The sexual life of copepods

3 figuresFor 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 egg