| Bivalves
include clams, mussels and oysters. They are abundant on the
rocky shore, and buried in the sand and mud. |
|
'Bivalve'
means 'two valves'. There are about 7,000 species of
bivalves.
|
|
Two Shells: A bivalve has a two-part
shell, that is, it actually has one shell made up of two parts. Each
part of the shell is called a valve. The valves are connected by a
hinge and kept shut by one or two large muscles (called adductor muscles).
When the bivalve relaxes its adductor muscles, a springy ligament
causes the two valves to open. In many bivalves, the hinge between
the two valves have teeth to prevent the shells from slipping sideways.
Bivalve shells have different shapes and textures to help its owner
better survive in its environment.
 Sanctuary
in the Sand: Most bivalves bury themselves. Here they are
safer from predators and keep cool and moist during low tide. They
use their foot to burrow, then stick out two siphons to the surface.
Water is sucked in through one siphon, and ejected through the other.
How do they dig in? A bivalve
has only one foot and no other limbs. Yet, it can dig into the sand,
some can do it very rapidly indeed. To dig in, the fleshy foot sticks
out between the shells. The end of the foot is then expanded into
a bulbous shape to form an anchor in the sand or mud. Water is then
expelled from between the shells to loosen the sand and mud and the
bivalve then quickly contracts its foot to pull itself deeper in.
It does this repeatedly until it is at a comfortable depth. Different
bivalves bury themselves to different depths. Those with more streamlined
shapes dig deeper.
Life in the Slow Lane: Bivalves
are mostly sedentary and don't move about as much as most gastropods.
Many are adapted to live buried in soft sea bottoms, some live permanently
attached to a hard surface. Being mostly immobile, peaceful filter-feeders,
most bivalves don't have a head or a radula. Burrowing bivalves have
a flattened, blade-like foot to burrow with. Oysters that stick to
hard surfaces don't have a foot.
Bivalve Food: Most bivalves use
their enlarged mantle cavity to suck water in and to filter out the
titbits from the water flow. Cilia (beating hairs) on their gills
generate a current through the gills. Mucus on the gills traps food
particles which are sent along a groove to the mouth. Fleshy pads
near the mouth then push the mucus-food misture into the mouth. The
gills also extract oxygen from the water. Oysters and mussels that
do not bury themselves simply open their shells a little to get a
current going through their bodies. Burying bivalves usually have
a pair of siphons, tubes made out of extensions of the mantle. These
stick out onto the surface for a one-way flow of water; water enters
one siphon and exits the other. In some, these siphons can be quite
long so that the bivalve can remain deeply buried and still feed and
breathe.
By a Thread: Many bivalves secrete
byssal threads, strong protein fibres that can be used to cement themselves
to hard surfaces and supports. Burying bivalves may use byssus thread
to literally root themselves to the surrounding sand or small stones.
The thread is produced by a gland near the foot. The foot gets a grip
of the surface and the secretion from the gland flows along a groove
in the foot. When the secretion hardens on contact with sea water,
the foot is withdrawn.
Bivalve Babies: Most bivalves
have separate genders. Bivalves generally practice external fertilisation,
releasing their eggs and sperm simultaneously into the water. Most
undergo metamorphosis and their larvae look nothing like their adults.
Free-swimming larvae develop, drifting with the plankton. These may
change as they float, developing a small shell. Eventually, they settle
down and develop into miniatures of their parents.
Human uses: Many bivalves are
eaten. Although all molluscs can produce pearls, pearls used commercially
come mostly from bivalves.
Red Tide: During some seasons,
filter feeding bivalves can be highly poisonous. This is because climatic
or other conditions favour a bloom of microscopic toxic organisms.
These toxins are concentrated in the bivalves. The toxins do not harm
the bivalves, but can be fatal to humans who eat the bivalves. These
toxic organisms multiply so quickly in ideal conditions that they
stain the water red; hence the name 'red tide' for this phenomenon.
At other times, filter feeding bivalves may also concentrate other
unpleasant chemicals and bacteria which could make you ill. |
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Window-pane shell
lies freely on the sand
Venus clam
buried in the sand
Green mussels
attaches to rocks
Rock oysters
attaches to rocks |
