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Phylum Mollusca > Class Bivalvia
Bivalves or clams
Class Bivalvia

updated Oct 2016

if you learn only 3 things about them ...
Clams have a two-part shell and NOT two shells.
Some clams produce a special thread to anchor to hard surfaces.
Clams can be dangerous to eat. Don't eat those found at the shore.

Where seen? Bivalves 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
barnacles 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 bivalves feed.

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 in length.

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 clams, Oysters, 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.

Status 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 large bivalve.
Chek Jawa, Nov 01

Drawing by Kelvin Lim
A wide variety of bivalves bury
themselves in sand or mud.


Oysters have a two-part shell too.
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


These tiny Nest mussels formed
vast beds on Chek Jawa in 2007
Chek Jawa, Aug 07


Ark clams are among our favourite seafood!
Changi, Jul 02

Bivalve 'leaps' to safety!


Changi, Jul 11

Bivalve escaping a moon snail!


Using its long foot, the bivalve 'leaps' away to safety.


shared by Neo Mei Lin on her blog

Bivalves recorded for Singapore
Text index and photo index of molluscs on this site
list of threatened molluscs in Singapore

Links References
  • 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.
  • The 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 Zoology Brooks/Cole of Thomson Learning Inc., 7th Edition. pp. 963
  • Pechenik, Jan A., 2005. Biology of the Invertebrates. 5th edition. McGraw-Hill Book Co., Singapore. 578 pp.
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