The largest number of species of aquatic animals live in seawater, a smaller number in fresh water and least

The largest number of species of aquatic animals live in seawater, a smaller number in fresh water and least of all in brackish water. What do you think is the reason for this?

Initially, life originated in salt water, it is also considered the least aggressive environment, because the concentration of salts dissolved in it is closer to that inside the cells than in fresh water. Therefore, there are many more species in seawater than in fresh water; most invertebrate species are exclusively marine (for example, cephalopods and starfish).

But many have adapted to fresh water as well. This was due to organelles or organs that prevent cells from swelling due to osmosis (for example, the contractile vacuole of protozoa), or due to the development of integuments.

In brackish waters, salinity is too low for marine animals, and too high for freshwater animals, so they contain mainly euryhaline species (which can live in a wide range of salinity). There are few of them, but they can reach large numbers.



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