOS ONE.
"The most surprising thing was the flexibility of the sharks' behavior," said lead author Jayne Gardiner, a post-doctoral fellow at Mote. "People talk about sharks as being giant swimming noses that can detect one drop of blood in a million gallons of water. But it's not all about smell."
For the study, researchers used 18 blacktip sharks, 10 nurse sharks and 16 bonnethead sharks. These species were studied because they use different habitats and different hunting strategies.
"Nurse sharks are primarily nocturnal, feeding on benthic, bottom-dwelling, prey," Gardiner said. "They come in slow and capture food by suction. Bonnetheads cruise just off the bottom looking for crustaceans. They're ram feeders: They make a rapid strike and overtake their food with their mouths open.
"Blacktips are rapidly swimming mid-water ram feeders that chase down fish."
Individual sharks were placed at one end of a 24-foot-long, 6.5-foot-wide, 48-inch-deep flow channel, or flume; a prey item was tethered at the other end of the flume (pinfish for the nurse and blacktip sharks, pink shrimp for the bonnetheads).
In some cases, sharks were allowed to use all their senses to track the prey item; in some cases individual senses were blocked.
Eyes, for example, were covered with black plastic, and nares (nasal openings) were plugged with cotton soaked in petroleum jelly.
"In a normal study of an animal's senses, you study one sense at a time," USF biology professor and study co-author Philip Motta said. "So, you look at the effect of blocking the vision of bees or smell in a moth. This was a multisensory study: We knocked out the sharks' senses in sequence to see what effects that has on how these animals stalk their prey."
Among the findings:
• When researchers blocked the sense of smell for blacktips and bonnetheads, the animals were able to find their prey using vision and the lateral line (sensory cells that run along a fish's sides and detect vibrations).
"Sharks have many ways to get food," Gardiner said. "It's not as simple as saying they're driven by smell or visual cues. A lot of different stuff comes into play. They have alternate ways to get there if they're missing their preferred cue."
• When researchers blocked nurse sharks sense of smell, they couldn't find the prey.
"Nurse sharks rely on their olfactory sense," Gardiner said. "That makes sense: They jam their heads into a reef to get at fish. They really need smell to know what they're looking at."
• With blocked vision, all species could locate prey with the lateral line.
• When researchers blocked vision and lateral lines in blacktips and bonnetheads, the sharks missed their prey.
• Sharks detect electrical impulses through sense organs around their heads called ampullae of Lorenzini. In all three species, electroreception triggered the fish to open their mouths to capture prey. With the ampullae of Lorenzini blocked, nurse and blacktip sharks missed the prey unless they actually touched it near their mouths. Bonnetheads without electroreception never opened their mouths, even when they touched the prey item.
"Some of these findings we figured were going to happen," Motta said. "Other things we didn't anticipate, like the fact that nurse sharks are olfactory-driven. And we were surprised that blocking the electroreception on bonnetheads caused such a drastic change."
Results from the study could help shark populations, particularly concerning bycatch, the unintended capture of a species by commercial fishermen, which is a major cause of the decline of shark populations worldwide.
"As humans, if we design fishing gear that is attractive to fish we're targeting and less attractive to sharks, that could decrease bycatch," Gardiner said. "We're hoping the engineering folks take our information and apply it."
Then there's the human angle.
"If we're trying to find ways to make ourselves less noticeable to sharks by focusing on one sense, visual camouflage or electrosensory camouflage, that probably won't be effective," she said. "They are good at finding alternate senses to use, so we need to think about the ways these animals get to food."