Clams may seem boring but we have some pretty cool ones, like this heart-cockle that is sometimes seen.
Clams or bivalves are commonly seen on our shores. Sandy and muddy shores are particularly rich in buried bivalves. On rocky shores, oysters are permanently stuck to hard surfaces. While on reefs, magnificent giant clams may be seen.
What are bivalves? Bivalves are molluscs (Phylum Mollusca) that belong to Class Bivalvia. Bivalves include clams, mussels and oysters. '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. 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.
Life in the slow lane: Bivalves are mostly sedentary and don't move about as much as snails. 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.
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.
Here is a razor clam digging in with its white muscular foot. The banded tube at the other end is the siphon.
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.
This large fan shell (Family Pinnidae) usually lies buried in the sand with the pointed end below. Here you can see the byssus thread that form on the pointed end to help anchor the animal in the sand.
The jingle shell clam (Family Anomiidae) usually settles under stones. It has a two part shell, with an opening on the part that is against the stone, through which the byssus threads emerge to cling to the stone.
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'. The photo on the left is the nests formed on Chek Jawa covering metres of shore.
Yet others like the Giant clam (Family Tridacnidae) are enormous and can reach nearly half a metre in length.
Giant clams are among the largest bivalves to have ever existed on our planet! The two-part shell is thick and usually has a wavy opening that never closes completely. Unlike most other bivalves, the giant clam harbours symbiotic zooxanthallae (a kind of single-celled algae) in its fleshy body. The zooxanthalae produce food through photosynthesis. To maximise its "farm", the clam exposes these alga to the sunlight by facing the shell opening (and thus the body containing the algae) to sunlight.
Bivalves are among our favourite seafood. These include Ark clams (better known as 'see ham'), 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.
Calamity Clam: 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.
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.
Adopt a unique Singapore star and learn more about Cyrene Reef, a major Knobbly Nursery!
