Everybody knows that sharks sniff out the tiniest drop of blood in the ocean and swim right over to its source and start chomping. Basically, if not for that telltale scent, the sharks would never find anything to eat, right?
A 6½-year study of three species of Florida sharks, published Wednesday, has found that in hunting prey they use all of their senses, not just their sense of smell.
"We've always heard that sharks are just giant swimming noses, and that's not the case," said Jayne Gardiner of Mote Marine Laboratory in Sarasota, lead author on the study, which also featured work by University of South Florida scientists.
University of Miami shark biologist David Shiffman, who tweets under the name "Why Sharks Matter," praised the Mote-USF study, published in the journal PLOS One: "Its findings are a big advance for behavioral ecology in general."
The findings show that a lot of what people think they know about sharks "is actually a lot more complicated than we think," Shiffman said.
The study focused on about 40 blacktip, bonnethead and nurse sharks, so the research would cover a range of feeding behavior and prey, she said.
The scientists built a flume 26 feet long and 2 feet wide so they could control the flow of water between the sharks and their prey. They included windows to enable them to film high-speed video alongside and above the action, then examine it later frame by frame.
Before releasing a shark and its potential prey into the flume, the scientists blocked off one of the shark senses, such as plugging the nose with pieces of cotton soaked in petroleum jelly or covering the eyes with small pieces of heavy black plastic.
Shark senses include a couple that humans don't have — the ability to detect faint electrical charges, and the ability to detect the flow of water — so they had to find ways to block those out, too. For instance, they painted over the pores that pick up electric pulses using either glue or silicone-rubber paint.
That's how they found that the sense of smell wasn't quite as important as had once been believed.
"We were surprised by the flexibility in behavior," Gardiner said. "There were so many ways for the animals to get to food."
When their noses were blocked, the blacktip and bonnethead sharks both used their eyes to spot fish or crustaceans swimming ahead of them. Then they used their unencumbered senses — their ability to sense electrical pulses such as a heartbeat, for instance — to line up for an attack on their prey.
Both are "ram feeders," racing up behind a fish and suddenly gulping it down.
The nurse sharks, though, needed a scent to feed. They normally slide up near their prey and slurp it down. But if they could not smell the fish swimming right in front of them, even if they could see it plainly, they would not even try to eat it. They would simply swim past it.
The findings of the study can help scientists determine the relative health of different types of shark habitat, Gardiner said. Chemical pollutants and even turbidity in the water can disrupt the sharks' senses, she said.
It probably won't be much help in assisting swimmers and surfers in avoiding shark bites, a major problem in Florida, known as the Shark Bite Capital of the World. Gardiner herself got a taste of that, suffering a small nip from a blacktip shark while taking off its vision blocker.
Now that this project is completed, Gardiner said, the researchers are focusing on determining why female sharks appear to return to the place where they were born to give birth to their own pups.
Craig Pittman can be reached at [email protected].com. Follow him on Twitter at @craigtimes.