Which organisms develop in the water

Biology and the properties of water

There is no life without oxygen, with the exception of a few types of bacteria. The air contains oxygen in abundance, the water, on the other hand, only contains the soluble form - an often scarce commodity. As if that wasn't already difficult enough, the oxygen content also decreases with increasing temperature. In summer, of all times, when the metabolism could get going, the energy supply drops: an often risky balancing act for aquatic organisms. It's amazing that life developed in water, of all places. Perhaps there is also something to be said for a life in water? This question again reveals a typical problem of understanding: We see the world constantly from the perspective of a country dweller. We actually have the bigger problems here. It is not without reason that life arose in the water around 3.6 billion years ago, the step on land only took place 500 million years ago.

Many life processes take place much better in water than on land. First of all, there is the problem of dehydration, which does not even arise in water. In the salt water, where life originated, body fluids and the external medium actually have almost the same mineral composition, so that sea creatures do not even need a particularly effective outer shell to protect their ion balance. In this respect, fresh water is much more hostile to life and requires advanced anatomical and physiological measures in order to protect the internal environment of the cells and organisms from the escape of ingredients.

But fresh water also shares many of the properties of the oceans. There is, for example, the climate: low temperature fluctuations, no frost - and if it does, the ice always floats on top. What things do not land creatures have to come up with in order not to freeze or dry out? And then there is locomotion: swimming in three dimensions without expensive aids, because water makes you (almost) weightless. Because weight hardly plays a role and there is also no risk of dehydration, many aquatic animals and plants do not invest in supporting structures or hard-wearing outer shells. On the other hand, gravity literally forces us earth dwellers to crawl under the weight of our skeletons and skin armor. Whether human, turtle or beetle, our way of locomotion does not look elegant.

Sex is also more effortless underwater than on land, the sex cells simply swim to one another. The often arduous mating acts are inventions of rural life, because the positioning of the sperm cells is precision work in most terrestrial animals. Aquatic animals do without it, unless they are those that, like aquatic mammals or aquatic insects, have secondarily said goodbye to life on the land. And what about aquatic plants? An interesting question: How do aquatic plants actually reproduce? But the answer to that question has already been covered: just take this link.

And then there are those Water anomaly. This is the name of the physical secret that makes the lush life in our ponds, lakes and rivers possible in the first place. While the density of most liquids increases with decreasing temperature, this is only the case with water above 4 ° C. At this temperature, water has its greatest density. One liter of water weighs exactly 999.97 grams at 4 ° C, 999.70 grams at 10 ° C and exactly 998.20 grams at 20 ° C. Even if the differences may seem small, the effect is clear: warmer water is lighter and floats at the top. In summer it is warmest on the surface of a pond and the colder water masses down to the bottom of the water as the temperature decreases.

This stratification is also retained when the air cools down in autumn. Because the surface water is initially still warm, the first temperature drops in late summer also lead to the formation of water vapor. Slowly, however, the temperature difference between the bottom of the water and the surface decreases until the entire body of water has cooled down to 4 ° C.

And now it comes: colder water is getting lighter again! At 3 ° C, one liter of water weighs 999.96 grams and at 1 ° C it weighs only 999.90 grams, which is what is called the anomoly of water. As a result, when the air cools down further, the now colder surface water is stored above the 4 ° C warm, heavier deep water. The result: In winter, the water in the pond is layered in the opposite temperature sequence than in summer: Now the coldest layer is on top and the 4 ° C "warm" water is below. And that is exactly the reason why a body of water does not freeze over from below, but from above! The ice, however, always floats on the surface, after all, it only weighs around 918 grams per liter.

This anomaly of the water is the secret of life in all fresh waters, because this circumstance is of existential importance for plants and animals: under the ice there is always liquid water. That is why fish do not freeze in winter, unless the water is very shallow and freezes to the bottom. But ice thicknesses of more than one meter are rare, even in the waters of the Arctic, because ice also has an ecologically important property: it insulates. At the very bottom of the pond there is therefore a temperature of 4 ° C throughout winter, and that is not as uncomfortable as it seems. Because with the exception of birds and mammals, all other animals are cold-blooded. This means that they adapt their body temperature to the outside conditions. In any case, they will not freeze to death at a water temperature of 4 ° C.

Unlike land animals, water dwellers do not have to protect themselves from ice formation in their cells, because that would be death. On the other hand, the life processes in cold water are greatly reduced. The so-called reaction rate-temperature rule (RGT rule) can be used as an approximation. It says that if the temperature increases by 10 °, the metabolic rate doubles. This also applies the other way around: the metabolism of a fish, a water flea or a dragonfly larva is reduced to around a quarter of the summer activity in the cold water of the pond floor. And it gets even better: At the same time, the solubility of oxygen in water increases from around 9 milligrams per liter at 20 ° C to almost 15 milligrams per liter just above freezing point. A quarter of the metabolism with almost a doubling of the availability of oxygen - these are conditions under which one can live quite comfortably.

Life goes on under the ice!


What sounds like a dry physics lesson ultimately brings to light a peculiarity of water, with which the evolution of animals was able to take completely new paths. The anomaly of the water paved the way for insects, which like dragonflies mainly live in the tropics, to our latitudes. By relocating the larval stage to winter, the offspring escape the hardships of the cold season. Even more: While many land insects have to take a break in winter, the dragonfly larvae also use this time for their further development. In spring, a new generation of these tropical-looking insects makes the air space above their native waters unsafe. Dragonflies, like their larvae, are voracious predators. Her parents, on the other hand, fell victim to the winter frosts months ago. In fact, this type of development is not a privilege of dragonflies, but stoneflies, mayflies and caddis flies also spend their youth under water in winter. A surprising idea: while we skate, life goes on inexorably under the ice.


The development of the larvae of the dragonfly takes one to two years ...

... until they leave their wet habitat in spring ...

... and dominate the airspace over your home waters for one summer.

Just like the aquatic plants, the aquatic insects have found their way back to a life in fresh water secondarily. Fossilizations from the beginning of the Cambrian 540 million years ago prove that the ancestors of all arthropods came from the sea. From these, the crustaceans, which now populate the oceans with around forty thousand species, and the insects, which dominate the land with almost a million previously known species, developed. A small group of crustaceans has also been able to conquer fresh water from the sea. These include our water fleas, hopscotch, flea and mussel crabs. With its eight inches in length, the freshwater crab looks like a giant between these tiny creatures. Insects first appeared on the scene in the Silurian, between 408 and 438 million years ago. This made them one of the first animals to leave the water. They followed the land plants, which almost simultaneously created the conditions for animal life on the mainland through their biomass production. The conquest of the country required a radical change in the oxygen supply. If the exchange of water-dissolved oxygen and carbon dioxide in marine animals took place through the skin or through specialized gills, the insects developed a trachea system, the trachea, as an organ of exchange between the atmosphere and body fluid.