Blue sharks steer straight for days, seemingly guided by Earth's magnetism
Off the northeastern United States, tagged blue sharks have held a dead-straight course for hundreds of kilometres over several days, far out in open ocean with no landmarks. The only cue available the whole time seems to be the planet's magnetic field, which these sharks may sense through tiny jelly-filled pores in their snouts.
Those pores, the ampullae of Lorenzini, each lead through a canal about a millimetre wide to a small chamber in the skin. They pick up faint electrical fields, responding best to slow flickers around one cycle per second. A shark uses them to find prey and dodge predators, and as it swims through Earth's magnetic field it may detect the weak voltages generated by currents or by its own motion, a possible compass. To notice a steady field, it has to be moving relative to the source.
The rest of its sensory kit is just as impressive. Eyes set on the sides of the head give an all-round view in the vertical plane and more than 300 degrees horizontally. Its hearing is tuned to low rumbles, most sensitive below about 100 hertz. A lateral line along the body feels water movement, telling it the direction and speed of currents and the vibrations of animals nearby.
Slender and countershaded, deep blue above and white below, blue sharks average around three metres and roam deep temperate and tropical waters off every continent except Antarctica. They mostly cruise lazily, drifting at little more than a kilometre an hour by day, but pick up the pace at night with short dives, and can sprint when chasing squid, their favourite food. Groups have been seen herding prey into tight balls to feed.
Mating is rough. Males nip females on the fins during courtship, so females have evolved skin three times thicker than males. Litters are big, anywhere from 4 to 135 pups after a pregnancy of nine months to a year, and newborns are left in nursery areas away from the adults. The species is abundant yet listed as Near Threatened, with many caught accidentally on longlines and nets.
Source: Blue shark
