Answer:
Changing their external environment
Explanation:
Estuaries can experience fairly extreme changes in abiotic conditions. For example, estuaries can at different times be dominated by the ocean when freshwater input from upstream is minimal, or it can be freshwater dominated when there is a lot of flow coming into the estuary from upstream. This means that organisms that exist within estuaries must be able to tolerate a wide range of salinities. These are called euryhaline organisms. Estuary ecosystems also receive biotic inputs from both the ocean and from the upstream river. There are therefore a lot of scavengers that thrive within estuaries, such as crabs, mud prawns and clams. One primary producer group that is important for estuaries are mangroves, but they are not found in all estuaries. Estuaries act as important nursery areas for ocean fish, who come into the estuary to spawn.
Selective permeability, made possible in cells by the phospholipid bilayer that makes up the plasma membrane, is basically monitoring and regulating what can and can't enter the cell. It is carried out by a series of membrane proteins that act as channels to polar molecules, opening to allow them in and closing to prevent their entry. This is possible because the hydrophobic tails of the phospholipid molecules repel polar molecules and so prevent their entry unless through a channel.
