Invertebrates
Copepod
Copepoda · also called copepods
A copepod is barely visible to the naked eye, yet by sheer numbers it may be the most abundant animal on the planet, found in nearly every drop of water from the open ocean to a puddle. Every night, uncountable trillions of them rise toward the surface to feed and sink back down by day — the largest synchronised movement of animal life on Earth, repeated on a daily schedule.
Overview
Copepods, class Copepoda, are tiny crustaceans found in nearly all aquatic habitats worldwide. Most are filter feeders, sweeping phytoplankton and bacteria from the water with finely branched limbs, often with a single, centrally placed eye rather than a pair.
An engine for the whole ocean
Their nightly rise-and-sink vertical migration helps drive the ocean’s biological pump, ferrying carbon from the surface toward the deep sea through sinking waste and bodies. As a foundational food source for larval fish, whales, and basking sharks, copepods underpin an enormous share of marine and freshwater food webs.
Status
Least Concern — vast, resilient, and virtually ubiquitous, though local shifts in ocean temperature, acidification, and pollution can ripple up to the many larger animals that depend on them.
Copepod: common questions
Why are copepods considered possibly the most abundant animal group on Earth?
Copepods occur in almost every body of water on the planet, from the deep open ocean to small freshwater ponds, and their population densities in productive waters can be extraordinarily high, with estimates of total global copepod numbers and combined biomass ranking among the largest of any animal group, rivalling or exceeding even insects when considered across all habitats. This combination of near-universal distribution and extremely high local abundance is why copepods are frequently cited alongside krill as strong candidates for the most numerous multicellular animals on Earth.
What is the significance of copepods' daily vertical migration?
Rising toward surface waters to feed under the cover of darkness and sinking to deeper, darker water during daylight to avoid visually hunting predators allows copepods to access food while reducing predation risk, a strategy so widespread and involving such enormous total numbers of animals that it is often described as the largest synchronised movement of animal biomass on the planet each day. This migration also has a significant ecological side effect, since copepod movement and waste production help transport carbon and nutrients between surface and deeper ocean layers.
How do copepods help remove carbon from the atmosphere?
Copepods feed on phytoplankton near the ocean surface, which itself has absorbed carbon dioxide from the atmosphere during photosynthesis, and copepods then produce dense faecal pellets and eventually die, both of which sink toward the deep ocean, carrying that carbon with them. This process is a significant component of what oceanographers call the biological pump, a natural mechanism that helps move carbon from the surface ocean, where it could otherwise re-enter the atmosphere, down into deeper water where it can remain locked away for long periods.
Are copepods dangerous or harmful in any way?
The great majority of copepods are harmless filter feeders or small predators of other zooplankton, posing no direct threat to humans, though some copepod species function as parasites on the skin or gills of fish, and in a small number of cases copepods can serve as an intermediate host for certain parasitic worms that can infect humans if untreated water is consumed. In general, however, copepods are far better known and far more significant for their essential, positive role at the base of aquatic food webs than for any harm they cause.