Tiny Critters Massive Migration

ROUND TRIP IN JUST ONE NIGHT

- Nora Nickum

The largest migration on Earth happens across all its aquatic habitats, from ponds to the ocean, every single night. Many of the travelers are zooplankton. This is a group that includes tiny crustaceans such as copepods and krill, as well as jellies and pelagic, or open-ocean, worms. Many fish and squid also make the journey. During the day, they hide in the dark depths. But at night, they move upwards in the water hundreds of feet or more, in about 20 minutes. Then, when sunrise nears, they head back down.

A Tough Journey

This diel vertical migration (diel means 24-hour) isn't an easy jaunt for zooplankton. They don't have a lot of power for movement and can be smaller than the white edge of your fingernail.

If you scaled it to a human, that'd be like running a 10K [6.2 miles] to get your dinner and then a 10K before you went to bed-and doing it at twice the speed of an Olympic marathon runner, says Kelly Benoit-Bird. She's a senior scientist at the Monterey Bay Aquarium Research Institute in California.

This migration takes a lot of energy. A copepod or krill uses its swimming legs to motor up and down in the water column, but it isn't strong enough to fight sideways currents or tides. It risks getting swept away from its home range. So, why do zooplankton migrate if it's difficult and risky? Zooplankton need to hunt for microscopic plant-like organisms called phytoplankton. These float near the surface and photosynthesize, or soak up sunlight to make energy. If zooplankton hang out at the surface, they'll find plenty to eat.

The problem is when the Sun is up, those surface waters can be dangerous. In the light of day, birds, salmon, and tuna can spot the zooplankton and devour them. The deep is much safer. Down deep, though, snacks are tough to find. The cold water and lack of food slows a zooplankton's growth. They can't stay here, either.