Most bivalves are adapted to a burrowing existence, living just beneath the surface or deep within the sediment.
Cockles (Cerastoderma edule) are shallow burrowing suspension feeders, feeding on food particles that are suspended in water. As is typical for bivalves with this mode of life, they have a shell with strong hinge teeth, well-developed sculpture (ribs) on the surface of the shell, a large foot and short siphons.
In contrast, Gapers (Mya spp.) are suspension feeders that live in deeper water, buried at a considerable depth beneath the sediment surface. Their siphons are long and partly or completely fused together. If these animals are disturbed they can pull their siphons down beneath the surface of the sediment but only a fraction of the way into the shell. The specimen of the Gaper Mya truncata in the photograph to the left has its siphon preserved.
The Peppery Furrow Shell (Scrobicularia plana) is a burrowing deposit feeder. Instead of filtering food from the surrounding water, deposit feeders get their food from the sediments around them. The Peppery Furrow Shell has long siphons that are separated from each other along their entire length. The inhalant siphon is cast onto the surface of the sediment, pulling in particles from the layer of deposited organic matter to eat. This activity leaves behind tell-tale star-shaped impressions on the surface of mudflats.
Razor shells (Solen and Ensis spp.) are extremely elongate and adapted for rapid descent. In several countries these are a delicacy but they can burrow almost faster than a collector can dig. Experienced fishermen often use salt to entice the razor-shells to surface. The razor shells ( Ensis sp.) are from Hunstanton, Norfolk.
The shell surface of many bivalves is often ornamented by complex sculpture including ridges, bumps, scales and grooves. These are thought to assist the bivalve in rocking motions in the sediment during burrowing.
The shells of some bivalves, such as Pitar (Hysteroconcha) lupanaria, bear long spines, which are thought to help protect the siphons from being bitten off by fish, as well as serving to stabilize the shell in shifting sediments.
How bivalves burrow
Burrowing in bivalves involves foot,shell and siphons. These which operate in sequence to bring about downward movement.
1. The foot first extends downwards in a probing motion and then expands to form an anchor.
2. The siphons close to prevent any water being ejected.
3. The adductor muscles close the valves rapidly, effectively expelling water from the ventral margin.
4. This is immediately followed by contraction of foot retractor muscles, pulling the bivalve downward towards the anchored foot.
5. Finally, the adductor muscles relax and the ligament opens the valves.