Plankton refers to a diverse collection of microscopic organisms that drift within ocean waters. These tiny life forms encompass both plant-like phytoplankton and animal-like zooplankton. Plankton form the foundational food source for nearly all aquatic ecosystems, supporting an immense variety of marine life.
Filter Feeders: The Ocean’s Living Sieves
Many marine animals consume plankton by filtering large volumes of water through specialized anatomical structures, allowing them to capture vast quantities of these tiny organisms.
Baleen whales, for instance, lack teeth and instead possess baleen plates which hang from their upper jaws. Species like blue whales engulf enormous mouthfuls of water and krill, then use their tongues to force the water out through the baleen, trapping the prey inside. Other baleen whales, such as right whales, are skim-feeders that continuously swim with open mouths, while gray whales sift through seafloor sediment to filter out small crustaceans.
Basking sharks also employ filter feeding by swimming with their mouths agape. Their enormous mouths allow water to pass through specialized gill rakers that strain out copepods and other zooplankton. They can filter thousands of gallons of water per hour.
Manta rays similarly feed by swimming with wide mouths and unfurling cephalic fins to funnel plankton-rich water towards their gill rakers. They often feed in groups, exhibiting coordinated behaviors like chain feeding or cyclone feeding to maximize their plankton intake.
Smaller filter feeders include various bivalves, such as clams, oysters, and mussels, which draw water through siphons and capture food particles on their gills. Sponges, immobile animals, rely on specialized cells that create water currents, trapping microscopic food items as water flows through their internal channels. Even corals, while also relying on symbiotic algae for energy, actively feed on zooplankton using tiny tentacles equipped with stinging cells to capture prey.
Active Plankton Hunters
Beyond passive filtration, many marine animals actively pursue and capture individual plankton, particularly zooplankton. Small fish, often called forage fish, like sardines, anchovies, and herring, form large schools and consume vast quantities of plankton. These fish actively target and consume individual plankters as a primary food source.
Jellyfish are carnivorous that primarily feed on zooplankton, fish eggs, and small fish. They use their tentacles, armed with stinging cells, to ensnare and paralyze their prey. Many jellyfish species exhibit daily vertical migrations, ascending to surface waters at night to feed on abundant plankton.
Tiny crustaceans, such as copepods and krill, also engage in active plankton consumption. Copepods are omnivorous and feed on phytoplankton, smaller zooplankton, bacteria, and detritus. Some copepods filter particles, while others actively seize larger prey like small fish larvae.
Krill, while often considered filter feeders of phytoplankton, are also omnivorous and consume smaller zooplankton and fish larvae. They use their front legs to comb through the water for food. The larval stages of many larger marine animals, including fish and various invertebrates, also depend on actively consuming zooplankton for their development and survival.
Plankton’s Central Role in Marine Ecosystems
Plankton’s importance extends throughout the marine food web, forming its fundamental base. Phytoplankton, as primary producers, convert sunlight into energy through photosynthesis, much like plants on land. This process not only supports their own growth but also produces a substantial portion of the oxygen in Earth’s atmosphere. Zooplankton then consume these phytoplankton, initiating the transfer of energy through the ecosystem.
This energy transfer continues as small fish and crustaceans feed on zooplankton, and these in turn become prey for larger predators, including larger fish, seabirds, and marine mammals. Without the abundance and productivity of plankton, the vast majority of marine life would not be able to sustain itself. The health and population dynamics of plankton are closely tied to environmental factors such as ocean temperature, salinity, pH levels, and nutrient availability. Shifts in these conditions can have cascading effects, impacting populations throughout the marine food web.