What are Copepods? Essential to the Web of Life

 What are Copepods? They’re tiny, ungainly crustaceans. Often barely visible to us, they aren’t on anyone’s must see list. But they’re everywhere in the oceans, and one of the foundations of the web of life in the sea. Appreciate them.

Copepod shells are so thin they’re essentially transparent, revealing the internal tissues within. Many females carry their eggs attached to their bodies.

NOBODY GOES DIVING TO LOOK AT COPEPODS, AND IF THEY DO SEE THEM THEY’RE LIKELY TO BE GROSSED OUT. Generically, they’re just tiny crustaceans, mostly almost invisible, vaguely similar to shrimps and crabs, that happen to be one of the main food resources for many of your favorite marine denizens.

But some are larger and are parasites, obviously and ghoulishly attached to fishes and other creatures, like whales.

If you like looking at seahorses, thank copepods. Seahorses may look lethargic but in fact they are lightening-fast predators of the copepods drifting past them in the currents – as many as 3,000 a day, by some estimates.

If you’re thrilled when you see an eagle ray scrounging in the sands for food, it’s copepods, small mollusks and worms they’re after. If you wonder what striped grunts do at night, when they’re not hanging around coralheads resting, they’re off in the sandflats looking for the aforementioned copepods, small mollusks and worms.

what are copepods-planktonic copepod, species unknown
A planktonic copepod, species unknown.


In fact, copepods are the most abundant multicellular creatures in the world’s oceans, all of them. There are, it’s suggested, as many as 24,000 species of these barely visible creatures, far more (and mostly smaller) than their crustacean cousins shrimps and crabs. Copepods are found in every oceanic environment worldwide, from the deep sea oases of thermal vents to the shallows of your favorite Caribbean dive resorts.

They also abound in freshwaters worldwide – in pools high in mountain ranges like the Andes, swamps in Louisiana, bogs in Britain and the New York City water supply. And, it’s not unusual to find copepods in habitats as diverse as damp moss and leaf litter, marine and freshwater caves and the water reservoirs of plants like bromeliads and pitcher plants.

what are copepods-blue chromis feeding on copepods
When fishes like these blue chromis, above, hover above coralheads facing into the current, it’s to catch  copepods and other plankton as they sweep along with the currents.
spotted eagle ray feeding on copepods
An eagle ray scrounges in the sandy bottom for copepods, mollusks and other benthic prey.









In the oceans, they’re one of the cornerstones of the food supply. Devourers of photosynthetic  phytoplankton (as many as 373,000 phytoplanktons a day), they become the next level up in the food web. The National Oceanic and Atmospheric Administration calls them the “cows of the sea.” When marine biologists talk about the zooplankton that sustain the world’s fisheries, copepods are far the most common.

What are copepod lifestyles? Mostly, they exist in one of three basic lifestyles. The most abundant are the open-water free swimmers – the ones who live in the plankton, dine on phytoplankton and are in turn dined on by planktivory fishes like blue chromis and creole wrasses. It’s not just exotic reef fishes. Anchovies and herring, Alaska pollock and other commercial fishery fishes are dependent on copepod abundance. And, of course, those are the fodder for the next larger fishes up the food chain, like tunas and sharks. And, other crustaceans, including krill, dine on copepods.

Baleen whales, like blue whales and right whales, earn their livings by sweeping in copepods and other planktonic creatures. Gray whales emulate stingrays by stirring up the seabottom sediments to uncover them. Copepod research, including how well they cope with warming and acidifying oceans and the implications for commercial fisheries, is a significant field. NOAA Fisheries maintains “The Copepod Project” as a data bank for monitoring copepod health for the benefit of fisheries.

A large proportion of copepods spend most of their lives as benthic organisms living in the sand and sediments, dining mostly on detritus they find there and being dined on by scroungers like trunkfishes and stingrays. But a substantial proportion are parasites, living on fishes and other hosts.

what are copepods - diverse body plans
An artist drew a gallery of amazingly diverse (and colorful) copepods, species unclear, for a 1904 book cataloging them. Public Domain photo via Wikimedia Commons.


With so many species, copepods display an amazing diversity of body size and design. They range in size from as small as the period at end of this sentence to as long as 8 in/20 cm. The free swimmers tend to be the smaller members of copepod world, the parasites that prey on the larger vertebrates the bigger ones.

What they all have in common is that, like all crustaceans, their bodies consist of three segments – a head, a thorax and an abdomen. Head and thorax are normally fused into a unified cephalothorax and feature six sets of appendages, including two sets of antennae, maxilliae that assist in feeding, and maxillipeds, swimming feet that they can beat like oars (the term “copepod” is from the Greek for “oar feet”). Their thoraxes are generally divided into segments, and for copepod specialists, the exact location of the articulation between body segments is a defining factor among species. We’ll ignore that. Their abdomens have no appendages except for a tail-like extension.

Most often, planktonic copepods are short and cylindrical in shape, with rounded or beaked heads, although plenty of variations exist. Members of Order Harpactpoida, who most commonly settle in sands or sediments, tend towards more elongated shapes – probably useful for maneuvering in a loosely solid substrate. Some species are distinctively elongated.

Like all crustaceans, they have a chitinous exoskeleton, although they are so small and the chitin so thin that the skeletal covering and the body underneath are largely transparent. Most have a single, compound eye, usually red, in the center of their transparent heads. But benthic species may lack eyes altogether.

planktonic copepod, species unknown
Variation in body shapes. Planktonic copepod, species unknown.


With as many as 20,000 species, it’s more enlightening just to describe the three basic lifestyles marine copepods follow:

What are pelagic copepods? The most plentiful groups of copepods in the world’s oceans are barrel-shaped free-swimmers in the 1,800 species in Order Calanoida. They feed on algae and other phytoplankton, meaning that they are found in the oceans’ upper levels. They’re also the largest of the planktonic copepods in physical size, as large as .71 in./18 mm but more typically .02-.08-in./0.5-2.0 mm range. It’s suggested that their cumulative weight outshines all other animal groups in the ocean plankton worldwide.

Since plankton density in any given patch of ocean will vary with ocean temperature and nutrient mixing due to season temperature variations, it’s estimated that Calanoids can make up 59 to 95 percent of the world’s plankton in any given region. That’s a major reason why planktonic “blooms” lead plankton-feeders like baleen whales to migrate among the world’s oceans.

As copepods go, their significant distinguishing characteristics are that their first antennae are about half their body in length and that they have a distinctive joint between the fifth and sixth body segments.

What are benthic copepods?  The largest group of benthic, or substrate-dwelling, copepods are the 3,000 or so species of Order Harpacticoida. They’re second only to nematodes in their abundance in ocean-floor sediments. In the Arctic, they’re often found in sea ice.

They tend to have relatively worm-like bodies with wide abdomens and a very short pair of first antennae. Typically, with mouth parts adapted for scraping and biting, sediment-dwelling copepods survive on organic detritus and/or the bacteria that grow in the decaying matter.

What are parasitic copepods?  A third lifestyle taken up by many species is the devouring or absorbing the tissues of other animals, either externally or internally. At least three of the known copepod Orders are entirely or nearly all parasitic. It’s said that parasite copepods hosts encompass virtually every animal group, from sponges to whales to human beings. Copepods that specialize  in larger vertebrates, like whales, are likely to be larger, reaching up to 8 in/20 cm.

Researchers working in Palmyra Lagoon in Indonesia studied 849 fish specimens in 44 species and found that 14 of the fish species were parasitized by at least one copepod species and several by two or three copepod species. Copepods that infest fish skin and gills represent serious problems for commercial fish farms.

In a turnaround, copepods are subject to parasites of their own. The best known are 12 species of marine dinoflagellates that inhabit the guts of many copepod species.

Detail from the 1904 catalog of copepods.


Most nonparasitic copepods spend their entire lives as free-swimming denizens of the planktonic soup. Benthic copepods, of course, spend their lives in the sands and muds consuming organic stuff and waiting to be found by hungry rays and fishes.

Copepods make more copepods by sexual reproduction, the male grasping the female with his first set of antennae and transferring his sperm to the female with his thoracic limbs. Some species release the fertilized eggs directly into the water column but the females of many species carry them in a sac on their bodies.

The nauplius larvae that emerges when the eggs are hatched have heads and tails but not thoraxes or real abdomens. Over a period that ranges from a week to a year, depending on species, a series of molts produces a more adult-like larvae, then and adult.


  • Additionally, home aquarium hobbyists (and presumably public aquariums) support a brisk market for copepods as food for their fish populations.
  • The presence of copepods in public water supplies is an issue for some U.S. cities but is generally dealt with by mechanical filtering and/or chemical treatment.
  • The ability of cholera bacteria to stick to adhere to copepod bodies creates a threat of that disease in the untreated water supplies of some tropical countries, such as Bangladesh. The problem can be minimized by filtering the water.
  • Again in untreated tropical water supplies, guinea worm disease can be associated with copepods, since a stage of the worm’s life cycle needs its larvae to develop within a copepod’s digestive tract.
  • On the other hand, some species of copepods have been found effective in eating the larvae of disease-bearing mosquitos and have been used as biological control agents in another tropical country, Vietnam.


Video Sidebar: Fish Bottomfeeding Strategies

PRINCIPAL SOURCES:  A Primer on the Different Characteristics and uses of the Major Copepod Groups, Algae Barn; Copepod Web Portal, luciopesce.net; Can Copepods Cope? Copepoda UConn; Copepod Research, Smithsonian National Museum of Natural History; Copepods: Cows of the Sea, NOAA Fisheries; Harpacticoida, Guide to the Zooplankton of South East Australia; Parasitic copepods (Crustacea, Hexanauplia) on fishes from the lagoon flats of Palmyra Atoll, Central Pacific, ZooKeys; Copepod, Britannica.comCopepod, Harpacticoida, et.al., Wikipedia.

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