THE MUSSEL FILE
They don't get about much but, hey, mussels have their own excitements, says Paul Naylor
A mussel will attach itself to a rock and its neighbours using "byssus" threads, planted like guy-ropes by its extendible foot. Marine engineers would love to replicate the amazing glue that fixes these threads so strongly to slippery underwater surfaces. These threads form the stringy "beard" when a meal of mussels is not prepared properly.
The mechanisms that make mussels super-efficient filter-feeders also lead to them accumulating toxic substances from the water. Analysis of mussels often forms an important part of studies on pollution. As well as pollutants, mussels can concentrate toxic algae and bacteria, so only those from reliable sources should be eaten.
A mussel can offer only passive resistance to most predators, but it takes an active stance against the "driller-killer" dog-whelk. While the dog-whelk is preoccupied with drilling, the unwilling victim and its neighbours may attach so many byssus threads to its shell that the predator is turned over, or at least immobilised. It will eventually starve to death.
It is not only humans who think mussels make a tasty meal. Starfish pull them open for a feast, dog-whelks will spend days slowly drilling through the shell, and crabs and lobsters smash them open. Some tiny crabs even ride into the mussel's interior on its feeding current and stay living in a permanent larder!
An open mussel can be seen to have two siphons (tubes) sticking out through its shell opening. Water is pumped in through the frilly-edged tube and out through the smooth one, after planktonic food has been extracted by the gills. A single mussel can pump 2 litres of water in an hour.
The thickness of its shell shows how much wave action a mussel has battled during its development. Mussels found or farmed in sheltered water may have very thin translucent shells, while those on surf-battered shores have heavyweight protection.