Wild Harris

Link to Rocky Shore Animals

Mussels are molluscs with 2 shells, a group known as bivalves. Mussels are filter feeders - when the tide is in they open their shells and protrude 2 small tubes (called 'syphons'). Water is sucked in through one tube, the food it contains is filtered out, and the cleaned water is ejected from the other tube. Mussels are attached to the rock by incredibly strong byssus threads Ð liquid string that is squirted out and hardens on contact with the outside world. Mussels breed by releasing sperm and eggs into the water Ð the eggs hatch into free swimming larvae which eventually settle on a suitable surface. Mussel farmers take advantage of this by putting out ropes in spring and autumn, providing the mussel larvae (called 'spat') with somewhere to settle and grow. Cultivated mussels are brown and stripey, as opposed to the blue mussels you see on the rocks, although they are the same species. Their shells are thinner too, which reflects the fact that the mussels on the shore are under much greater stress, with drying out, battering by waves and attacks by predators among problems of daily life which cultivated mussels do not have to endure - they have an easy life in comparison with food available 24 hours a day, rather than just when the tide is in.

Mussels feeding underwater - the 2 syphons are clearly visible    (Photo: Sue Scott)

Despite its great strength, byssus threads are no match for the power of big waves. On exposed rocky shores, particularly on the west coast, mussels never grow to full size. Once they reach a certain size they can no longer resist the force of the waves and are ripped off the rock by storms. That is why you often see carpets of tiny mussels up to a centimetre or so in length, but no big ones growing on rocks in places such as Horgabost or Scarista.  

No old-timers here - small mussels growing on exposed rock at Horgabost    (Photo: Paul Tyler)

The mussel has many enemies. Birds such as oystercatchers will chisel them off the rock and crack them open, while gulls and crows will often drop a mussel onto rocks or the road to smash it open. When the tide is in the mussel is attacked by starfish and dogwhelks, and eider ducks will dive down and swallow mussels whole before crunching them up in their powerful throat muscles (called the gizzard). You will often see pulverised mussel shells on the shore where eider dicks have rested, passing out the shell fragments in their droppings. Mussels are not completely helpless however, they can sometimes use their sticky byssus threads to fight back against dogwhelks. If they succeed in snaring one it will be trapped under the hardened glue-like threads and will never escape Ð if you look carefully you may find a dogwhelk trapped amongst the mussels that has met this sticky end. 

by Paul Tyler

Link to Rocky Shore Animals