you learn only 3 things about them ...
Clams have a two-part shell and NOT two shells.
clams produce a special thread to anchor to hard surfaces.
can be dangerous to eat. Don't eat those found at the
are commonly seen on almost all our shores. Sandy and muddy shores
are particularly rich in buried bivalves, seagrass meadows are also
teeming with them. On rocky shores, bivalves such as oysters are permanently
stuck to hard surfaces. While on reefs, magnificent bivalves such
as giant clams may be encountered.
What are bivalves? Bivalves are molluscs
(Phylum Mollusca) that belong to Class Bivalvia. Bivalves include
clams, mussels and oysters. There are about 7,000 species of bivalves.
Two shells? 'Bivalve' means 'two
valves'. Actually, a bivalve has one shell. It is more correct to
say that it has a two-part shell, i.e., 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
valves from slipping sideways. This keeps the valves well aligned,
thus providing a tight seal when the valves are shut. The Gladys Archerd
Shell Collection website has a labelled
photo of parts of a typical bivalve
For some of our favourite seafood such as scallops, it is the adductor
muscles that we eat and not the body of the animal. These muscles
taste sweet because of the proteins found there.
Bivalve shells have different shapes and textures to help the various
animals better survive.
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 even have a foot. Some bivalves
like scallops, however,
can 'swim' for a short distance by clapping their shells together.
Sometimes confused with
and limpets. Here's more
on how to tell apart animals
with conical shells stuck on rocks.
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,
and some can do it very rapidly indeed! To dig in, the fleshy foot
sticks out between the valves. 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 valves 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.
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 mixture 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. The Gladys Archerd Shell Collection website
has a diagram and description of how
Hanging by a thread: Many bivalves
secrete byssus threads, strong protein fibres that can be used to
cement themselves to hard surfaces and supports. Burying bivalves
may use byssus threads 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. Byssus
threads are extensively studied to better understand how to create
similarly strong synthesic threads.
Bizarre Bivalves: Bivalves come
in a vast array of shapes and forms. Some like Nest
mussels, are 1cm long or less but can form vast 'nests'. Yet others
like the Giant clam
(Family Tridacnidae) are enormous and can reach nearly half a metre
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: Bivalves are among
our favourite seafood. These include Ark
Green mussels, Venus
clams and tragically, even the large, beautiful Giant
clams. Although all molluscs can produce pearls, pearls used commercially
come mostly from farmed and not wild bivalves. Please don't vandalise
our wild clams in the vain hope of finding valuable pearls.
Clam Calamity: Bivalves that are
ordinarily safe to eat can at some seaons be highly poisonous to eat.
This happens during a red
tide or harmful algal bloom. Filter-feeding animals such as bivalves
concentrate the toxins produced by these organisms. The toxins do
not harm the bivalves, but can be fatal to humans and other animals
such as otters that eat the bivalves. The toxins are not destroyed
by cooking. At
other times, filter feeding bivalves may also concentrate other unpleasant
chemicals and bacteria which could make you ill.
and threats: Sadly, many of our beautiful and fascinating
bivalves are listed among the threatened animals of Singapore. Like
other marine creatures, they are vulnerable to habitat loss due to
reclamation or human activities along the coast that pollute the water.
They are also vulnerable to trampling by careless visitors and over-collection
for food and for their shells can affect local populations.
The Giant clam
is among our largest bivalves.
Pulau Hantu, Aug 03
The Fan shell is another
Chek Jawa, Nov 01
A wide variety of bivalves bury
themselves in sand or mud.
Oysters have a two-part
Chek Jawa, May 04
Clams have siphons that
extend out of their shells.
Changi, Feb 02
Mussels can grow
in large numbers.
Changi, Jan 04
tiny Nest mussels formed
vast beds on Chek Jawa in 2007
Chek Jawa, Aug 07
clams are among our favourite seafood!
Changi, Jul 02
'leaps' to safety!
Using its long foot, the bivalve 'leaps' away to safety.
- Tan Siong
Kiat and Henrietta P. M. Woo, 2010 Preliminary
Checklist of The Molluscs of Singapore (pdf), Raffles
Museum of Biodiversity Research, National University of Singapore.
- Tan, K. S.
& L. M. Chou, 2000. A
Guide to the Common Seashells of Singapore. Singapore
Science Centre. 160 pp.
- Wee Y.C.
and Peter K. L. Ng. 1994. A First Look at Biodiversity in Singapore.
National Council on the Environment. 163pp.
- Ng, P. K.
L. & Y. C. Wee, 1994. The
Singapore Red Data Book: Threatened Plants and Animals of Singapore.
The Nature Society (Singapore), Singapore. 343 pp.
Bivalves section by J.M. Poutiers in the FAO Species Identification
Guide for Fishery Purposes: The Living Marine Resources of the
Western Central Pacific Volume
1: Seaweeds, corals, bivalves and gastropods on the Food and
Agriculture Organization of the United Nations (FAO) website.
- Abbott, R.
Tucker, 1991. Seashells
of South East Asia.
Graham Brash, Singapore. 145 pp.
- Edward E.
Ruppert, Richard S. Fox, Robert D. Barnes. 2004.Invertebrate
Brooks/Cole of Thomson Learning Inc., 7th Edition. pp. 963
Jan A., 2005. Biology
of the Invertebrates.
5th edition. McGraw-Hill Book Co., Singapore. 578 pp.