On these beaches the individual sand grains, thrown around by the waves and currents, move over and around each other and can grind away soft-bodied animals. Most animals bury themselves safely away from this dangerously mobile surface layer. Once deep in the safety of the sand, the animals are in a far more constant environment. It is not just the risk of mechanical damage that makes the sand surface a difficult place to live. There can be a considerable change in temperature as the tide goes in and out and small animals can easily dry-out or overheat.
Rainfall is also a major danger to marine animals that have bodies adapted to saltwater.
Even a few centimetres down, it is cooler and as fresh water seldom penetrates far, the water between the sand grains is as salty as the open sea. Finally living at the sediment surface exposes animals to predatory fish and birds. Often all the holidaymaker ever sees of the animals that live in the sand are their tracks and burrows. No matter how safe it is to be buried in the sand, many animals need to be in contact with the sea as much of their food is carried suspended in the water. Tube-dwelling species have ways of feeding that mean that they never have to leave their burrows in the sand.