Why the animals that lives in burrows usually wander during night time only?

Organisms of the oceans have lesser salt content in their bodies than the seawater which has around 3.5%. The fluid could drain out of the body of the organism into the sea. This could be dangerous and fatal to the organism. How do they survive under such conditions? Can fish in estuarine ecosystem survive in the river as well as in sea?

What do you understand by adaptations in organisms and why do they adapt?

With the help of two examples, explain how these organisms have adapted themselves in the ecosystem?