Coastal Biodiversity in Ranong, Thailand
Coastal Biodiversity in Ranong
The Sandy and Muddy Shore
Sandy and muddy shore sampling in Ranong.
Mud or sand covers most of the continental shelf and virtually the entire deep ocean floor. Both are composed of tiny mineral particles eroded from rocks and often moved vast distances by water. The faster the water moves the larger the mineral particles it can carry. In very fast currents all the particles are carried in suspension but as the currents slow down, the coarsest material falls to the sea floor. Slowly moving water still carries fine-grained muddy material, but as it slows down even more of this also drops to the bottom. This is the reason that sandy beaches are found on the open wave-exposed coast while mud is found in the shelter of estuaries and in deep water.
Sand dollar Echinodiscus bisperforatus found from the low tide mark down into shallow water.
Sandy seashores can appear to be a hostile environment for animal life. 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.
A ghost crab
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. Some can extend net-like feeding organs into the current and strain their food from it. Others, particularly clams can stretch out long tubes to suck up food material deposited by the current on the surface of the sand. However, there are animals that never need to expose themselves to the dangers of the surface. Many beach-worms feed on the micro-organisms that coat the mineral particles that make up the sand or mud. All these species have to do is to eat the sediment and digest away any nutrients it contains.
The fine organic matter, upon which many sediment-living animals feed, drops from suspension most readily in quiet water, in the same areas that mud accumulates, and hence mud flats have a rich food supply. To this is added a vast production of micro-organisms on the surface of the very small mineral particles that make up the mud. The large amount of food that is available nourishes enormous numbers of worms, crustaceans and clams which in turn are the food of adult and juvenile fish and wading birds. Sand flats are generally far less productive as a combination of physical disturbance and low levels of food allow fewer animals to survive. Nevertheless some species of fish and some wading birds specialise in feeding on the invertebrate animals of sandy beaches.