Many species of mollusc live on the sea shore. The bivalves of sandy shores feed on plankton and detritus which they suck in through their flexible siphon. They use their muscular foot to dig into the sand and can pull their shell beneath the surface to avoid predators and desiccation.
Bivalve Siphons
Water is sucked in through a flexible tube (inhalant siphon), passes over the enormous gills and exits through the exhalent siphon. Oxygen is extracted from this water, but the gills also trap small particles of food. For most bivalves this food is plankton, but some species of bivalve have very long siphons which effectively ‘hoover’ particles (detritus) from the surface of the sand.
The Bivalve Shell
As the name suggests the bivalves have two valves to their shell. When closed tight this shell protects the animal from mechanical damage, desiccation and most predators, but when it gapes slightly open the two siphons can poke out from the front end and the muscular foot can be pushed down into the sand.
The Bivalve Foot
The muscular foot is a hydraulic structure – that is to say it changes shape and size as fluid is pumped into it. When digging the bivalve first pokes a wedge-shaped foot out of the shell and gently probes the sand with rhythmic thrusts. Sand is relatively solid for a small animal, but vibration causes it to become almost liquid (this phenomenon is known as ‘thixotropy’). Once the sand takes on liquid properties the foot is pushed into it and changes shape to form an anchor.
Bivalve Digging Cycle
The cycle begins with the foot entering the sand and then forming an anchor (above) and continues with the rapid partial closure of the valves. Closing the valves rapidly squirts water out around the shell and makes it easier for the shortening of the foot (with the anchor still in the sand) to pull the shell down a bit. This sequence – probe rhythmically, extend foot, form anchor, partially close valves, contract foot against anchor – is repeated until the animal is at the required depth below the surface. Digging can be surprisingly efficient and rapid, and some of the ‘razorshells’ can dig through sand so fast that they are difficult to catch. (This is a pity because they taste very good indeed!)
Bivalve – Palial Sinus (NOT Pallial)
Inside the shell (of a dead bivalve) it is easy to see marks where the muscles once attached. The palial sinus (look on the drawing for the ‘sinus palial’) is an indentation which indicates how long the siphons were when the animal was alive. If a bivalve burrows deep into the sand it needs long siphons and therefore has a pronounced palial sinus, but if it is only a shallow burrower the siphons will be shorter and the palial sinus less pronounced. This knowledge allows one to pick up shells on the beach and amaze people with one’s erudition!
Ref: Personal communications from my old Professor, E.R.Trueman, and The Journal of Experimental Biology
Other articles by John Blatchford
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