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 fact 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 in the form of eggs that are either 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 of all in finding a suitable partner, that is, 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 begin to think about the difficulty involved in finding the half orange. Quite a challenge! Evolution, however, is very 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 

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 start to 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, on the other hand, 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 do not normally 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 as a whole, 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 tale advantage of the opportunity of ideal conditions, or by the release of new cells from of 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 accelerating its growth rates, on the other hand, marinas act as centers of aggregation and initial seed for proliferations. Moreover, the release into the sea of ​​excessive nutrients from city sewage, agricultural activity, etc. gives the necessary 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 true 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 consists of 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.

Global warming and plankton

Today, it is undeniable that the earth is warming at a faster rate than it should due to merely natural causes. From the anthropogenic point of view, that is to say purely human, one might think that nothing happens if the temperature rises a couple of degrees. We could even believe that global warming is positive, because it will not be so cold in winter, and in summer a little more heat is not noticed if you are under the air conditioning. Well, nothing could be further away from the truth. The consequences of a rise in average temperature, even if small, can be catastrophic for our current lifestyle. Extreme droughts, heat waves, more frequent and violent stormy episodes, torrential rains, sea-level rise, changes in species distribution, mass deaths of some animals or plants (remember that plants cannot escape), etc. 

Is there a solution? The short answer is no; however, what we can do is to slow down the temperature rise. In the Paris agreement, signed in 2016 by most United Nations countries (unfortunately some of the most contaminating countries refused to sign), the subscribers to the agreement promised to reduce emissions by a certain percentage of CO2, one of the usual suspects among others of global warming. This gesture was intended to dampen the rise in temperature on the planet. It seems, sadly, that only a few countries are doing what was promised and that the temperature continues to rise. Think that only during the first wave of the SARS-Cov2 pandemic in 2020, when the economy, transport, and industry were reduced to unprecedented levels for two months, was the annual emission reduction required in the Paris agreement. We must therefore prepare for the worst. 

It is important to understand, however, that the climate of the earth cannot be understood without the ocean and the other way around. Water stores heat much better than the soil, and releases it more slowly, which generates daily sea breezes, among other climatic phenomena. Temperature gradients between polar regions and the equator are the engine of a planetary water circulation that transports heat (energy) and nutrients around the planet (ocean circulation). This transport, however, may be affected by variations in temperature and atmospheric circulation. For example, the periodic thawing of the Arctic plate in the spring causes the resulting cold water to sink near the east coast of Greenland generating a displacement of water masses that generates the Gulf Stream and helps move the entire global oceanic circulation (the conveyor belt). In the unlikely event that an increase in global temperature significantly reduces the surface area of ​​the northern polar plaque, cold water resulting from thawing in the spring may not be enough to activate the Gulf Stream. The climatic consequences of such a phenomenon are very difficult to predict. We may not get to the extremes of the movie “The day after tomorrow” where the United States freezes in hours, but that there would be major weather changes is quite certain. 

Schematics of the effects of el Niño in the Peruvian upwelling

Climate change could also affect the periodicity and intensity of upwellings, and have important consequences for the world’s major fisheries and overall productivity for the entire marine ecosystem. Warming of the oceans can lead to variations in the direction and intensity of currents and affect the distribution of marine species. As we said in the post “The rhythms of plankton“, phenomena such as El Niño depend directly on climatic conditions and control the outflow of nutrients, and therefore fisheries, from the west coast of South America. At more local scales, the invasion of new species (a process sometimes aided by the transport of organisms in the ballast water of ships or by the exchange of species in aquaculture), the increase in the frequency and amplitude of harmful algal blooms (also related to other anthropogenic impacts), or the expansion of anoxic zones in the seas and oceans are a small sample of the changes that await us in the immediate future. Not all is lost, though. The species have some plasticity and adapt to changes in temperature, especially if they are gradual. In fact, in the laboratory, it has been shown that both algae and copepods (and I suppose also protozoa), after a period (long, about a year) of genetic adaptation to higher temperatures, end up regulating their metabolic rates and offsetting the effects of temperature. If, for example, we expose an alga to a temperature 5 degrees Celsius above the temperature it normally lives, its respiration rate will exceed its photosynthesis rate. This is because respiration is more sensitive to thermal changes than photosynthesis. In science, the rate at which a process, or metabolic rate, responds to temperature is called Q10 (not to be confounded with coenzyme Q10). Each metabolic activity of each species is associated with a Q10, which is defined as the increase in that rate as the temperature rises by 10ºC. Thus, the Q10 of respiration is higher than that of photosynthesis. However, after many generations in the new temperature conditions, the two rates are returning to their original balance. Then you will ask me why we care so much? The problem is that during this adaptation process, which can last for months and even years, the species in question is in metabolic imbalance and it is not competitive with other better-adapted species. A clear example is a displacement of the copepod Calanus finmarchicus (cold-water) by the Calanus helgolandicus (warmer water) in the North Sea. The first species is very prolific and nutritious, and thanks to it all the cod fishery in the area is maintained. It seems that C. helgolandicusdoes not reproduce so fast and that it is not enough to sustain cod populations, so the collapse of these important fisheries may be a reality soon. The question also reminds regarding the fate of species adapted to high Arctic conditions, such as polar bears and also some copepods, e.g., Calanus hyperboreus.

Calanus hyperboreus, a high Arctic copepod that may disappear after global warming

We also find a similar case in the Mediterranean. Whether it is due to the change of species in plankton or its poorer nutritional quality due to the temperature, it is being seen that there are fewer and fewer sardines and anchovies in the Mediterranean and that these have less nutritious fats. Especially the sardine is quite endangered on our shores (NW Mediterranean). Obviously, to this process we must add the impact of overfishing, making it increasingly difficult for this species to recover if we do nothing to prevent its collapse. 

We find our final example in the tropics, where the inhabiting species are already at the limit of their thermal capacities. Will the animals and plants of these ecosystems survive a temperature rise like the one expected at the end of the century? It’s hard to predict, but surely many will be lost in the way. 

Are there carnivorous plants in the sea?

Whether because of Audrey from Frank Oz’s musical Little Shop of Horrors, or because of The day of the Triffids of John Wyndham’s, or even because of the little venus fly trap plants sold in the flower shops, everybody knows what carnivorous or mixotrophic plants are. You may even have tried to grow one at home, probably with little success. This is because they are plants adapted to very particular environments, usually characterized by acidic soils and very poor in nutrients, high humidity, and a very precise temperature range. But if you keep all this in mind, it’s not that hard to keep a few species at home; I have over twenty different ones on my balcony. 

Due precisely to the environments in which these plants live, they have had to find evolutionary ways that allow them to grow taking advantage of what they had at their disposal. And that’s why these plants extract the nutrients they don’t find in the soil from insects and other critters that they attract and capture with modified leaves in the form of traps of different kinds. However, in environments that deviate minimally from their peculiar requirements they die or are quickly excluded by faster-growing competitors. In fact, we find carnivorous plants in very few places on earth, while non-carnivorous plants, the strictly photosynthetic ones, are everywhere. 

Mesodinium rubrum. Drawing Albert Calbet

But what about the sea? Well, there are a lot of mixotrophic plants in the sea that eat other organisms. What happens is that they are unicellular and invisible without a microscope and that is why they are not so well known. Apart from diatoms (algae with a siliceous skeleton) and very few representatives of other groups, the other planktonic algae can feed on live prey. Can you imagine that almost every plant on Earth was carnivorous? There would be no insects left! At sea, however, the range of prey they have is quite wide. Most constituent mixotrophs (plant-eating organisms capable of eating prey) eat other algae and do so either to obtain inorganic nutrients, such as nitrogen or phosphorus, to replenish the stock of chloroplasts, as a carbon source, or simply to eliminate competition for resources. Many marine mixotrophs, despite being vegetarian, do not despise a good animal prey, whether unicellular or even pluricellular, which immobilize and kill with the help of venom-bathed spears or releasing toxins into the water. In the marine ecosystem, we also find another type of mixotroph that does not exist on earth, apart from some science fiction films and comics, such as “The thing from another world” or “The swamp thing”. They are animals with plant characteristics (non-constitutive mixotrophs). These characteristics are acquired by capturing plant prey to incorporate their chloroplasts (or whole algae) and thus be able to do photosynthesis. Can you imagine a rabbit green as lettuce? There are, in fact, a few marine multicellular animals that have this ability as well; some corals, sponges, worms, or bivalves catch symbiont algae. Even the sea green slug Elysia chloroticacan synthesize a rudimentary chlorophyll. But the kings of non-constitutive mixotrophy in the sea are unicellular. Ciliates, dinoflagellates, foraminifera, radiolarians, etc., are some of the groups capable of capturing and enslaving whole algae or their chloroplasts. And not only that, but there are also those that, in addition to chloroplasts, incorporate the nucleus of the taken cell into their cytoplasm to help them in the duplication process. 

Mixotrophic Acantharida.  Lithoptera sp. Drawing Albert Calbet

An incredibly complex process in a single cell. Some reach impressive levels of specialization, preying only on one species of prey, or even only on one species of mixotroph that has previously captured the chloroplasts from a given prey, such as the dinoflagellates of the genus Dinophysis, which feeds on the ciliate Mesodinium rubrum, which in turn eats and captures chloroplasts of a given group of algae. The reason for this plasticity and ease of incorporation of foreign organelles from many marine protists is probably due to the evolution of the eukaryotic cell in the sea. It is believed that the origin of algae began with a cell without phototrophic capacity that was able to capture and retain an autotrophic bacterium (the first chloroplast). This occurred ca. 1500-2000 million years ago, at the beginning of life on earth. As you can see, despite being a widespread phenomenon, we are still far from understanding all the factors involved in mixotrophy in the sea, because we find that each species is a world, or even that each strain of the same species behaves differently. We are indeed still not entirely sure how relevant mixotrophs are in marine trophic food webs, because having the ability to use a particular metabolic pathway does not necessarily imply its use. 

Process of capture and ingestion of an alga (Rhodomonas salina) by a mixotrophic dinoflagellate (Karlodinium veneficum). Albert Calbet

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

THE FOUR SEASONS OF PLANKTON

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.