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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?Slide 2 of 15
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.Slide 3 of 15
How many species exist?Slide 4 of 15
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.Slide 5 of 15
Why do sharks migrate?Slide 6 of 15
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.Slide 7 of 15
What are they doing underwater?Slide 8 of 15