Zooplankton are tiny animals that drift through oceans, lakes, and rivers, too small and weak to swim against currents. Most zooplankton are so small you'd need a microscope to see them clearly, though some, like jellyfish, can grow quite large.

These drifting creatures include baby crabs, young fish, microscopic shrimp-like animals called copepods, and countless other species. They spend their lives floating through the water, eating even tinier organisms called phytoplankton (plant-like drifters) or other zooplankton.

Zooplankton form a crucial link in ocean food chains. Small fish eat zooplankton by the millions. Larger fish eat those smaller fish. Eventually, the energy from zooplankton reaches sharks, whales, and the fish we eat for dinner. Without zooplankton, most ocean life would collapse. Some whales, despite being the largest animals on Earth, survive by filtering enormous amounts of zooplankton directly from the water.

Scientists study zooplankton populations carefully because changes in their numbers can signal problems in ocean health, like pollution or warming temperatures. These tiny drifters, invisible to most beachgoers, keep the ocean's food web running.