The beach is a product of
weathering rock and the seas drift. Countless molluscs and worms live inside
the sand, while animals on its surface range from tiny sand hoppers to bird
sand seals. Sand consists of tiny particles of rock and shell, and is the result
of years of erosion and weathering by the sea. The erosion begins with the sea
gnawing away at cliffs and breaking off rocks and stones. These are carried
away by the sea and waves batter them against rocky foreshores, breaking them
and their attached shells into ever smaller pieces. Thus the sea produces the
raw materials for a sandy beach. So, the next stage in the creation of a sandy
beach takes place once the particles of rock and shell are small enough to be
suspended in the water, when they are carried along the coast by a process
known as “longshore drift”. The direction from which waves approach a stretch
of coast is determined by the direction of the prevailing wind, since the waves
are created by winds out at sea. As the waves hit the shore they run up the
beach, usually obliquely (depending on the wind direction), carrying their
cargo of sand with them. This is called swash. The water returns the back wash
by the line of least resistance, which is straight down the beach. By this
to-and-fro action, waves after wave, the sand is shifted along, guided by the
direction of the prevailing wind. This is longshore drift. If the swash is more
powerful than the back wash then the sand gradually builds up to form a beach.
If the backwash is dominant the sand moves on along the coasts, round the head
land, and perhaps the next until it comes across a bay in which the conditions
are right for the formation of a beach.
Beneath the surface, like all
habitats dominated by the tides, sandy beaches have their favorable and unfavorable
periods for the creatures that live there. The water brings a fresh source of
food on each tide but when it retreats the inhabitants invertebrates such as
worms, crustaceans and molluscs have to seek shelter against the sun, the wind
and predators. For this reason the great majority live beneath the surface of
the sand. One of the most obvious inhabitants is the common lugworm, which
advertises its presence with the familiar coiled worm casts. This worm loves
permanently in a U-shaped burrow, where it swallows sand to extract the organic
content, after which the sandy waste is ejected as the coast. A few inches away
from the cast a shallow round depression in the sand marks the entrance to the
worm’s burrow.
Where the feeding activities of
the head end are causing the sand to sink the only way you can see the worm
itself is to dig for it as anglers do for bait or hope to see it being dragged
out by an oystercatcher or a gull feeding at low water. The damp sand of the
inter-tidal region contains several different marine worms besides the
lugworms. The tube building worms are particularly satisfying to find because
they build beautiful shelters against the difficult environment. The sand
mason, for example, secretes of sticky mucus from its body, to which sand
grains adhere. This sandy tube extends down into the sand and also pokes above
the surface. When the tide is out the worm withdraws, leaving only the top of
the tube open. As the tide comes in the worm extends itself up the tube to collect
tiny food particles drifting past its waving tentacles. The tube provides the
worm with a fixed home in the unstable medium of sand and allows the worm easy
movement up and down in response to the changing rhythm of the tide.
Burrowing Bivalves This kind of
movement up and down in the sands in response to the tide is also performed by
several different molluscs. The molluscs of the sand are mostly bivalves, in
contgrast to those of rocky shores which are mainly creatures such as limpets
and winkles, which seal their single shells to the rock surface to keep in
moisture and maintain an anchorage during the hazardous period of exposure. The
bivalves have a different strategy to survive this dangerous period they
burrow, using a powerful foot muscle to drag themselves out of sight into the
damp sand. A good example of a burrowing bivalve is the common cockle.
Moreover, when the tide is in the cockle lies half buried at the surface with
its two shells slightly apart to allow its siphons to extend into the water for
feeding and respiration. One siphon draws in water, along with its complement
of plankton food, while the other siphon ejects the sieved water as waste. When
the ebbing tide signals the end of the cockle’s feeding time it tightly clamps
its two shells, sealing itself in until the next flood tide, and hides itself
just below the surface of the sands.
Hidden predators sand also harbors
some active hunters, such as the masked crab and the burrowing starfish. When
the tide is in the masked crab digs and scoops its way into the sand with its
powerful hind legs. Once hidden it breathes by drawing water down through long,
tubed antennae and waits for the tide to go out so it can begin hunting. It is
active only at night along the lower shore and shallow water, walking without
the sideways scuttle typical of most crabs. The burrowing starfish adopts a
different strategy. It hides just below the surface of the sand and seizes
whole molluscs or worms, swallowing them into tis central stomach. A bivalve
may seem indigestible, but the starfish has a way of dealing with it. It
succeeds in prising open the molluscs shell inserting its own stomach. The
starfish’s digestive juices then reduce the meat inside to a ready meal. In due
course the empty shell is rejected on to the surface of the sand.
Birds on the beach are on the
lookout for molluscs, and they have some highly successful methods of
extracting the meat from within the shells. Even the tough shell of a cockle is
no protection against the powerful bill of an oystercatcher. Hunting in the
shallows just before the cockle runs out of feeding time on a falling tide, or
before the incoming tide renders the water too deep for the bird to wade, the
oystercatcher looks for an open gaping shell. When it sees one it strikes the molluscs
adductor muscle, after which the molluscs is unable to close its shell in self-defense.
Even if the shell does manage to close round the beak, the strong red bill can
easily force it open again. Alternatively the oystercatcher can smash its way
into the shell by using its beak like a hammer. Gulls also use brute force when
feeding on molluscs, but a certain amount of cunning as well. Herring gulls
sometimes force cockles to the surface by paddling their feet up and down on
the surface of the sand. They have also learned to pick up cockles from soft
sand, fly with them until they are above a hard surface such as a rock or even
a promenade and then drop them so that they smash open below.
Sandy beaches do not exist in
isolation but are flanked by rocky headlands or shallow rocky foreshores. In
sheltered parts of the beach, where the water currents are slack, the sea often
carries a large cargo of debris from these neighboring habitats, and from
further out at sea itself; seaweed, dead animals and shells, for instance. At
the high water mark much of this organic material piles up to form the familiar
strandline with its own wildlife of scavenging sand hoppers, kelp flies,
turnstones, crows rats and foxes. The same strandline also supports most of the
few plants found on the lower reaches of a sandy beach. Here you may find
plants such as saltwort, sea rocket and orache. Further inland, above the high
tide, level, the most obvious sign of plant life is often sand dunes. These are
an extension of the sandy beach, formed when onshore winds carry the sand
inland. Once there maritime plants notably marram grass stabilize the sand to
form semi-permanent dunes.
Going in the other direction,
away from the land, low tides sometimes create temporary offshore islands which
because of their isolation are excellent habitats for seals. Common seals often
give birth to pups on such islands, even though the pups must be able to swim
before the next tide comes in to engulf them. Once in water the baby swims with
its mother and then both return suckling on the sand bank or in the shallows
when the island reveals itself again. Common seal spend a lot of time out of
the water humping on to the sand in large social gathering as well as to drop
pups. Their passage along the sand is marked by the characteristic tracks
created by their bodies and flippers. As the tide drops the seals move with it
towards the water. From the seals point of view, one stretch of sand is as good
as another for their purposes their preference for offshore banks is a direct
response to human disturbances, for on land people are their only major
enemies, and people flock to beaches in their thousands for the recreation they
offer so well.
Source: Charismatic Planet
