Marine Biology

1.Substratum

Substratum

While stability of the substratum is the most characteristic feature of the rocky shore, a labile substratum is typical of sandy shores. As most of the sandy shores are exposed to strong wave action, the substratum will naturally be unstable. Owing to this instability of the substratum, the beach varies in extent with the state of the tide, both from day to day, and in areas where the tidal cycle is very pronounced from fortnight to fortnight. Besides, there will be more pronounced changes throughout the year due to the increased wave action or with change in wave action which may remove most of the sand and thus these changes significantly modify the slope or profile of the beach.

During monsoon periods, extensive erosion of beach occurs due to the action of strong winds and waves. Such erosion of the beach causes the reduction of the width of the shores, which in turn affects the distribution of sandy shore animals. Due to the churning effects of waves, the infaunal organisms along with bottom sediments are disturbed and thrown into the upper levels of the beach or put them in suspension in the water. So, many of the forms are known to exhibit tidal migration and may move up and settle in the narrow beach. For example, bivalve species such as Donax incarnates, and D. cuneatus, the crustaceans such as the mole crabs of the genera Hippa, Emeritta and Albunea and the gastropods such as Bullia melanoides are known to exhibit such tidal migration. During the periods of calm weather, re-deposition of sand particles occur, which facilitate the re-establishment of fauna present at the normal period.

Besides the tides and waves, size of the particle or sand grains on the shore also influences the profile of the sandy shore; the larger the particle size, the greater will be the slope and finer the particle size, the lesser will be the slope. Thus, the steepest slopes are found on the shingle and coarse sandy beaches. Alternately, the sandy beaches with mild/moderate wave action will tend to have fine grains and slope of the beach will be gentle in nature and the width of the beach will be broader than the beach with steep slope.

The size of the sand grains also governs the amount of water that is retained by capillary action within the minute spaces between the individual grains of sand. In the coarse sandy beaches, the interstitial spaces of the grains are more, which cannot retain interstitial water in its spaces and hence such beaches are devoid of any infaunal groups as there is not sufficient moisture inside the sand particles. In the case of fine grained beaches, the water is raised to a considerable height above the sea level due to the capillary action of the sand particles and thus more of infaunal organisms will be able to live as there is sufficient moisture in the sand particles even during the low tide period.

Apart from the size of the sand particles, its shape is also known to play a major role in the distribution of sandy shore fauna as it decides the suitability of the substratum for the burrowing forms. The newly formed particles with sharp edges are not the suitable substratum for burrowing animals as they create difficulties in the penetration or burrowing into the sandy substratum. The same is the case with the substratum with the particles of broken shells of molluscs and echinoid fragments.

In sandy shores, the seaweeds and the sessile macrofauna are completely absent as the substratum is not stable and constantly shifting in nature. However, the fine sand particles offer good scope for the inhabitation of some macrofauna as well as infauna called meiofauna or interstitial fauna as the soft sand particles are easy to penetrate and also allow good gaseous exchange, particularly oxygen, for the respiration of the animals.