Ask any shark biologist a question about sharks, and chances are, the answer will begin with, "We're not really sure, but…"
That's because researchers know remarkably little about these deep-ocean creatures. There are more than 400 species of shark, and many of them fare poorly in captivity, making it difficult to observe their mating, navigational, learning and social (or anti-social) behavior.
In celebration of the Discovery Channel's "Shark Week," here are seven mysteries that scientists have yet to solve about sharks.
How do sharks navigate?
The open ocean has few visual cues, so how do sharks know where they're going? Some sharks travel great distances, such as the great white sharks that swim across the Indian Ocean, from the west coast of Australia to South Africa, said Andrew Nosal, a postdoctoral researcher at the Scripps Institution of Oceanography and the Birch Aquarium.
"It is an enduring mystery how sharks find their way through the ocean, which environmental cues they use, and how exactly those cues are detected and integrated," Nosal told Live Science.
There is compelling evidence that some sharks use geomagnetic navigation, meaning that they can sense Earth's magnetic field, orient to it and use it as a navigational tool, he said. Olfaction (smell) may be another navigational tool that some sharks use. But perhaps other factors — such as water temperature, sound and even vision (to some extent) — may help sharks navigate the deep, Nosal said.
How many species exist?
Researchers are still discovering new species of shark, especially from the deep ocean.
"These habitats are so mysterious because they are so removed from human activity," Nosal said. "We still don't know what lives there."
For instance, the so-called ninja shark (Etmopterus benchleyi) was announced to the scientific world in December 2015; a newfound species of hammerhead shark (Sphyrna gilberti) off the coast of South Carolina was identified in 2013; and a new species of "walking" shark (Hemiscyllium halmahera), shown here, made headlines after researchers discovered it in a reef off an Indonesian island, according to a 2013 study in the Journal of Ichthyology.
Moreover, sharks can range greatly in size, from as small as a cigar to as large as a school bus (such as the whale shark, a plankton feeder). They also live in diverse habitats, so a newfound species could be uncovered anywhere, Nosal said.
Why do sharks migrate?
It's clear that many sharks migrate seasonally, different trackers show. But why these fish migrate is still a mystery.
"We've got great technology that tells us where they are going," said Gregory Skomal, a fisheries biologist with the Massachusetts Division of Marine Fisheries. "But we don't know what drives the migration."
Do sharks migrate for food, mating, temperature or perhaps a mixture of all three? It's hard to say, even after studying a handful of sharks within the same species, Nosal said.
"I might track 10 leopard sharks, and they all do something that's a little bit different," Nosal said. "It's hard to pull out the patterns to explain what's going on."
Only by studying vast numbers of a single species of shark can researchers find overall trends and perhaps tease out the reasons behind each migration, he said.
What are they doing underwater?
It's anyone's guess what sharks are doing deep in the ocean, Skomal said.
Trackers can tell scientists where the sharks are swimming, but once the fish dive deep into the water, it's hard to follow them without disrupting their behavior, he said.
"We have plenty of data on white sharks that shows that some of them go out to the middle of the Atlantic Ocean, wander around and dive down to depths as great as 3,000 feet (900 meters) every day," Skomal told Live Science. "But we don't have any clue what they're actually doing there."
Recently, Skomal and his colleagues sent down an autonomous underwater vehicle (AUV) to spy on great white sharks at night. The footage suggested that the sharks were resting. "I dare not say 'sleep' because it's hard for us to determine if and when these sharks sleep," Skomal said.
What role do sharks play in the ecosystem?
Most people say that sharks are apex predators and are essential for maintaining balance in the food web. But not all sharks are at the top of that web, Nosal said.
"Much of this is theoretical, and it's still a mystery exactly how sharks fit in," he said. "Surely they are important, and many species are indeed apex predators. But food webs are very complicated, and so it is very hard to predict the effects of population declines of sharks."
How smart are sharks?
Studies on shark brains suggest that they're complex beings.
"The easiest thing to study is the brain," said Jelle Atema, a professor of biology at Boston University Marine Program. "But that doesn't mean we know how they use that brain. By all accounts, it's very large compared to the size of [the brains of other] animals." [Related: Are Big Brains Smarter?]
Sharks don't have many folds in their forebrains (an area associated with decision making and reasoning in people), but they do have lots of folds in their cerebellum (a region associated with coordinating body movements), Atema said.
Moreover, Atema has studied the olfactory senses of sharks, largely because they have two well-developed olfaction bulbs — one on each side of the brain, he said.
In a 2010 study in the journal Current Biology, Atema and his colleagues found that dusky smooth-hound sharks (Mustelus canis) turned toward odors stimulated first in their nares (nose), even if the second smell stimulation offered to them was higher in concentration. This trick may help the sharks stay connected to an odor plume, even if another smell in the busy ocean is of higher concentration, he said.
Are sharks social animals?
Some sharks swim in schools of various sizes, and others gather in groups of hundreds to thousands. But it's unclear whether sharks are attracted to one another or whether they're simply in the same spot because it's a nice location with good temperatures and food availability, Nosal said.
"Almost certainly, it's going to be a combination of the two," he said. "But we don't really know the extent to which sharks are social animals. There's more and more evidence that they are, but the details are forthcoming."