Copepods: Good things come in small packages

Imagine a group of organisms more abundant than insects; so abundant that if we place them in a row touching each other by the antennas one could reach the sun and return. That they were the main source of food for fish, to the point that without them there would probably be none left in the ocean. That they were the fastest animals on Earth, and that they carried out the largest animal migrations on the planet every day. It all sounds like science fiction, right? But the truth is that these beings do exist, they are the copepods.

Male of Labidocera sp. Picture Albert Calbet

What are copepods? 

Copepods (from the Greek cope: paddle; poda: leg) are a subclass of aquatic crustaceans that inhabit virtually all seas and oceans. They are small, from less than a millimeter to almost a centimeter, but most are about a millimeter in length. There are so many that they rank as the most abundant group of metazoans on the planet. The species described are around 12,000, and they have certain characteristics that make them unique and essential for the functioning of marine food webs. Most are free-living, but there are many parasitic species of other organisms, such as mollusks, fish, and even cetaceans. The parasites have bizarre shapes, adapted to their function, and completely distant from those of their free-living relatives, which are like small shrimp with a more elongated abdomen and large antennae; although some are flattened or very thin and elongated. 

The fastest animals 

They move by a combined effort between the antennae, 5 pairs of flat legs on the ventral part of the cephalotorax, and the buccal appendages. They can move very fast, up to 3-6 km per hour; it does not seem much, but in relation to its size is equivalent to a thousand times your body in a second. Imagine, for example, jumping over 10 building blocks in a second! These speeds, however, are only reached when hunting or scaping from predators, which they detect with the mechanosensors of their antennae. They are not only fast but persistent in their displacements. Most species daily migrate (between 100 and 1000 meters) in a few hours from the deep ocean, where they remain during the day away from predators, to the surface at night, where they feed on phytoplankton and microzooplankton. 

What do they eat?

Because of their size, copepods move between what physicists call the viscous and the inertial worlds. This means that although they are large enough to perceive water as we do, liquid and fluid, the prey they capture is so small that by the laws of physics their environment becomes much more viscous (e.g. like honey). This makes movement as we humans understand it difficult and it requires particular adaptations, either to move (flagella and cilia) or to eat. Therefore, copepods have special structures near the mouth called mandibles, maxilla, and maxillipeds that help them in capturing and eating prey. The mechanisms they use depend on the species of copepod and the size and motility of the prey. For large prey (such as large protists or other copepods), they ambush; they wait immobile and when they detect a clueless prey, they jump on it and capture it. For small prey, as they move they create currents that attract the particles to the oral cavity, where the appendages I mentioned earlier pick them up and, if appropriate, carry them to the mouth. Until recently, it was believed that the feeding process was simpler, that they filtered the water and ate indiscriminately everything that fell into their mouths. However, research conducted around the 1980s made it clear that the mechanism was more complex, as I just explained.

Monstrillidae copepod. These are parasites in their younger stages. Picture Albert Calbet

Finding the right mate

For two copepods (male and female of the same species) to find each other in the immensity of the sea takes either a lot of luck or some evolutionary mechanism developed for that purpose. Usually, what works best for males is to follow the pheromone tracks left by females. Once the two mates meet, a courtship process begins, based on the odor or rhythmic movement of both sexes, characteristic of each species. Think they have no eyes, just a small ocular patch that serves as a photoreceptor. If there has been luck and the pair is the right one, the male, with the help of a modified antenna and the last leg of the cephalothorax (the fifth leg) will insert a bag loaded with sperm, called spermatophore, in the female’s genital orifice. A whole juggling show! 

A life going from larva to larva 

Once fertilized, the females reproduce by laying eggs, which, depending on the species, are released in the water or are carried until they hatch in a pouch at the base of the abdomen. Depending on the temperature and the species, this process can take more or less time. At 20oC, for example, an egg of Centropages typicus(a species common in the Mediterranean in spring) can take a day or two to hatch. From the egg, it hatches a larva called a nauplius, which will undergo 11 moltings until it becomes an adult (twelfth larval stage). Once a copepod is an adult, it no longer grows and devotes all its energy to eating, escaping from predators, and reproducing. To give you an idea, the whole development process under the above conditions can take about 12-14 days, and then an adult copepod can live for about a month. However, there are species adapted to polar environments that take two years to complete their life cycle. 

Copepod naupli. Acartia grani. Picture Albert Calbet

Why are copepods so important? 

Their relevance is not the result of all the peculiarities of their anatomy and physiology, but of the key role they play in marine trophic food webs. Because they feed on protists (either algae or protozoa) and are the main prey of many fish species (especially in their larval stages) they act as a link between primary producers and fisheries, being the estimates of their abundance and production essential to understand and predict future fishing stocks and regulate fishing effort. There are fisheries, such as the cod one, in the North Sea, which depend exclusively on a single species of copepod, Calanus finmarchicus. Due to global warming, this species of cold water is moving further north and its former distribution areas are being occupied by other species with lower reproduction, and probably different nutritional quality. This fact will surely have repercussions on the future of cod fishing. Copepods are also important as fish food in aquaculture and recreational fish growing. It has been shown that its lipid composition, rich in omega 3 among others, and its nutritional quality are incomparable, and that the survival success of larvae of delicate fish species is much higher with a diet based on copepods. Unfortunately, it is not easy to breed copepods in large numbers. That is why today there are many laboratories researching new methods of cultivation of this exciting group of organisms.

Calanus hyperboreus, an Arctic copepod. Picture Albert Calbet

The rhythms of plankton

The dichotomy between day and night drives most rhythms in nature. The sun activates photosynthesis and with it, thousands of biochemical, physiological, and ethological processes. Humans sleep mostly at night, but many beasts thrive in the dark. During the night we see the moon, which also has its rhythms around 28-29 days. The duration between day and night in the different latitudes of the planet is directed by the seasons, which with greater or lesser accuracy are repeated year after year. In short, we are conditioned by rhythms. From the daily TV news to the winter of the Yogi bear, or from the appearances of the werewolf and other lunatics at the World Mobile in Barcelona (although this year the pace has been broken by the COVID). All of them are rhythms that are repeated with a periodicity. Because plankton could not be less, it is also deeply marked by rhythms of different amplitude and intensity. Here, are a few examples, but I do not intend to be exhaustive, nor do I want to go into too much detail about the mechanisms that trigger them, simply because we do not know many of them. In most cases, either it is an external factor that adjusts the rhythm every day (for example the hours of light) or, due to its periodicity in the evolution of the species, an internal clock has been created that works disregarding the presence or absence of light.

Circadian rhythms 

Phytoplankton (unicellular planktonic algae) actively photosynthesize during the day and breathe at night; this causes many species to take advantage to divide at night. Darkness is also when large zooplankton organisms, such as copepods and krill, migrate from deep, dark areas of the ocean to the surface, to feed on phytoplankton. These movements of organisms are considered the largest migrations on the planet; and they happen every day! By migrating for food at night, zooplankton members prevent their predators, fish, from seeing and attacking them. Copepods also consume microzooplankton (unicellular animals) that are about the same depth as algae. Microzooplankton, to minimize predation by copepods, mostly feeds on algae during the day, when copepods are not present. 

As you can see, everything is in order and balance, mostly because of the millions of years of the joint evolution of predators and prey. 

Circadian rhythm of plankton. Figure Albert Calbet

Circalunar rhythms

In the Arctic night, and supposedly in the Antarctic Ocean as well, the depth at which the zooplankton are located is marked by the illumination of moonlight; the brighter the moon the deeper zooplankton are. This behavior of zooplankton occurs to avoid being consumed by fish adapted to very dim lights. Moon effects similar to those described in the poles, also occur in other oceans, where the depth at which the zooplankton are found at night is modulated by the moonlight. Even eclipses, whether lunar or solar, disrupt the migratory patterns of zooplankton. 

Circalunar rhythm of copepods. Figure Albert Calbet

Circannual rhythms 

The four seasons are another example of periodicity that is more or less accurately repeated every year. As expected, the different weather conditions associated with the length of day and night in each of the seasons mark the dynamics of the marine ecosystem. Synchronous spawning of corals and polychaetes, or whale migrations, are some examples of the thousands of cases we find at sea. However, perhaps the most notable one for its global relevance is the succession of plankton organisms throughout the seasons. This succession, together with the physicochemical characteristics of the water associated with each season, is responsible for the spring phytoplankton bloom. This bloom at its turn will support a flourishing zooplankton community that will serve as food for fish and other marine creatures. 

Multi-year cycles 

We may not be able to strictly call them rhythms, but there are great climatic phenomena that are repeated every few years. The best known is probably “El Niño”, which, although it mainly affects the Pacific Ocean, also has its consequences worldwide. “El Niño”, and its opposite phenomenon “La Niña”, are cyclical variations in temperature that normally occur every 4 years (year up and down) in the central and eastern tropical regions of the Pacific Ocean. Its consequences go beyond changes in temperature and rainfall, as they have a direct effect on the upwelling of the west coast of the America continent. Under “El Niño” conditions, trade winds that normally favor the upwelling of nutrient-rich deep waters reverse their direction and weaken the upwelling. This has consequences for the phytoplankton that feed on these nutrients and climb up the food web to reach fish. All in all, it has very serious socio-economic implications for the area. 

Another similar phenomenon is the North Atlantic Oscillation, which is characterized by a change in pressure between the Azores and the subpolar zone of the North Atlantic. Positive oscillations involve high temperatures in northern Europe and usually the opposite in southern Europe. The Gulf Stream is affected and, with it, a whole set of planktonic species and fish. 

There are many rhythms or cyclical processes as you see in the ocean, each with its idiosyncrasies and characteristics. We may not be aware, but nature always moves at the rhythm of its rhythms.

Carton on what would be the life of a werewolf copepod. Albert Calbet


Vivaldi’s Four Seasons, composed in 1721, are among the most famous classical music works of all time. By listening to each of the concerts, we instinctively move into nature in all its splendor and severity. In the same way, plankton live and vibrates in response to the sound of the seasons in a harmonious way that is repeated year after year. Here, I will give you a short summary of what are these seasons for a standard temperate ecosystem. I hope that the next time you look at the sea, either in summer, fall, winter, or spring, you can imagine how the little creatures that live there interact, relate and struggle to survive. You will notice that in the sea, biology, physics, and chemistry always go hand in hand and that without one we cannot understand the others. I hope that these few lines serve to share with me the need to take care of the fragile and invisible plankton ecosystem, which nourishes us by being the base of the marine food web and gives us half of the oxygen we breathe.

Before I begin, I recommend you to read my first post A teaspoon of seawater, a tiny ecosystem because there, I explain the different components of plankton and their trophic interactions. If you don’t want to, it is also OK, I give you a summary next: 

The basis of the trophic food web in the sea is phytoplankton, a multitude of unicellular algae that by photosynthesis incorporate CO2into their living tissues and produce O2. These tiny, but vital, beings are the food of microzooplankton (unicellular predators), and mesozooplankton (larger multicellular predators), such as copepods. Copepods are the basic food of many adult and fish larvae. 

Having said all that, let’s start with the cycle of the seasons. In order to facilitate comprehension, I have made a small drawing with a summary of the most outstanding facts of each season. We will start with winter when the water is cold and mixed by the wind action, and the light of a sun, which does not rise much above the horizon, penetrates with little intensity the surface of the ocean. The few phytoplankton we find, despite being full of nutrients because the intense mixing of surficial and deep waters, cannot grow much because it is limited by light availability and low temperatures. Towards the end of winter and the beginning of spring, the light is more intense and the temperature begins to rise. The water heats up slowly and a thin thermocline (a layer that separates two bodies of water with different temperatures and densities) develops, which will confine the mixing layer to more superficial areas. These conditions favor the phytoplankton bloom, which will be accompanied by a growth of the populations of microzooplankton first and larger zooplankton (for example, copepods) later. Time passes, the temperature rises, and summer enters. At this time, the already well-formed thermocline clearly separates two areas, like oil on a pan full of water: a shallow one (about 50 m deep in open waters), hot and poor in nutrients because de consumption of those by phytoplankton in spring and beginning of summer, and one deep, cold and full of nutrients. Early in the summer, nutrient-limited phytoplankton depleted by zooplankton consumption give way to a particular, and less numerous, community of algae adapted to these conditions. Summer algae are either small in size, with a high surface-to-volume ratio that facilitates the use of the few nutrients available, or they are large but motile (e.g. dinoflagellates) able explore the micro-patches of nutrients that may remain. These latter algae, the dinoflagellates, under favorable conditions (for example, within confined areas such as harbors and similar) can begin to grow to form harmful algal blooms, formerly misnamed “red tides” (they are neither tides nor often red). Consumers in these summer communities are either microzooplankton or a combination of small particle filter zooplankton, such as some gelatinous organisms or marine cladocerans (water fleas); we also find some carnivorous predators, such as some species of copepods, and some detritivores. When the first autumn storms arrive, and the wind intensity rises, the thermocline breaks down and allows nutrient-rich waters to reach the surface. Sometimes, if the weather conditions of the year allow it, there may be another small bloom, but often low irradiances and temperatures make the phytoplankton unable to take advantage of the fresh nutrient input. Winter is back and the cycle starts again. 

As you can see, life cycles are repeated year after year in plankton as well, and this has led many marine organisms to reach a kind of internal clock that tells them when to lay eggs, for example. With climate change, unfortunately, many of these clocks get out of adjustment, which affects the natural functioning of the ecosystem.  

Zooplankton’s most feared enemy

They are beautiful, they may be smart, we want to protect them, but from the perspective of our tinny friends they are terrifying. The whales!

I hope you enjoy my weekend’s drawing 🙂

An average-sized humpback whale can eat 2000-2500 kg of krill each day during the feeding season in cold waters (about 120 days). And they eat twice a day!

Help me! Kill the whales!

Other major predators are fish, jellyfish and other zooplankton

The most dangerous predators of the ocean, the protists

Mr. Vaughn, what we are dealing with here is a perfect engine, an eating machine. It’s really a miracle of evolution. All this machine does is swim and eat and make little sharks, and that’s all

This is how oceanographer Matt Hooper (Richard Dreyfuss) described the white shark in Steven Spielberg’s 1975 film Jaws. Certainly, when we think of an efficient and terrible marine predator, the shark always comes to our mind, and more specifically the white shark, which is the worst famous. However, if Mr. Spielberg had known of the terrible ways in which marine protists kill and devour their prey, perhaps the film would have been called “The Protist”. Jokes aside, if there are any creepy and dangerous beasts for their congeners in the ocean these are the protists.

Protists are eukaryotic (i.e., with nucleus) unicellular organisms ubiquitous in seas and oceans. We can classify them in various fashions, for example, by their way of obtaining energy: those that do photosynthesis (autotrophs or algae), those that eat other organisms (heterotrophs, also called protozoa) or those that combine both strategies (the mixotrophs). Here we will focus only on those that eat live prey, i.e. protozoa and mixotrophs.

Of the approximately 50 gigatons (50 billion tons) of carbon that algae produce annually in the seas and oceans, protozoa (perhaps with the help of mixotrophs) consume about 30- 60%. We think that the next relevant consumers, the copepods, only eat about 6 gigatons. Protozoa and mixotrophs do not eat only algae, they also feed on bacteria, other protozoa and even some animals much larger than them. But how do these tiny, mouthless unicellular beings eat them? The truth is that they have different prey capture and feeding strategies depending on the group, and they are all very curious.

The main feeding strategies in protists

Filtration: Many microorganisms use whale-like feeding systems, either by attracting prey into the oral orifice or by swimming and collecting prey. This feeding strategy is usually used for very small prey, such as bacteria or flagellates.

Engulfment: When prey begins to gain considerable size, many protists can catch and ingest them whole in a process resembling that of a boa eating a goat. In fact, some protists, such as the dinoflagellate Gyrodinium dominans have a very flexible body (cell) and can ingest chains of diatoms much larger than theirs (Fig. 1). Some foraminifera, distant relatives of amoebas and provided with an outer cover formed by calcium carbonate, can swallow even large copepods. The process is slow but effective (Fig. 2). Many protists use venom-laden stingrays or release toxins into the water to immobilize prey. Some toxins are so efficient that they can kill fish and other organisms, and even once accumulated by filters, such as mussels, they can lead to serious poisoning in humans.

Figure 1. Process of swallowing a chain of diatoms by the dinoflagellate Gyrodinium dominans. The red arrow indicates a G. dominans with a chain of diatoms inside. The blue arrow shows the size of the same species without prey inside. Photo by Albert Calbet


Figure 2. The image shows a foraminifer that has just captured two copepods. Photo by Albert Calbet

Tube or peduncle: Certain dinoflagellates have a retractable tubular structure that they insert into the prey to suck its contents, as if it were a straw on a Margarita Cocktel (Fig. 3). They use this mechanism to eat prey similar in size to theirs, but also to kill and devour, like tiny leeches, animals much larger than themselves, such as copepods, worms, and so on.


Figure 3. Peduncle feeding of a Dinophysis sp. on a myxotrophic ciliate (Mesodinium rubrum). Drawing Albert Calbet

Pallium or veil: This is perhaps the most curious and complex mechanism. Like sea urchins and starfish, they evaginate their stomachs (well, not an actual stomach indeed, but a membrane with digestive characteristics) in order to slowly digest their prey, such as large diatom chains (Fig. 4). Gradually, the trapped cells are consumed and the predator incorporates the dissolved nutrients into the membrane. Once finished, only a siliceous skeleton will remain.


Figure 4. Protoperidinium sp. dragging a chain of diatoms into its pallium. Photo by Albert Calbet

Piston: Not long ago, a very odd dinoflagellate was discovered and named also with a very odd name, Erythropsidinium. It has a small piston that can be expanded and hided very fast. It seems Erythropsidinium uses it to detect and, by a suction mechanism, catch prey that will be eventually swallowed (Fig. 5). The most interesting thing about this unicellular creature is that it also has a kind of primitive eye (ocelloid), with its lens and all. The function of this ocelloid is still subject of debate among the scientific community, but it could be used, in a very rudimentary way, to locate prey. Remember, we’re talking about a single-celled organism!


Figure 5. Representation of an Erythropsidinium showing its ocular lens and piston. Drawing Albert Calbet

Luckily for us, all these creatures, which could be taken from the most terrifying of Stephen King’s books, are no more than a few tens of thousandths of a millimeter. Imagine what would happen if they were our size!

The biological pump and the role of ocean plankton in mitigating global warming

The biological pump is a process by which the ocean, with the help of marine organisms, captures CO2 from the atmosphere and buries it in sediments, where it will remain for hundreds or thousands of years. Through this process, the ocean helps to mitigate the effects of global warming, as it captures and integrates into the living matter the same CO2 as all plants on the planet’s surface. And all this is done mostly by tiny unicellular beings called phytoplankton. Phytoplankton consists of microalgae of a few thousandths of a millimeter, but of great relevance because they are responsible for photosynthesis in marine planktonic trophic food webs.

Schematic representation of the biological pump. One of the many possible pathways has been exemplified. Drawing Albert Calbet

To understand how the biological pump works, imagine for a moment that we are a carbon atom that, together with two oxygen atoms, forms a molecule of CO2, the dreaded byproduct of burning fossil fuels. Maybe we came straight from a car’s exhaust pipe, we came out of the chimney of an industry, or just out of the lungs of our neighbor, no matter what. We, in the form of carbon, fly happily by the proximity of the sea enjoying the view, but one day, we enter into the water through a process called diffusion. In the water we are quickly trapped by a small algae that turns us into living matter with the help of the sun’s energy and some or other inorganic nutrient. Although we feel proud to be part of something bigger and more organized than a simple molecule (we are now part of a sugar chain), our joy does not last long, because a small mixotrophic dinoflagellate swallows us. Within the digestive vacuoles the complex thing we had become disintegrates again into small fractions and is used to create other complex structures. Well, not so bad, now we are part of something even bigger and that makes us happy. However, a ciliate that passed by makes us part of his diet and the digestion process starts over again. But the odds decided that, this time, we did not finish the digestion process because a copepod chews us and we end up in the beast’s stomach. With time and patience (i.e., by catabolic and anabolic processes), we move to a lipid chain that goes to the cephalothorax of the copepod in the form of a drop of fat. Our host migrates to s deeper zone during the day to avoid being consumed by fish, which as we all know are visual predators. At sunset, we ascend to shallower layers, where there are algae and other prey, but along the way, a euphausiacea (krill) attacks us and the copepod of which we were part ends up split in two. The part where we are is not ingested and we are slowly settling to the depths of the ocean — if what had attacked our guest had been a fish or a jellyfish we might still be wandering the food web and our history would be different. On the way to the abyss, bacteria and other microorganisms begin to decompose the remains of the copepod and each time we find ourselves into a smaller piece. Suddenly, there is a adrupt change of speed. Several decomposing particles have been added together and now we fall more quickly stuck to a piece of sea snow. Upon reaching the bottom, after what have seemed days, we still have some chance of being part of the benthic trophic network again by the action of crabs, worms, or other critters. However, whether by chance or because we were in a difficult-to-chew bite, our carbon atom is respected and little by little we go deeper into the sediment, where the lower pH will keep us for years, maybe decades, centuries or even millennia. By this way, that carbon atom that was part of a CO2 molecule has been trapped in the depths of the ocean.

Mixotrophic dinoflagellate. Ceratium sp. Photo by Albert Calbet.
Copepod. Calanus minor. Photo by Albert Calbet

The pathways by which a carbon atom passes from the atmosphere to the ocean floor are unlimited and of very different durations, from a few days, like the one I have represented here, to hundreds or thousands of years, if it never gets there. Of course, in the process, this carbon atom will surely contribute its grain of sand for life to continue.

Microzooplankton, terrible predators of the oceans: The movies!

The two previous videos show different species (Protoperidinium spp.) of dinflagellates pallium feeding on diatom chains. Similar to the seastars, the dinoflagellates evaginate a sort of “stomach ” to slowly digest the poor prey.

Heliozoan feeding movie
Tube feeding of a Lessardia sp.

This snake-like thing that you see swimming all over the place is a Gyrodinium dominans that has engulfed a diatom chain.

Phoraminiphera can look peaceful unicellular guys. However, they are voracious predators. The one on the movie is trying to eat two copepods at the same time!

The plankton and the marine food web

Here, I will post few drawings of plankton and their role in the marine food web. All drawings and pictures belong to me, Albert Calbet. If you want to use any for teaching or non-comercial use let me know. In future blogs I will add some pictures and videos. Enjoy.

This is my representation of the main trophic interactions within a planktonic food web. There are many different ways to represent it and you may add as many compartments as you wish. But I think it is a nice one 😉
Planktonic food webs cannot be understood without nutrient and organic matter recycling. I opted to present it in a separate drawing to avoid excessive complexity.
Pelagia jellyfish
Calanus hyperboreus
Mesodinium rubrum
Lithoptera sp
Oithona davisae
Aurelia aurita
This one is a joke. However, given all pelagic organisms depend on plankton…