Copepods are tiny crustaceans, often no larger than a grain of rice, that drift in the water column of nearly every aquatic environment on Earth. These organisms represent the most numerous multicellular animals globally, with populations across the oceans reaching into the nonillion range. Phytoplankton are microscopic algae and cyanobacteria that form the foundation of the marine food web by performing photosynthesis. Copepods are the primary consumers that graze upon phytoplankton. This interaction drives the energy flow for the entire ocean ecosystem, connecting the sun’s energy captured by algae to virtually all higher marine life.
The Copepod-Phytoplankton Connection: Primary Herbivores
Copepods consuming phytoplankton defines their role as primary herbivores in aquatic food webs, converting plant energy into animal biomass. This grazing activity regulates the vast blooms of phytoplankton that occur seasonally. Without this steady consumption, the rapid growth of phytoplankton could lead to unchecked algal blooms, potentially depleting oxygen and creating detrimental conditions in the water column.
These small crustaceans are highly efficient grazers, with some species consuming a significant fraction of their own body weight in phytoplankton daily. The three major groups of copepods—Calanoids, Cyclopoids, and Harpacticoids—all participate in this herbivorous role. Calanoid copepods, often the most abundant planktonic form, are specialized for this floating, grazing lifestyle.
Diverse Feeding Strategies Among Copepods
The mechanism by which copepods ingest phytoplankton varies significantly across species. Many planktonic copepods, particularly Calanoids, employ suspension feeding. This involves the rapid, synchronized movement of specialized mouthparts, such as the second maxillae, to create precise water currents. These currents draw water toward the feeding appendages, allowing the copepod to actively select and capture individual phytoplankton cells.
The copepod can sense the size, shape, and chemical signature of particles to accept or reject them. In contrast, other copepods, such as many Cyclopoids, utilize a raptorial or ambush feeding strategy. These species often target larger prey, including microzooplankton and large, motile phytoplankton cells.
Raptorial feeders have stronger, agile appendages adapted for seizing rather than filtering. They rely on chemical or mechanical cues, like water movement, to detect a potential meal. Once detected, the copepod attacks and immobilizes the prey before ingestion. Many copepod species are also omnivorous, switching between suspension feeding on phytoplankton and raptorial feeding on small animals depending on food availability.
The Vital Role of Trophic Transfer
The copepod-phytoplankton interaction serves as the bridge for nearly all oceanic energy transfer. Copepods act as a bottleneck, packaging the energy stored in microscopic phytoplankton into a size and form digestible by larger organisms. This transfer supports commercially important fish populations, such as herring and cod, whose larvae rely heavily on copepods for survival. Marine mammals, including baleen whales, and numerous seabirds also depend on these small crustaceans as a primary food source.
Copepods also play a role in regulating global climate through carbon cycling. When they consume phytoplankton, they produce dense, carbon-rich fecal pellets. These pellets sink rapidly out of the surface waters, transporting carbon to the deep ocean floor, a process known as the biological pump.
In high-latitude regions, certain Calanoid copepods contribute to the seasonal lipid pump. These species gorge on spring phytoplankton blooms, storing the energy as large reserves of lipid. They then migrate hundreds of meters deep to hibernate, slowly metabolizing these fats over the winter. This respiration releases carbon dioxide at depth, sequestering carbon away from the surface layers and influencing the ocean’s capacity to absorb atmospheric carbon.