The Edge of The Ocean
To be able to thrive on the shoreline, the animals must cope with the quick crashing waves, immersing into water and exposure to air. They experience impulsive changes in temperature as the tides continuously envelope them and uncover them. While these tiny animals try to protect themselves against that, there are two sets of predators that they must avoid too. Predators that live in the water, and predators that fly or live on land and breathe air.
The majority of the shoreline animals are ocean dwellers that are adapted to life in the surf. They face great danger when the tide goes out as they are in need for the oxygen contained in the water. Some such as prawns and fish, avoid the potential danger by flowing away with the water tides. However can get stuck between the large rocks, but as long as they remain surrounded by water they can still breathe until released again by the returning tides.
Other animals have methods to survive both in and out of the ocean such as Mudskippers, for example, absorb dissolved oxygen to breathe underwater but when they are about to be exposed to air they enlargen their gill chambers and fill them up with water. This ability enables them to head out on land and find food resources from which are completely out of the reach of other fish and of course, survive.
Most shoreline animals are, however, are dependent solely on water to breathe. Majority of crabs, even though having to swiftly move about on land, have to regularly return to the ocean to top up their oxygen levels. Inside their carapaces they carry water but the water often has to be flushed out and replaced every time the oxygen inside it is used up. Much like Scuba divers except the opposite.
Many shoreline animals are too slow moving or spend their lives attached to one spot. However with the crabs it differs as they are able to nip in and out of the water whenever they please. For the slow and fixated creatures, life depends on being able to survive on that little amounts of oxygen that surrounds them until the tide returns again.