When most people think of animal migrations, they conjure up images of bird flocks flying south for the winter, hordes of wildebeests thundering across the savanna, and giant caribou herds trampling through the Yukon. Yet each day, there's a silent migration underway within the world's oceans, lakes, and ponds that eclipses all others: the vertical movement of billions of fish, squid, shrimp, and various forms of zooplankton—floating creatures, including jellyfish, tiny crustaceans, and protozoa. Collectively, these animals climb toward the surface at dusk, only to descend to the murky depths at dawn. Planktonic creatures, by definition, cannot swim against horizontal currents, yet they can still propel themselves hundreds of meters upwards and downwards, moving at speeds up to 200 meters per hour.
Vertical migration is often called the largest mass movement in the animal kingdom. Although it's been known about since the late-1800s, the extent of vertical migration was not appreciated until the 1940s, when sonar used to look for submarines during World War II came across a "false bottom" in the sea—a strongly reflecting layer that rose toward the surface every night. Rather than hitting the ocean floor or an enemy submarine, the sonar waves were bouncing off the bodies of countless organisms rising toward the surface.
Sixty years have elapsed since that discovery, and many mysteries remain about this widespread, and seemingly universal, phenomena. In particular, investigators would like to know what triggers the daily up-and-down surge. Marine biologists assume that animals lurk in deep, darker waters at day to avoid being seen by predators, swimming upwards under the cover of night to feed on rich surface waters teeming with phytoplankton. A related factor may be a preference among species for light of a certain intensity, as vertically-migrating animals appear to follow "isolumes"—lines of constant intensity. Evidence also suggests that zooplankton calibrate their movements to limit exposure to damaging ultraviolet radiation. What's more, they save energy by staying in cool, deeper layers, where metabolic rates are lower, until it's time for dinner.
Efforts to save whooping cranes, the most endangered of the world's 15 crane subspecies, offer a rare instance of animals learning about migration by watching what human do. Starting in 2001, humans flying ultralight aircraft have led whooping crane chicks on their first migratory flight south from a wildlife refuge in Wisconsin to a newly-created wintering habitat in a Florida preserve--a 1,200-mile journey.