In: Biology
Describe the main zoning levels of organisms on sandy coasts.
Sandy coasts or shores are those coasts that are dominated by sand size sediments and are formed as a function of both sand sources and coastal processes. Sandy coasts generally lack a proper pattern of vertical zonation and apparently appear barren and devoid of life. The most important factor determining the distribution of organisms in this habitat is the wave action. Although it is much less defined as compared to other habitats, zonation does exist on sandy coasts. A typical sandy coasts can be divided into three zones: supralittoral, midlittoral and infralittoral (subtidal) zones. These zones are discussed as follows:
(i) Supralittoral zone- It stretches from the high tide to the point of terrestrial vegetation origin. Sand dunes cover the top most layer of this zone and much of this area is unsuitable for habitation as the scorching heat of the sun bakes the surface continuously, raising its temperature and drying it. Just below this level is a zone of drying sand which remain moistened only during the high tides and most of the organisms inhabiting in this zone are termed infauna because they burrow in the sand to escape the dry periods and intense heat of the sun. They obtain oxygen and other nourishments through their skin or gills that draws water by an elaborate mechanism via siphons. The dominant macroorganisms of the supralittoral zone of temperate regions include insects, isopods and crustaceans (beach hoppers). In the tropics or warmer parts of temperate region, there is a slight difference, as the beach hoppers are replaced by Ghost crabs (Ocypodinae) and fiddler crabs (Uca).
(ii) Midlittoral zone- Most inhabitants of this zone are burrowers. During high tides, they emerge from their burrows in search of food, to find mate and reproduce or sometimes, they extend their appendages to filter out desired nutrients and oxygen from the water. During low tide, they return back to their burrow into the moist sand. The midlittoral zone also exhibit vertical zonation. The dry sand and drying sand are less stretched as compared to supralittoral zone. Just below the drying sand, there is a zone of retention that retains moisture at low tide due to capillary action of water. This zone is inhabited by various isopod crustaceans. This zone is followed by zones of resurgence and saturation which are closer to sea. In the zone of resurgence, water is retained down the surface and provide supports to a variety of assorted crustaceans and polychaete worms. Farther down is the zone of saturation which is highly diversified and supports a wide range of organisms. It contains maximum water content. Burrowing animals like polychaete, bivalves like coquinas, tellins and venus clams are found in this region alongwith some amphipods and other small crustaceans. The sandy bottom part also reveals the presence of echinoderms like sand sea star. Moon snails and olive snails are common in this habitat and they leave a trail while crawling along the bottom of the sand. Another prominent organism found in this habitat is the lungworm. This are deposit feeders and digest the organic matter present in sand.
(iii) Infralittoral zone- The infralittoral or subtidal zone is a truly marine environment where the variety and distribution of organisms depends on the characteristics of the bottom sediments. Where the bottom sedimets exposes barren sands only. the organisms are predominantly burrowing in nature, similar to those inhabiting the midlittoral zone. Several species of bivalves, tube worms, and other polychaete either filter food particles or search sand particles for detritus. This regions also contains animals like heart urchins that move slowly along the sandy bottom feeding on the detritus mixed with sand particles. In addition to arthropods and molluscs, the sandy coasts of the subtidal zone reveals the presence of seagrass beds which host sea urchins, sea stars, sea cucumbers, brittle stars and anemones. The presence of coral and introduction of rocks increases the complexity and diversity of life at the bottom. As this zone exposed only during the lowest spring tide, many species of fishes are found here that occasionally move into the midlittoral zone at high tide in search of food. The activity in this zone remains relatively constant and pace of life remain undisturbed for most of the time.