Many animals in the ocean begin their life as drifters, or plankton. The word plankton comes from the Greek work planktos, meaning to wander. Some plankton wander their entire lives—they’re called holoplankton and include diatoms, dinoflagellates, krill and copepods. Others, called meroplankton, wander for a short time until they grow up and settle to the bottom or grow into a free-swimming form—these include most larval forms of echinoderms, crustaceans, and most fish.
Plankton are generally divided into three groups: zooplankton, phytoplankton and bacterioplankton. Zooplankton are animals; sometimes the eggs or larvae of larger animals. Phytoplankton are plants; they live near the water’s surface where there’s enough light to support photosynthesis. And bacterioplankton are, simply, bacteria: one-celled organisms that live their whole lives as plankton.
Survival is the key to success
Most zooplankton are very small but have adapted many ways to survive. Some have transparent bodies, bright colors or bad tastes. Scientists have also discovered that many tiny larval fish and crustaceans quickly become excellent swimmers with incredible endurance.
Plankton on the move
Many types of zooplankton migrate deeper into the water during the day (maybe to avoid predators and lower metabolism in colder water to save energy) and come up at night—to feed on phytoplankton. Plankton are heavier than seawater and depend on locomotion to stay in the water column. Some move through action of cilia, antennae, jointed appendages, muscle contractions and/or jet propulsion.
Making a living
Phytoplankton produce their own food through photosynthesis. Sunlight allows phytoplankton to bloom, which is then a major food source for zooplankton. Barnacle nauplii (larvae) use appendages to sieve particles from water. The young of clams and snails (veligers) use cilia to filter food particles into a "food groove" that leads to the mouth. Some developing echinoderms, such as sea urchin and sand dollar larvae, use well-developed arms to sweep food into their mouths. Some larvae may not feed at all while living as plankton, such as the blood star, which relies on energy stored in the egg.
Strength in numbers and diversity
A teaspoon of seawater can contain millions of plankton. The smallest (called picoplankton) measures less than two micrometers (micrometer = one millionth of meter) and the largest (called siphonophore) can grow 130 feet long (40 meters).
We’re all connected to plankton
Copepods, perhaps the most abundant type of animal in the world's oceans, sustain many marine fish populations. Baleen whales, such as the humpback whale, consume 1.5 tons of planktonic animals and small fish each day.
Want to learn what other animals depend on plankton? Visit the Seattle Aquarium! And learn more by reading our plankton fact sheet.