Marine Biology

III. Organisms of the sandy shore

Organisms of the sandy shore

           A clear-cut zonation, as is evident on the rocky shores, is not evident along the sandy shores.  This is because a majority of the sandy shore animals live buried in the sand and thus are not visible to the observer.  Besides this, sandy shores have a shallower gradient than rocky shores, so that the corresponding intertidal zones are widespread, broader and less obvious.  However, a zonation actually exists in the sandy shores also, and this has been described based on larger crustaceans and gastropods.

            The supralittoral fringe zone is populated by two main types of crustaceans, the ghost crabs (Ocypode spp.) in the tropics and the talitrid amphipods (beach hoppers) in the temperate regions: the middle region (mid-littoral) is dominated by the isopods of the sub-family Cirolania and the lower region, towards the sublittoral fringe, is occupied by anomuran crabs in the tropics and amphipod crustaceans in the temperate regions.

            Zonation is especially characteristic of many other groups of animals also.  But the formation of different zones of various animals as in rocky shores, is indeed difficult. The upper regions of almost all the sandy shores are occupied by the talitrid amphipods or ocypode crabs.  Both these have much in common ecologically, being scavengers on animal or vegetable debris thrown by the tide and both are predaceous to some extent. Talitrus saltator of the temperate region can live an almost true terrestrial life like the littorines of the rocky shores.  The same is the case with ocypode crabs of the tropical shores.  In India, crabs of the genus Ocypode viz.  O. ceratophthalma, O. macrocera and O. platytarsis are very common.  These crabs inhabit the dry sands just above the surf zone and lead an almost terrestrial life.  They return to the water only occasionally to moisten their gill chambers.

            The mid-littoral region of the sandy shores possesses a dense fauna of amphipods such as Bathyporeia and Urothoe, and isopods like Eurydice.  The amphipods, Bathyporea (Bathyporea pilosa  , B. pelagica) and Urothoe are capable of both swimming and burrowing and they feed on minute particles of organic matter present in the intertidal water. The isopod, Eurydice is known to migrate shoreward when the erosion of beach occurs.

             Many polychaetes like Nephthys and Glycera are present in the mid-littoral zone.  These polychaetes are carnivorous forms having jaws borne on a relatively enormous introvert, which may assist in burrowing as well as seizing the prey.  The same zone of the sheltered sandy shores having some admixture of mud with sand is occupied by tubicolous polychaetes worms like Arenicola and Pectinaria. Arenicola is responsible for the conspicuous worm casts seen on many sandy shores.  It lives in characteristic ‘U’ – shaped burrows, the walls of which are consolidated with mucus.  The burrow comprises a head shaft, a gallery and a tail shaft.  The head shaft contains loose sand, and the sand from this section supplies the food.  At intervals, the worm retreats tail first to the tail shaft and liberates the rod-shaped excreta to the surface.  During high tide, when the burrow is filled with water, circulation is maintained by the pumping action of the animal, in at the tail shaft and out at the head shaft.  This current of water is used for respiration.  As Arenicola is a burrower consuming the deposit for food, the organic content and the particle size of the substrata are important factors governing their incidence.

            Pectinaria is another remarkable sandy shore form which builds a delicate, tusk-like tube, made of a single layer of fine sand grains cemented together.  Lying buried upside down in the burrow, it excavates a conical depression in the sand with its head at the bottom and feeds on particles of food that slips into the depression, by picking them with the feeding tentacles.

            Besides these amphipods, isopods and polychaetes, the mid-littoral zone is occupied by a number of bivalve molluscs which are highly adapted for a life in sand.  Of the many species known , the most important include Cardium, Donax, Tellina, Macoma, etc.  They are all burrowers and are comparatively slim when compared to those in muddy shores.  Tellina and Donax prefer clean sand while Macoma and Cardium live in more muddy sand.

            In the sub-littoral fringe area, below the low water level of neap tides, the sandy shore becomes much richer and crowded with organisms.  The most characteristic organisms of this region are anomuran crabs belonging to the hippidae (eg. Emerita) for the tropical shores and amphipods like Bathyporeia and Haustorius for the temperate seas.  Both forms are nocturnal and are found in the region of breaking waves.  Both crabs and amphipods are filter feeders and they even look alike.  Emerita holthuisi, E. asiatica and Albunea symnista are the anomurans, most common along the sandy shores of India, and they are all well-adapted to live in the wave-beaten shores.  As the tide rises and falls, these crabs follow it so as to be located continuously in the region of the wave-wash.  They burrow backwards facing the sea and lie buried, keeping only their plumose antennae outside.

            The sub-littoral fringe zone of the sandy shores is inhabited by a number of animals.  These include many molluscs like Oliva, Harpa and Tonna.  The characteristic mollusc of this zone is the razor clam, Ensis which is by far the most highly specialized of burrowing bivalves, having unique power of rapid downward movement.  The shell is elongated because of the prolongation of the hind part.  The straight smooth shell offers only minimum resistance to passage through the sand, so that the movement is exceptionally rapid.

            The tubes of the sand-mason Lanice, a terebellid is a typical feature of the lower intertidal zone.  These tubes are seen in tufts projecting from the surface.  If disturbed, they quickly, retract into their tubes which are considerably longer than their body. The heart urchin, which is found near the low-tide level, is another burrower representing a unique instance of a sea-urchin adapting to burrowing life.  Echinocardium is heart-shaped.  With the help of the spines, they actively dig down straight to a depth of nearly 15-20 centimeters.  The walls of the burrow are made firm with mucus, secreted and applied by tube feet.  A respiratory current is maintained inside the burrow by an extremely long tube foot which extends up to the surface.

      Other animals found in this zone include the worm-like sand cucumbers (Leptosynapta), enteropneusts (Saccoglossus, Ptychodera), sea-anemones (Peachia and Halicampa), the shrimp Crangon and shore fishes like Glossogobius. Besides these permanent inhabitants of the intertidal sandy shores, many others visit the sandy beaches.  These include terrestrial forms like beetles, and birds like sand-pipers and sea-gulls which visit this region in search of food.  There are many other true marine animals which visit the intertidal sands mainly for breeding.  Most notable among them are the king crabs, the grunion and the sea-turtles.  Two species of Xiphosuran crabs are known to occur off the east coast of India.  They are Carcinoscorpius rotundicauda and Tachypleus gigas from the Gangetic delta and along the shores of Chandipore in Orissa.  These curious king-crabs visit the sandy shores sometimes in hundreds.  The female excavates a hole in the sand deposits the eggs which are soon fertilized by the males.  These eggs are soon covered with sand by the action of waves.

            The grunion, Leuresthes tenuis of the California coast, visits the intertidal sandy shore for breeding.  Thousands of these fishes move towards the shore, and on reaching the shore the females quickly burry into the sand, tail first.  Lying half-buried, they lay the eggs, during this time the males curl around them on the surface of the sand and deposit the milt which run down the sand and fertilize the eggs.  After breeding, the fish return to the sea.  The eggs are covered with sand by the receding tide.  It has been found that the breeding migration of this fish is well-correlated with the lunar cycle and that they lay the eggs only along the highest spring tide level, so that the developed eggs may be washed back into the sea by the next spring tide.

            The females of sea-turtles also perform such migration to the sandy shores to deposit their eggs.  They excavate pits in the sand, bury their leathery-shelled eggs there and then return to the sea.  The young after hatching crawl down and swim off.