Sharks are the tabloid fodder of the animal kingdom. They dominate headlines, are the subject of overblown or fabricated claims, and inspire titillating biopics. In his 20 years of communing with sharks at laboratories in Boston and Woods Hole, Jelle Atema, a College of Arts & Sciences professor of biology and an adjunct scientist at the Woods Hole Oceanographic Institution, has found much to respect about the creatures, and not so much to fear. Atema is weary of alarmist media coverage. While fatal shark attacks are extremely rare, he says, humans kill millions of sharks every year, and swimmers who venture near a pod of seals, which can attract sharks, should know better.
Much of Atema’s work has focused on the workings of sharks’ sense of smell, which is sophisticated, complex, and finely tuned to survival in a world where visibility is poor.
At his Woods Hole laboratory one broiling hot summer day, Atema hovers over an immense saltwater tank that serves as a kind of lap pool for odor-detecting experiments conducted by Ashley Jennings (GRS’14), who is working toward a master’s in biology, with a concentration in ecology, behavior, and evolution.
The sharks at Atema’s lab are young, smooth dogfish, smaller than their approximate adult length of four feet. Not only are they nonthreatening, they are downright lovable; Atema refers to them as “beautiful creatures.” The sleek gray fish have broad, canine-like snouts and small rounded teeth for crushing crustaceans and shellfish. In the lab they are fed on squid. Atema and Jennings can easily grab the fish with their bare hands out of the inflatable, outsized kiddie pools where they live. Abundant in northeastern Atlantic waters, dogfish are hunters and scavengers who prefer shallow water. When they’re active, the research subjects circle their pools in undulating motion, and Jennings’ curious dog sometimes rests his head on the pool’s edge and watches them raptly.
Atema has experimented with devices mounted on sharks’ heads or implanted in their nostrils. The sharks are caught locally and live happily in the lab pool for months, sometimes even giving birth. They have learned the drill well: get lifted out of the pool with a net and dropped in the experimental flow tank, which is long and narrow like a lane in an Olympic pool. After some acclimation, they are gently herded into a barricaded downstream pen while upstream a small pump starts to deliver a stream of squid odor that slowly travels downstream in a narrow wake plume. (In Atema’s lab, odor plumes are manufactured from the pungent contents of a pile of squid stashed in the laboratory fridge.) Then the barrier is lifted, and the shark heads out like a racehorse at a starting gate, cruising along in classic lateral style, zigging and zagging its way toward the odor source, humans (and video cameras) monitoring its every move. Each experiment involves a parallel control plume consisting of nothing but injected water.
This morning, Atema and Jennings are observing a shark as it pursues a laboratory-generated plume. Although odor plumes—produced by anything, large or small, that has a scent—are ordinarily invisible, their shape and extent are measured in separate experiments with a control plume colored with red dye. From this, the scientists can measure the point at which the shark begins to track the plume. After hundreds of experiments, Atema’s team has learned a great deal about how sharks use their acute sense of smell as well as their ability to detect underwater turbulence to pursue prey not close enough to see. What the scientists are trying to determine now is how much odor is needed for a shark to locate the odor source.
“All animals, dead or alive, give off some kind of odor,” says Atema. “The science here is to understand how odor is dispersed into the water, and how many molecules does a shark need in his nose to start tracking that odor.” The tracking itself is actually not guided by the nose, but by the shark’s lateral line, a turbulence sensor, as established earlier by Atema and Jayne Gardiner (GRS’06) in their work at Woods Hole. Some shark species depend more on vision, others on smell. However, even in a well-lit sea, water scatters the light into a fog that limits vision; at depth and at night there is of course even less visual information. Introducing smells into a carefully controlled laboratory environment—while mimicking the way ocean currents carry the plumes as they break up and drift into ever smaller patches—Atema and his team are honing in on the limits of this shark’s use of the sense of smell.
More recently, Atema and Gardiner discovered that sharks are guided not by concentration differences between their two nostrils, as everyone had assumed, but by odor arrival time differences: the shark turns in the direction of the nostril that first detects the prey’s odor. The finding, published in July 2010 in Current Biology, suggests that this fast reflex response steers the shark into the odor patches that make up a chaotic turbulent plume and thus keeps it in the wake that leads to the odor source. This means that sharks can decipher very quickly, in a matter of seconds as opposed to minutes, where their next meal is, no matter how chaotic the dispersed odor plume.
It’s still an open question whether sharks can actually sense a single drop of blood a mile away. But there’s another bit of shark lore swimming around on the internet: for every two humans killed by sharks, humans kill two million sharks. That one, says Atema, just might be true.