Feeding mechanisms in zooplankton

Strange numbers and viscous media

Who doesn’t remember the famous movie “Fantastic voyage” 1966, starring the iconic Raquel Welch and Stephen Boyd? The plot is about a scientist from the East who falls into a coma because of an attack. The only way to save this character, who keeps tremendously important secrets in his mind, is to miniaturize a submarine to the size of a little more than a blood cell and get inside his body to get to the brain and remove a life-threatening clot. You may also remember that the submarine used a propeller to move, as with most water vehicles. However, a submarine of this size would not advance even a couple of millimeters using a propeller. Why? Because of a magic number called a Reynolds number.

No, I didn’t go bananas. Reynolds number is a mathematical concept separating fluids with laminar regimes from turbulent ones. In other words, it describes the relationship between viscous and inertial forces. Tiny particles have low Reynolds numbers and are moving in the viscous world, where the water acquires a density similar to honey or syrup. Large particles have high numbers, which correspond to the inertial world, which we all know, and where water is fluid and slips between our fingers. What are the implications of this? The “Fantastic voyage’s” submarine (with low Reynolds) would make little use of a propeller to move forward because it would turn and turn going nowhere. To move, it would benefit from other methods, such as a long flagellum or, why not, thousands of cilia. Within zooplankton, we find organisms that live in the viscous world (e.g., protozoa) and others in the inertial one (salps, jellyfish, etc.), or even some that straddle the two worlds, such as the copepods. Being in one or the other of the two worlds will mark the different feeding strategies.

Oithona and Clausocalanus are two genera of copepods with different feeding mechanisms. Images Albert Calbet

Prey, where are you?

But before I go into detail to explain how zooplankton feeds, I would like to mention a small fact that all members of zooplankton share; all of them have to get their food in a highly dilute environment. For example, to eat a ciliate (the equivalent in size to an apple for us, and found at an abundance of less than one individual per milliliter in our waters), a copepod would have to scan at least 1000 mm3. It doesn’t seem like much, but if we think that a copepod is only a millimeter long, it means that it has to scan and travel at about ten times its length in any direction. Imagine that to eat an apple you had to seek into a sphere of 10-20 meters diameter. If you had to do it with closed eyes (copepods don’t use sight to locate prey), you’d probably be starving. Fortunately, copepods, and other members of zooplankton, have different mechanisms for detecting and attracting prey.

Zooplankton feeding mechanisms

Feeding by diffusion of inorganic or organic solutes

Many members of the unicellular zooplankton, especially mixotrophs, can feed like algae, absorbing dissolved nutrients into the cell. Diffusion feeding is usually linked, though it is not a necessary condition, to motility to break gradients and prevent solutes from depleting around the cell. There are organisms such as Karlodinium veneficum that can incorporate solutes through their membrane and eat prey simultaneously.

The dinoflagellate Karlodinium veneficum can feed by spreading dissolved substances and capturing live prey.Image Albert Calbet

Passive ambush feeding

Jellyfish, ctenophores, and some protozoa, such as radiolarians and foraminifera, keep adrift on the lookout for prey. If you want to see a video of a foraminifera eating a copepod and other videos of protozoa’s prey capture, here is the link of a previous post: https://planktonocean.com/2020/10/05/microzooplankton- terrible-predators-of-the-oceans-the-movies.

These types of feeding are usually accompanied by toxic structures responsible for immobilizing the prey and by other capture mechanisms, such as sticky pseudopods, facilitating the attraction of the prey to the mouth/vacuole. A particular case is that of some pteropods (tiny planktonic snails) that, like spiders, segregate nets of adhesive mucus to collect prey.

Cladocerans and doliolids are filter-feeders. Images Albert Calbet

Active ambush feeding

Some ciliated species, such as Didinium nasutum or Mesodinium rubrum, many copepods, such as the ubiquitous Oithona spp., and also chaetognaths, among others, are some of the members of this group of organisms. They all share a capture mechanism similar to that of large felines in the savannah or the jungle: they await on the lookout for prey, and when they locate one, they jump over it. However, the difference between lions and zooplankton (apart from size and fur) is that the former are based on sight to detect prey and the latter have mechanosensitive structures that guide them towards them. At long distances, also specific chemical receptors (similar to the olfactory ones of large terrestrial predators) can guide zooplankton to the most favorable direction to find prey.

Filter-feeders and cruisers

Appendicularians, salps, doliolids, cladocerans, and some choanoflagellates use water currents to attract small prey, such as bacteria and tiny phytoplankton, to highly efficient filter structures. Many crustaceans, such as barnacles, some calanoid copepods, and Krill, also create feeding currents to attract prey. They do so without these currents involving a displacement of the organism. In contrast, other species of copepods and protozoa also create feeding currents but at the same time use them to move (cruising feeding). 

The particular case of copepods is quite curious because it was believed they were filter-filters until early eighties. We now know that they are suspensivorous and manipulate prey one by one or in small groups in an active way. Dr. Alcaraz and collaborators, in 1980, were the first to describe the mechanism by which copepods attract, manipulate, capture, and sometimes even taste, and reject prey. They do all this with the help of mouth appendages called maxillipeds. The first movies of the mechanism were made with a Eucalanus crassus attached by the thorax to a dog’s hair (the pet of a member of the group) with Crazy glue. These movies are on the website of one of the article’s co-authors: (http://www.planktonsafari.net/video-archive).

Foraminifera feeding on two copepods it has captured. Image Albert Calbet

As you can see, the ways by which zooplankton detect and capture prey can be very diverse and are not exclusive of any group. Evolutionary convergences to either mechanism are common. If you want a more scientific view on the subject, I recommend an article by Professor Thomas Kiørboe in 2011, which I quote below.


Alcaraz, M., Paffenhöfer, G.-A., and Strickler, R. (1980). Catching the algae: a first account of visual observations on filter-feeding calanoids. Evolution and ecology of zooplankton communities, ed. W.C. Kerfoot. (New England: University Press), 241-248.

Kiørboe, T. (2011). How zooplankton feed: mechanisms, traits and trade-offs. Biol. Rev. Camb. Philos. I am. 86, 311-339.

Holoplankton and meroplankton: two strange names to describe ordinary creatures

We, biologists, tend to use Greek or Latin roots when choosing a name. This habit leads to some very curious and strange cases, such as the two that concern us today, holoplankton and meroplankton. From previous posts, we already know that plankton refers to something that it is drifting. But what do the two last prefixes mean? Both “holo” and “mero” come from Greek; the first means “complete” and the second “part of”. No, this does not mean that some are whole beasts and others just factions. This actually means that a holoplankton organism spends its entire life cycle on plankton and that a meroplankton organism spends only part of it.

Members of the holoplankton. From left to right, the copepod Centropages typicus, the cladoceran Penilia avirostris, and the tintinnid ciliate Favella sp.

We know of holoplankton organisms from previous posts: copepods, cladocerans, many protozoa, some species of jellyfish (such as Pelagia noctiluca), salps, etc. But meroplankton may sound more unfamiliar to us. But this is not entirely true. In fact, we see many meroplankton organisms in the fish market, and even when we go to the beach. Starfish, sea urchins, crabs, and sea snails have larval stages in the plankton. Some of these larvae are very strange and do not remind their adult representatives.

Members of meroplankton. Left, a larval stage of a brittle star. Right, a crab larva

Others bear a closer resemblance between adults and larvae. For example, many fish species spend part of their larval life in plankton. A very curious case in fish is that of flatfish, such as turbot and sole, which I explain in a previous post. Many jellyfish also reproduce by strobilation from benthic polyp stages. Even some protists, such as many dinoflagellates that produce harmful algal blooms (the so-called red tides), can encyst and spend much of their life cycle in sediment outside of plankton.

As you can see, there are many meroplankton organisms, and they are well known. Holoplankton are also numerous; remember that copepods are probably the most abundant multicellular animals on the planet and that there are viruses, bacteria, and protists in the holoplankton’s basket. 


Itchy jellyfish, appendicularians with luxury chalets, filtering barrels, and other extraordinary creatures

Jellyfish is perhaps the most popular plankton group, although I am not sure everybody would classify it as plankton. Jellyfish clusters together with other organisms to form what we call gelatinous plankton. In this hodgepodge, we can find chordates, such as appendicularians or salps, some fish eggs, jellyfish, and even some seaweed. Here, however, we will focus on those groups of metazoan zooplankton whose body structure is made up of some jelly. The main groups are appendiculars, pyrosomes, salps, doliolids, jellyfish, and ctenophores.


The appendicularians are no more than a few millimeters long and are shaped like a tadpole. Although they seem very simple and little evolved, these plankton animals, like other tunicates of (salps, doliolids, and pyrosomes), are of those nearest to us evolutionarily speaking (outside, of course, of fish larvae). They are chordates (they have a notochord or nerve dorsal cord), and with a bit of imagination, one could find some resemblance to them in the early stages of an embryo. But the most fantastic thing about these creatures is not this fact, but the house where they live. A few times a day, the appendicularians build a gelatinous house that is a highly efficient filtration device, allowing them to filter particles of thousandths of a millimeter. The houses are usually less than a cm long, and the appendicularians that live inside, with rhythmic beats of the tail, create water currents to swim while attracting food (small protists and bacteria); with this system, they can filter a few liters of water a day.


Salps and doliolids

Although they are different groups, salps and doliolids share many characteristics, and I will explain them together. Both are tunicates, like the appendicularians and filter-feeders. They look more or less like a barrel and can often be mistaken for a piece of clear plastic. They range from a few millimeters to a few centimeters in length. We often find them alone, but they can form colonies of several meters long. They inhabit virtually all seas and oceans, but in Antarctica, particularly salps, they play a key role in the ecosystem, consuming and packaging the algae that proliferate in the spring. Actually, in the Southern Ocean, they share relevance with the famous krill, to the point of talking about years of salps or years of krill.



Pyrosomes are one of the rarest and most difficult groups to find in gelatinous plankton. They are also tunicated, and consist of colonies of clonal organisms of a few millimeters, which joined by a gelatinous matrix can form structures from a few centimeters to about 20 meters long, always in the shape of a wind sleeve. All the individuals in the colony collaborate to swim and seek food. We can find them in surface layers of tropical seas, but also at great depths. As if all this were not enough, they are also bio-luminescent.


Everyone knows jellyfish. What not everyone may know is that they are close relatives of corals. In fact, many species during their life cycle go through a polyp phase (such as those of corals), sessile, from which small jellyfish (ephyra) will emerge (strobilize). There are many types, shapes and colors of jellyfish. According to the taxonomy, we have 4 classes: schifozoa, cubozoa, hidrozoa and staurozoa. The schifozoa would be the real jellyfish, with their umbrella (umbrella) and tentacles. Many of them sting with the action of stinging cells called cnidocysts. By the way, never touch a jellyfish; even if it is dead on the beach, its cnidocytes could still be functional. The Cubozoa, cubic in shape, as the name implies, and small but very dangerous. Many Australian box jellyfish, despite being a few inches long, can be deadly. A species of box jellyfish has been described in the Mediterranean, but while it can do a lot of harm, it is not as dangerous as those in Australia. Hydrozoans are colonial organisms, Portuguese man of war or tiny blue jellyfish that often invade the beaches of the Catalan coast are examples. One of the individuals in the colony acquires buoyancy and becomes the nectophore, others engage in reproductive functions, and others hunt and digest prey. A complicated colony where everyone has their role. Finally, we have the Staurozoa, not very common, small in size, and sessile.

Jellyfish ephyra

The presence of jellyfish on the Catalan coasts has long been a topic of research and debate. The main question is predicting whether a year will be jellyfish or not? Well, the thing is not simple, because it depends on many factors. In Catalonia, perhaps the most common jellyfish in summer is the Pelagia noctiluca, which has a life cycle without a sessile phase, reproducing in the open sea, far from the coast. This suggests that their presence will depend on currents and whether or not open seawater can reach the coast. On the Catalan coast, these water exchanges between the open sea and the coast rely on a density front (like a water barrier) formed parallel to the shore by the currents that flow through it. This front is stronger or smaller depending on the freshwater inputs from the rivers. Years of heavy rainfall make the differences in salinity on either side of the front strengthen, and the front prevents jellyfish from entering coastal areas from the open sea. Drier years, or those in which, for whatever reason, less freshwater reaches the sea, make the density front weak and the jellyfish reach the beach. This is one explanation, but I guess there are others, and the presence of jellyfish on our coasts depends on many factors. 

In our country, fortunately, jellyfish only represent a problem for tourism and certain water activities. However, there are places, such as the seas of Japan, where much more problematic species have proliferated, as they affect fishing activities. Nomura jellyfish, which have recently increased their abundance in these waters, can be more than two meters in diameter. There are so many that they often collapse fishing nets and the whole catch has to be thrown back to the sea. In addition, they compete with fish for food (zooplankton) when they are small and eat fish when they are grown up. Not a very good picture, indeed.



Ctenophores are similar to jellyfish, but apart from not biting (or not much), they have other characteristics that differentiate them. To begin with, its locomotion is due to eight bands of ciliated combs that beat together to generate displacement. They are bioluminescent and of various shapes, predominantly spherical with or without tentacles or ribbon-shaped. Perhaps the best-known species is Mnemiopsis leidyi, an invasive species that can wipe out fisheries wherever it is introduced. The crudest example is the Black Sea, where the species was accidentally introduced in the 1980s. In a decade, M. leidyi reached about 400 individuals per cubic meter and decimated local fish species. To remedy the ecological (and economic) disaster, another ctenophore, Beroe ovata, was introduced, which preys on M. leidyi. It seems to be working, and although M. leidyi has not been completely eradicated from the Black Sea, its abundances are under control. The problem is that for some years now, we have M. leidyi in the Mediterranean.


After this review, one wonders about the usefulness of grouping such different beasts into what we call gelatinous plankton. The life, food, evolution, and ecology strategies of each group we’ve seen are so different that this unification may not make much sense. Of course, they all look like jelly desserts.

Phronima: a plankton organism that came to Hollywood

Today, I want to introduce you to some small crustaceans, the Phronima. They belong to the order Amphipods and have a rather curious life; in addition, they are very famous, but we should not advance events.

The Phronima do not exceed four or five centimeters (a couple of inches), have long legs, very thick compound eyes, and are semi-transparent, with some red spots. So far, very common for a crustacean. What makes Phronima unique is its life strategy, as it swims across the deep ocean until it finds a salp, a doliolid, or any other gelatinous planktonic tunicate — free-living gelatinous, barrel-shaped and semi-transparent organisms-. Once it finds a suitable one, it moves in, like someone moving into a summer house.

Painting of Phronima sedentaria. Author Miquel Alcaraz

Equipped with terrible claws, the Phronima cuts the inside of its guests to leave an empty barrel structure. Although the final form bears little resemblance to the original host, it still keeps some cells alive. The Phronima then navigates the sea from the inside, feeding when it finds suitable prey. Its transparent shed serves as protection and to lay the nearly 600 eggs a female can produce. The eggs hatch in this sort of nursery and develop inside it until they reach pre-maturity when they leave the house and seek life in the oceans.

Very curious indeed. However, I’m sure you’re wondering why I said Phronima are famous. Well, it is said that the terrifying creature that killed almost the entire crew of the Nostromo (except Lieutenant Ellen Ripley and her cat, Jonesy) from Ridley Scott’s Alien (1979) was inspired in a Phronima. If you look closely, you will see the similarities.

Picture of Phronima from Solvin Zankl. Alien from the movie, 20th Century Fox.

When the eyes do not stay in place: the peculiar case of sole larvae

I bet you know most fishes have larval stages belonging to the plankton (actually, to the ichthyoplankton). This is not surprising, considering many marine organisms pass part of their live in the plankton (we call this group meroplankton). What is indeed surprising is the peculiar behavior of some fish larvae. An extreme and very characteristic case is the development of the sole, and that of many other flat fish. This fish usually rests in the ocean’s bottom, and because of that, it developed a physiological adaptation: having both eyes on the same side of the face. By doing so, it can be aware in 3D of all what is on top of it. Well, I guess it would be also rather inconvenient having an eye facing the bottom and full of sand all the time. Its larval stages, however, have one eye on each side of the face. This is because they are planktonic and need a 3D view of what it is in front and on the sides. During larval development, one eye migrates from one side of the face to the other one. When both eyes are on the same side of the face, the fish, even if still tiny, adopt a benthic behavior. In the picture below, you can see the eye migration progress along the different larval stages. Cool, isn’t?

Larval developement of several species of flat fish showing the migration of the eye

Why are there no insects in the sea?

Before I start this post, I would like to clarify that I am not an entomologist, so I apologize if I say anything very wrong; I hope not. Although I am sure many of you, indeed connoisseurs of the subject, are already thinking, “there are insects in the sea”. Certainly, several species of Halobates live on the surface of the ocean, and some other insects in interstitial areas of beaches, but they are quite rare and do not go into the sea depths. The fact I want to point out here is that compared to the richness and abundance of insects on land, it is surprising that there are virtually none in the sea. The reasons can be diverse, and theories are not lacking:

To start with, the insect’s respiratory system is aerial and does not allow the exchange of gases in the water. This, however, has been solved by some beetles, or larval stages of dragonflies and mosquitoes, to name a few examples, which live in lakes and rivers. Then, what happens at sea? It has been speculated that insects, being aerial, would not have the ability to migrate to deep areas of the ocean during the day to avoid predation, as do groups of similar size to the sea.

Insects are everywhere on land

We also have evolutionary reasons for the absence of major numbers of insects at sea; It is believed that insects evolved from crustaceans more than 400 million years ago and that their evolution was closely linked to that of plants. For example, winged groups such as butterflies, beetles, and bees have a parallel evolution to the appearance of flowers. There are very few flowering plants in the sea, which would explain the reason for the lack of many groups of insects there.

Finally, it is important to consider not only the group itself but its functions in the ecosystem. Insects include herbivores, parasites, decomposers, etc. These functions at sea are conducted, among others, by a group of small crustaceans that is an old friend of those who follow my blog, the copepods. Copepods are large herbivores, acting together with worms and other organisms as decomposers, and there are many parasites of fish, mollusks, and other marine animals. They perform their functions with exquisite meticulousness and efficiency and have no rival in abundance or biomass within the world of metazoans.

At sea, it seems copepods are doing insects’ roles

The evolutionary origin of copepods is widely debated, as there are very few copepod fossils, but recent evidence indicates that its origin was found in the Cambrian, about 500 million years ago. It is very difficult, then, for a group like insects that spread ashore to return to the sea millennia later and take the place of the already well-settled copepods in the ecosystem, although rarer things are they have seen (whales, seals, turtles, etc.). Looking closely, also seeing the evolutionary success of copepods, perhaps we should rethink the question that entitled this post to “Why are there no copepods on land?”

PLASTIC: You are plankton and to plankton you shall return

Today, I’m going to talk about public enemy number 1, plastic; and more specifically, I’ll tell you about their interaction with marine plankton.

Plastic, from the Greek plastikos: which can be shaped, is a word that includes a multitude of products of different origin and chemical composition. Some are more or less natural, but most are purely artificial. What you may not know is that many plastics are made from petroleum products, which originated from plankton millions of years ago.

Imagine copepods were made up of plastic ;–)

Is plastic as bad as they want us to believe?

Well, honestly, plastic isn’t to blame for who it is at all. In fact, it’s one of the most revolutionary inventions of the last few centuries, along with Sunday afternoon movies and popcorn. What is bad is the abusive and irrational use we make of it. Most plastics are made to last, and we give them a single-use. And the worst thing is that most times, once used, they end up where so many things end up, the sea. There, plastics begin a process of degradation, usually quite slow, depending on their chemical composition and environmental conditions. For example, a plastic bottle can take about 500 years to degrade completely. However, from around 1860, when the first plastic was produced (a billiard ball, by the way) until today, many of the containers and plastic materials that have ended up in the sea have decomposed into small particles (microplastics) of a few microns (thousandths of a millimeter) that are now suspended in the water column in greater or lesser concentration or have ended up in the sediments. Moreover, our daily life generates a lot of plastic fibers and microparticles. For example, in every wash of a washing machine, which you do on Sundays if you are single or almost every day if you have children, more than 700,000 particles are released, most of which will surely end up in the sea. I should say here that water treatment plants cannot cope efficiently with microplastics. Just in case you still need more examples: many cosmetics still carry microplastics in their formula, microscopic fragments of tires that come off when you circulate with your car, by-products of industrial activity, etc., are also other of the multiple origins of marine microplastics.

Why are microplastics important?

The most serious problem with microplastics is that they end up entering the marine food web, either though plankton or through fish, among other organisms. This is because these particles are within the size range of plankton or fish prey. For example, 60% of the sardines and anchovies we consume in Catalonia (NW Mediterranean) have a piece of plastic incorporated into their digestive tract. But don’t suffer. We probably ingest more plastic every time we get into our brand-new car (mostly made of plastic in its interior) than eating 1 Kg of sardines. In fact, we ingest about 5 grams of plastic a week from different origins, which is about the equivalent of a cookie (a plastic one; yummy!). The same thing that happens to sardines happens to zooplankton. They sometimes confuse microplastics with prey and ingest them. Laboratory experiments show that both copepods and protozoa consume plastics when they are in high concentrations. Fortunately, the present concentrations of these particles in the sea are still quite low, so the problem does not seem that serious. Yet, if we consider that every year about 8 million tons of plastic enter the sea (i.e. about 500 Eiffel Tower) we can think that in a not so distant future the concentration of microplastics in water can represent a real threat, even for plankton.

Are microplastics harmful?

Usually, the ingested plastic particles pass through the digestive tracts of the organisms and the thing does not go further than a certain degree of constipation. However, some products used to produce plastics (plasticizers and other additives) are toxic and certain plastics have an affinity for pollutant compounds (such as hydrocarbons) and accumulate them. Toxicity of microplastics is being studied in laboratory experiments, but a field approach, with much more precise analytical techniques than current ones, would be needed to properly assess whether we face a real hazard. 

In short, if you use plastic, reuse it and at the end of its useful life, recycle it properly. Together, we should try to ensure that our sea does not end up being a sea of ​​plastic.

Sapphirina, an astonishing show of light and color

Today I present to you a copepod that has been called the most beautiful animal in the world, the Sapphirina. Sapphirins are parasites of gelatinous plankton, especially doliolids, into which they enter and devour. While this is terrifying and would make them worthy of a post within this blog, this copepod also has other peculiarities that make it unique within plankton and even within the animal kingdom. To start with, its segmented body is very depressed, thin as a sheet of paper, and goes from being completely transparent to showing iridescences of all the colors of the rainbow. In fact, the name Sapphirina comes from sapphire, which is why they have also been called sea sapphires. The changing colors of their body are achieved thanks to hexagonal crystals of guanine, a chemical compound that most cells use to make the fundamental elements of DNA and RNA. Guanine crystals are also used by other animals, such as chameleons to change color, but in Sapphirines, these changes occur in seconds and are of astonishing intensity and beauty. The role of this iridescence is still a matter of debate in the scientific community. The most widely accepted hypothesis is that since only males can change color and females have more developed eyes, this is a mechanism for attracting the mate. In the last post (The Sexual Life of Copepods) I talked to you about how copepod males detect and trap females to reproduce. Here, it seems to be the female who is attracted to the male, but who would not be attracted to such a splendid light and color show?

Sapphirina sp

The sexual life of copepods

For those of you who haven’t read my post “Copepods: Good things come in small packages”, I recommend you do so before you delve into the exciting reading of this new one. In any case, for the laziest, copepods are small crustaceans of vital importance in marine food webs, as they are the food of many fish species and are key consumers of primary producers and protozoa. Besides, they are possibly the most abundant group of multicellular animals on the planet. That said, I do not doubt that it is important to talk about how these creatures reproduce. To begin with, they are organisms with two different sexes, which need copulation to reproduce. Reproduction is as eggs that are released into the sea or carried over (at the base of the abdomen) until the larvae, called nauplii, break the shell and emerge. So far, it seems easy. The problem lies first in finding a suitable partner, of the same species and opposite sex. If we consider that in the first 1,000 meters (where we can find copepods) of the world’s oceans there are about 361 million km3of water, and that a copepod on average is not much longer than a millimeter and has no eyes, one can think about the difficulty involved in finding the half orange. Quite a challenge! Evolution, however, is wise and has provided copepods with strategies to help them in this arduous task. Depending on the species, two strategies can be used to find a mate: pheromones or swimming patterns. 


Pheromones are chemicals with different functions in living things, including reproduction. Copepod males are no exception, and many species rely on locating these substances released by the female. This is indeed not easy, because pheromones are short-lived and any small turbulence can dilute and make the signal disappear. Copepods locate and identify the pheromone molecules of females of their species with special receptors in their antennae. Once a chemical signal is positively identified, they follow the path taken by the female until they find her. 

Pheromones are chemicals with different functions in living things, including reproduction. Copepod males are no exception, and many species rely on locating these substances released by the female. This is indeed not easy, because pheromones are short-lived and any small turbulence can dilute and make the signal disappear. Copepods locate and identify the pheromone molecules of females of their species with special receptors in their antennae. Once a chemical signal is positively identified, they follow the path taken by the female until they find her. 

Swimming patterns

Other species have a more curious system for identifying suitable females. Males swim constantly until they detect hydromechanically (also by receptors in the antennae) another copepod. Then they perform a kind of ritual dance based on small boats of a certain length and frequency. If the other copepod follows the rhythm, it means that she is the right female and can start copulation, otherwise he has to start again and keep looking. This system of locating females is less efficient than that of pheromones and occurs only in species that have high abundances in the marine environment and usually with similar proportions of males and females. 

What happens when you find your perfect match? 

Once the couple has been identified, copulation must take place. In crustaceans, this is not simple. In the case of copepods, males catch and hold the female with a modified antenna for this purpose, usually more muscular than the other. When the female is immobilized, they use the last pair of legs of the thorax (the fifth pair), which is also very modified and is different in each species, to stick a packet of sperm at the genital orifice of the female. An act of juggling! That being said, we can only hope that the end will be happy and that a few days later we will have a new lay of fertilized eggs.

Plankton, mussels with french fries, and diarrhea

The Belgians have the culinary tradition of eating mussels with French fries; it is all a matter of taste. In Catalonia we serve them alone, in sauce, or in paellas or other equally delicious dishes. Mussels, apart from being a gastronomic delight, available to everyone, are incredibly efficient filtering machines. In their daily activity, they can filter and clear of plankton near 200 liters of seawater per day. And this is where the problems begin. Plankton are made up of a multitude of organisms either plants (phytoplankton) or animals (zooplankton) that rarely pose any danger to humans (I encourage you to consult the post a teaspoon of seawater, a miniature ecosystem). However, certain groups of phytoplankton are toxic. These include many dinoflagellated algae and a few species of diatoms and cyanobacteria. At the concentrations we find, nothing happens if we accidentally swallow a sip of seawater. However, if they are concentrated by a filter-feeder, such as mussels or oysters, or if they have been climbing the food web and bio-accumulated in fish, they can be harmful to humans, and even cause death. 

What are the so-called red tides? They are nothing more than accumulations of unicellular algae, usually toxic, in a certain area, of a greater or lesser extent. Many of these algal blooms indeed have a reddish coloration, but there are several other colors, such as brown or green, which makes it strange to call them red tides when the water is, for example, green. Therefore, to be more precise, scientists have long adopted the term harmful algal blooms (HABs). These proliferations differ from the typical spring blooms (see post the four seasons of plankton) because they usually adversely affect the ecosystem or at least one or another organism, including us. 

Gymnodinium impudicum, a close relative of the very toxic dinoflagellate G. catenatum. Picture Albert Calbet and José Manuel Fortuño

Why do HABs occur? Well, unfortunately, their origin is not clear. Whether by physical accumulation in semi-confined areas, such as ports or estuaries, promoted by sea currents, by exaggerated growth of some species that can suddenly take advantage of the opportunity of ideal conditions, or by the release of new cells from cysts accumulated in the sediment, the fact is that more or less every summer (and even some winter) we have on our shores some of these proliferations. What is certain is that human activity contributes to the increase of these phenomena. On the one hand, global warming is speeding up its growth rates. On the other hand, marinas act as centers of aggregation and initial seed for proliferations. The release into the sea of ​​excessive nutrients from city sewage, agricultural activity, etc. gives the food to these algae and also promotes HABs. All in all, we are helping to create growing broths suitable for HABs. Fortunately, some of these issues (such as sewer efficiency and control over agricultural fertilizer) are currently being addressed and the results are promising.

How do they affect marine and non-marine life? There are many types of toxic algae and toxins. Some directly attack mollusks and fish (among other organisms), causing massive deaths in nature and aquaculture facilities. Others, such as the diatom Pseudonitzschia , which produces a potent neurotoxin (domoic acid), can drive animals crazy; Remember Alfred Hitchcock’s “The Birds?” It is believed that the famous director was based on a genuine case of seagull poisoning by this diatom after eating contaminated fish. There are also cases of sea lions affected by this toxin that act like zombies. Other HABs affect the food web from its base, acting on zooplankton and other algae species. Some are harmless, and their effects are simply aesthetic and harmful only to tourists looking for crystal clear water on the beaches. The worst, however, from a human point of view, are those that go more or less unnoticed through the food web and that, ultimately, reach us. From the well-known summer diarrhea, vomiting, convulsions, to even death, are the range of effects of these organisms on our species. A rather curious, and unfortunate, case happened in a laboratory in North Carolina in 1993 where two scientists who cultivated Pfiesteria, a toxic dinoflagellate attributed to fish mortality, were progressively showing symptoms of memory loss and disorientation. They never completed their study, and to date, they still have problems in their cognitive abilities. 

Drawing of a HAB on a shore line. Albert Calbet

As you can see, the risks from the proliferation of toxic algae are considerable. Fortunately, regular monitoring programs have been established in places that may be affected by these algal blooms, and especially near aquaculture facilities. Monitoring today comprises a handful of experts who look at water samples under a microscope and identify the usual suspects. Surely, soon this will be done automatically with machines that do real-time molecular or chemical analysis. But for now, we have to trust our experts.