Video: Daphnia (Daphnia Magna)
There is no aquarist who is not familiar with daphnia. In the summer, at the Bird Market in Moscow, you can see the sellers in front of whom there are troughs with millions of tiny crustaceans teeming in them. From this huge mass of living beings, the water even acquires a reddish-red color. Well, in the cold season, Daphnia is sold dried. But it would be unfair to speak of daphnia only as food for fish. Indeed, from the point of view of biology, they are one of the most perfect representatives of freshwater plankton, creatures ideally adapted to "soar" in the water column.
So, daphnia are cladocerans from the genus Daphnia. Only in Europe there are more than a dozen of them, and therefore in one jar with "live food" there may well be several species at once. All of them are very similar in basic features. Like any planktonic organisms, daphnia spend their whole life as if in a suspended state. However, "hanging" in the water is not easy even for these tiny animals. No matter how light daphnia is, its weight is always a little more than the buoyant force that acts on the crustacean according to Archimedes' law. That is why, in stagnant water, dead daphnia, although slowly, goes to the bottom.
Great Daphnia (Daphnia magna)
A live crustacean is quite another matter. Its long, forked antennae are covered with pubescent bristles and act as a parachute that slows down the fall. As soon as the daphnia wave these antennae, it jumps up and returns to its original position. By regulating the frequency of the antennae flapping, daphnia can not only "soar", but also rise into the upper layers of the water, or, conversely, go to a depth. Thus, they make vertical movements (migrations) associated with the search for food, changes in water temperature or time of day.
Perhaps the elusiveness of the fast-jumping crustaceans reminded scientists of the legend of the nymph Daphne almost overtaken by Apollo, but never caught by him? Or maybe the mustache of the crustaceans seemed to someone like the branches of an evergreen laurel, into which a beautiful nymph turned.
Related article Water Fleas (Daphnia and Moina)
Овидий в поэме «Метаморфозы» рассказал, как однажды златокудрый бог света Аполлон неосторожно посмеялся над сыном Афродиты Эротом (или, как его еще называли греки, Эросом). Обидевшийся бог любви из золотого лука поразил среброликого покровителя муз в самое сердце. Встретив как-то красавицу Дафну, дочь речного бога Пенея, Аполлон влюбился в нее с первого взгляда, но прекрасная нимфа, которую Эрос поразил стрелой, убивающей любовь, пустилась бежать от него с быстротою ветра. Погнался тогда за ней Аполлон, но нимфа лишь быстрее и быстрее бежала от прекрасного бога. Когда же силы ее стали иссякать, начала Дафна умолять своего отца лишить ее облика, доставляющего ей только горе. Сжалился над дочерью старый Пеней. И в тот миг, когда, казалось, Аполлон уже догнал красавицу, превратилась она в лавровое дерево.
Saddened Apollo did not want to part with his beloved. He decorated his quiver and cithara with laurel leaves, and placed a wreath of laurel branches on his head, the aroma of which always reminded him of the elusive Daphne.
Let's return, however, to our crustaceans.
Luxurious antennae are not the only external feature of daphnia. With the exception of the head, the entire body of these "water fleas" is enclosed in a transparent bivalve shell made of thin chitin, which daphnia periodically sheds and replaces with a new one. Such molts come in handy for daphnia. The fact is that in the hot summer months, the water in reservoirs warms up strongly and, according to the laws of physics, its specific gravity decreases. Accordingly, the buoyancy force acting on the daphnia also decreases. Crustaceans "get heavy" and, in order not to drown, they have to flap their antennae much more often, wasting energy.
Daphnias "solved" this problem just brilliantly. As the water temperature rises, they become larger with each new molt: spines, sharp processes appear on the shells, a pointed "helmet" appears above the rounded head. As a result, with almost unchanged weight, the dimensions of the crustacean increase, and the specific gravity decreases. Daphnias "lighten".
By autumn, everything is repeated in the reverse order: the cooling water becomes heavier, and daphnia, shedding, take on their original shape. Now, it is probably worth telling what our "heroine" looks like under a microscope. There is also something to be surprised at.
The head of the Daphnia is extended in front into a sharp "beak" or "sock", which makes its owner very pretty. True, she has only one eye, but this is the fate of all cladocerans. But it consists of at least two dozen simple eyes, and this is quite enough to detect an accumulation of bacteria or algae - a common food for daphnia.
When daphnia approaches food objects, the abdominal legs of the crustacean hidden under the shell come into play. Making hundreds of strokes per minute, some of them drive water under the sink, while others, covered with bristles, filter out of the water what is good for food. Eventually, a small lump is formed from bacteria or algae, which is pushed to the mouth.
With a lack of food, daphnia cease to multiply and die. That is why "live food" is easier to catch in a small, muddy pond or ditch than in a lake with clear water: there are very few bacteria in clean water. But an excess of bacteria is also not to daphnia's liking: microorganisms release so many toxic substances into the water that the life of crustaceans becomes impossible.
Oddly enough, daphnia even have a heart. Contracting several hundred times per minute, it pushes blood first to the head, and then to the gills and the posterior end of the body. According to some observations, the color of daphnia depends on the amount of hemoglobin in the blood: the more pigment, the "redder" the crustacean. Well, the greenish color is caused by the green algae in the intestines.
As a rule, all daphnia caught in a net turn out to be females. This is not surprising, because during the summer crustaceans reproduce parthenogenetically, carrying unfertilized eggs on their backs, in the so-called brood chamber. Daphnia males are rare, usually appear by autumn and are always much smaller than females.
Source: V. Alekseev, I. Danilevskikh, magazine "Aquariumist", 1993 - 6