The Life of Animals: The Freshwater Flea
By Jean Painlevé
Very frequently an audience to whom one is speaking about the water flea will exclaim: “Oh, yes, I know it well; it’s a little creature which jumps around the edge of the waves at low tide, on wrack and in the sand!” Well no: that is the sandflea, a large creature one or two centimetres in length. The water flea or daphnia lives in freshwater and it grows no bigger than two millimetres. Despite its size, there is much to be said about it, not only because its transparency allows us a clear view of its unexpected structure as well as the complicated phenomena which occur inside it, but also because its existence is rich in bizarre developments and strange outcomes.
This little creature, known particularly by fish enthusiasts who give their fish daphnia in its powdered form as food,[1] is more interesting in its fresh state. They can be collected from the water with a fine-mesh net, from all freshwater, whether stagnant or flowing, large limpid pools or ponds full of foul-smelling slurry. One can reduce the creature’s jerky movements by keeping it in a small amount of water and, with a magnifying glass, or even better a microscope, observe that the leaping which makes it resemble a flea and gave it its name comes from violent blows in the water made by a pair of legs called antennae. These legs are located at the join between the head and the body (a join which is not very clear in some species). They are located outside the shell, which is formed from two valves surrounding the whole creature, jointed on its back and almost meeting under its stomach.
The head comprises the mouth and above this a small moustache, possibly with a sensory function; the part of the digestive tract which is connected to this has a large appendage similar to the liver; finally, in front of the head and directly under the shell, a large striking compound eye, totally unique, with a sparkling tiara and below it a mass of nerves which carries nerve fibres, strings, muscles to all points of the eye, making it swivel continually in all three spatial directions. In fact, this central eye is made up of two side-facing eyes which merge down the middle, forming a black-pigmented mass with a crown of ten crystalline lenses, five for each eye. In the body, the intestine follows its path, carrying hard chitinous appendices towards its exit.
Above the intestine in the female daphnia a space is clearly visible where the eggs develop: this is the brood chamber. Underneath the intestine, small organs in the shape of flat broad strips move continuously. This is where breathing takes place: these are called branchial “legs” because of the similarity with the position of the legs in other animals, but they are not used for walking or, except in very few species, for swimming.
When the conditions for life are good, the daphnias reproduce extraordinarily quickly. They do not waste time with the usual reproductive techniques; they are all female. The eggs, of which a dozen or so move into the brood chamber, develop into normal little daphnias – identical to their mother – and start to thrash about inside.
With its hollow turned toward the inside of the mother, the curved appendix positioned at the rear of the intestine stops the little daphnias from escaping; but at a certain point in time the concavity bends towards the outside, leaving open a big enough passage for the little daphnias to escape, helped along by the evacuating movements of the mother’s intestine. The little daphnias already have eggs in their own brood chambers: these are starting to develop and have sometimes even taken on the recognisable form of daphnias… It’s just like Russian dolls! This continues at a rate of 10 to 15 births every week. Each daphnia is able to give birth to a new generation within a week of its own birth, and this amusing novelty carries on for about a month. There are of course some losses; the meal, on the other hand, is much appreciated. In short, there are never too many of them.
When the circumstances become unfavourable (extremes of temperature), and if the daphnias do not all die at once but instead adapt, then the change in temperature causes bizarre changes to take place. The result is that in the final batch of daphnias (which until now have been all female) some male daphnias appear. These are recognisable by certain external clues, the form of the appendices and the presence of male sexual organs in the place of female ones.
In this generation, fertilisation will take place, and instead of laying a dozen normal eggs, the fertilised female daphnias will lay two or three eggs with a very tough shell which, when the daphnia sheds its shell (the daphnia is a crustacean and moults like all crustaceans), will remain in the brood chamber which fell off before the moult; this brood chamber, known as an ephippium, is very strong, and enables the eggs to withstand both the winter ice and the total drought of the summer. They will only develop when the conditions are favourable again; there then result generations of perpetual females (reproduction by parthenogenesis) while these conditions last. And then the cycle will start all over again.
Jean Painlevé
[1] Some unspeakable dealers substitute sawdust for all of part of this powder – this is easy to do but of little nutritional value.
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