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.
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.
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.
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.