Zooplankton are tiny, drifting aquatic animals that play a fundamental role at the base of aquatic food webs in both marine and freshwater environments. As heterotrophs, they consume other organisms to thrive, often feeding on phytoplankton, which are microscopic plant-like organisms. Zooplankton vary in size, ranging from microscopic protozoans and rotifers to larger crustaceans like copepods and even macroscopic jellyfish. Their abundance makes them a crucial food source, linking microscopic life to larger animals throughout aquatic ecosystems.
Invertebrate Predators
Many invertebrate species directly consume zooplankton, forming important links within the lower trophic levels of aquatic food webs. Jellyfish and comb jellies (ctenophores) are prominent examples, often using their tentacles or sticky cells to capture zooplankton from the water column. These gelatinous predators can significantly impact zooplankton populations, sometimes competing with larval fish for food.
Arrow worms (chaetognaths) are another group of invertebrate predators that hunt zooplankton. These transparent, torpedo-shaped animals seize prey with specialized grasping spines. Larger copepods, tiny crustaceans, also feed on smaller zooplankton, including other copepods and protozoans.
Fish and Bird Consumers
Numerous fish species rely heavily on zooplankton as a primary food source, especially during early life stages or throughout their entire lives. Small schooling fish like herring, anchovies, and sardines are well-known zooplankton consumers, often filter-feeding large volumes of water to capture prey. The larval stages of many commercially important fish species also depend almost entirely on zooplankton for nutrition and growth. First-feeding larval fish often consume copepods and other small zooplankton, with their diet diversity increasing as they grow.
Various seabirds also forage for zooplankton, particularly in areas where these tiny organisms are abundant near the surface. Petrels, shearwaters, and some puffins are examples of seabirds that capture zooplankton through surface feeding or shallow dives.
Marine Mammals and Giants of the Ocean
The largest animals on Earth, including several marine mammals and massive fish, depend on zooplankton for their immense energy needs. Baleen whales, such as blue whales, humpback whales, and fin whales, are iconic examples. They possess specialized baleen plates in their mouths that filter vast quantities of krill and other smaller zooplankton from the water. A blue whale, for instance, can consume up to 3,600 kg (8,000 lbs) of krill daily during its feeding season.
Other giant filter-feeders, including whale sharks, basking sharks, and manta rays, also consume significant amounts of zooplankton. Whale sharks, the largest fish in the world, feed on zooplankton, including copepods, krill, and fish eggs, by either swimming forward with mouths open (ram filtration) or actively sucking in water. Basking sharks, the second-largest fish, are passive filter feeders, swimming with mouths agape to sieve zooplankton, particularly copepods, from the water. Manta rays similarly consume zooplankton, such as copepods, shrimp, and crab larvae, by funneling water into their mouths with specialized cephalic lobes.
The Vital Link in Ocean Ecosystems
The consumption of zooplankton forms a crucial energy transfer pathway within aquatic food webs. Zooplankton act as intermediaries, converting energy from primary producers like phytoplankton into a form readily available for higher trophic levels. This transfer is fundamental for supporting populations of fish, marine mammals, and seabirds, making zooplankton a foundational component of aquatic ecosystems.
Zooplankton also contribute to carbon cycling and nutrient distribution in the ocean. They help regulate carbon dioxide exchange by consuming phytoplankton and producing fecal pellets that can sink to the deep sea, exporting carbon. Disruptions to zooplankton populations or their predators can have cascading effects throughout the food web, impacting fisheries and overall ecosystem health.