Copepods eat mostly phytoplankton, single-celled algae that drift through oceans, lakes, and estuaries. But calling them simple plant-eaters undersells their dietary range. Depending on the species, copepods also consume bacteria, protozoans, other tiny crustaceans, fish larvae, and even decaying plant matter. Some are strict predators. Most are opportunistic omnivores that eat whatever is available and the right size.
The Core Diet: Algae and Microorganisms
Phytoplankton form the backbone of copepod nutrition across nearly every aquatic habitat on Earth. These microscopic algae provide the carbohydrates, fats, and proteins that fuel copepod growth and reproduction. In the open ocean, diatoms and dinoflagellates are primary targets. In freshwater lakes and ponds, green algae and cyanobacteria fill the same role.
Beyond algae, copepods regularly eat bacteria, protozoans, and ciliates. In both oceans and lakes, copepods are actually among the main predators of planktonic ciliates, tiny single-celled organisms that would otherwise graze unchecked on bacteria and algae. This predation shapes the entire microbial food web from the top down. Some species also consume yeast, and lab studies have shown copepods can survive on unconventional foods like rice bran, corn starch, and soybean flour, which hints at how flexible their digestion really is.
How Different Copepod Groups Feed
The roughly 13,000 known copepod species fall into three major groups, and each has a distinct feeding style that determines what ends up on the menu.
Calanoid copepods are the classic filter feeders of the open water. They generate a feeding current using their head appendages, drawing water across fine bristle-like structures that strain out particles. But they’re not passive strainers. Calanoids are mixed-type feeders: they filter small particles like bacteria and tiny algae while actively grabbing larger, more nutritious items like ciliates and other protozoans. The marine genus Acartia and the freshwater genus Eudiaptomus both use this dual strategy.
Cyclopoid copepods take a completely different approach. They are exclusively ambush feeders, sitting motionless in the water for long stretches before lunging at prey that drifts within striking range. Studies using high-speed cameras show species like Oithona nana and Oithona similis spending long periods of inactivity, punctuated by sudden leaps of motion. Cyclopoids target relatively large, moving prey: ciliates, other copepod larvae (called nauplii), and other small zooplankton. One tropical cyclopoid, Mesocyclops thermocyclopoides, can reach densities of 100 individuals per liter in warm, nutrient-rich waters and exert heavy predation pressure on the entire microbial community.
Harpacticoid copepods live mostly on or near surfaces, crawling along the seafloor, on sediment grains, or on aquatic plants. Their diet leans toward bacteria, microalgae growing on surfaces, and organic detritus. They scrape and graze rather than filter or ambush.
Detritus: The Overlooked Food Source
Not everything copepods eat is alive. Decaying plant material, often called detritus, plays a surprisingly important role in copepod nutrition, especially in estuaries and coastal wetlands where dead grasses and leaves wash into the water. Research in the San Francisco Estuary found that the calanoid copepod Eurytemora affinis actively consumed detritus from tule (a type of bulrush) even when phytoplankton were available and detritus was scarce. The copepods weren’t just eating it by accident; chemical biomarkers and DNA analysis confirmed they were selectively choosing it.
More striking, copepods fed a mixed diet of algae and terrestrial plant material (at a 1:3 ratio of algae to plant carbon) actually survived longer than those fed algae alone. One explanation is that copepods use the energy from plant material for basic metabolic needs, reserving the more nutritious fatty acids from algae for growth and reproduction. This dietary flexibility helps explain why copepod populations thrive in detritus-heavy environments where phytoplankton production is low, particularly during winter storms and spring snowmelt when rivers flush terrestrial particles into coastal waters.
How Much a Copepod Eats Per Day
Copepods are voracious for their size. Studies of tropical copepods found daily food intake ranging from about 3% to 33% of their body weight, with the exact amount depending on species, water temperature, and food availability. Warmer water speeds up metabolism, driving higher consumption. A copepod in tropical seas eats proportionally far more than one in cold polar waters, though cold-water species compensate by being larger and storing more fat.
Filter-feeding calanoids process impressive volumes of water to collect enough food. The species Temora longicornis, under normal conditions, filters about 14 microliters of water per minute. That sounds tiny, but scaled to the animal’s body size, it’s the equivalent of continuously running water through a fine-mesh net all day long.
Why Copepod Diets Matter for the Ocean
What copepods eat, and what they do with it nutritionally, has outsized consequences for marine food webs. Copepods are one of the few animal groups that can manufacture their own long-chain omega-3 fatty acids, specifically DHA and EPA, from simpler dietary fats. The cyclopoid species Apocyclops royi maintained DHA levels above 20% of its total fat content even after being fed a DHA-free diet for two generations. When fed algae rich in precursor fats, its DHA content climbed to nearly 49%.
This biochemical talent makes copepods nutritional powerhouses for the animals that eat them. Fish larvae, juvenile fish, whales, seabirds, and coral all depend on copepods as a concentrated source of these essential fatty acids. In effect, copepods act as tiny nutrient factories, converting simple algae into the high-quality fats that fuel the rest of the ocean food chain.
Microplastics and Feeding Disruption
Because copepods are indiscriminate about particle size, they readily ingest microplastics. Lab experiments show this has real consequences. Filter-feeding copepods exposed to polystyrene particles ate 11% fewer algal cells and took in 40% less carbon than unexposed individuals. The plastic fills the gut, blocks digestion, and forces the animal to waste energy attempting to break down something indigestible, a condition researchers call toxic anorexia.
The effects are dose-dependent. Temora longicornis filtered water at normal rates when no plastic was present, but at high microplastic concentrations (10,000 particles per liter), its filtration rate dropped by nearly 90%. Another species, Centropages typicus, saw its feeding rate fall from 34 algal cells per hour to fewer than 10 when surrounded by high concentrations of plastic particles. Since copepods sit at the base of so many food chains, reduced feeding efficiency ripples upward, potentially shrinking the energy available to fish, seabirds, and marine mammals that depend on them.