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An adult edible mussel is 2 to 5 inches long. It is typically blue or purple, sometimes brown or with radial dark brown or purple markings. Near the front end is the hinge and the elastic ligament that pushes the shell open when the muscles inside relax. There is one of the adductor muscles at each end, running from one valve to the other, that at the hind end being largest and strongest. When the mussel is submerged, its long brownish foot can be seen protruding from between the valves. The foot is used for moving about, as well as for making the tough protein (byssus) threads with which the mussel is normally anchored.
Distribution and Habitat
This species is distributed throughout most of the temperate and subtropical coasts of the northern hemisphere, where it is found in very large numbers on the shores and down to about 30 feet, principally near low tide mark, on rocks, pier piles or stones lying in mud or gravel. Mussels can live in sea water considerably diluted with fresh water, and can tolerate even further dilution for a while by clamping their shells.
Concentrations of up to 16,000 individuals per square foot are not uncommon, and in some areas there may be 20 million in a mile stretch.
Adult mussels rarely move about, but the young do so fairly often. Movement can be done freely by means of the long extensible foot, or by throwing out fresh byssus threads to pull themselves along with and then cutting loose the old threads.
Mussels feed by straining fine particles from the water together with small floating organisms like diatoms. They do this by means of four "gill plates," two on each side of the foot. Each gill plate bears countless cilia that beat in such a way as to draw water in at the hind end of the body, through the gill plates and out through a special opening at the rear. As the food particles hit the gills they are caught up in mucus given out by the gills and are driven with it by cilia to the edges. Here the mucus enters special ciliated channels and moves forwards towards the mouth. The particles are sorted by smaller gill-like flaps at the mouth. Mussels are so efficient at filtering microscopic particles out of the water that they can actually purify the water around them.
The breeding season varies with location and water temperature. A single female may release 5 to 12 million eggs at a time. The eggs are grouped together in short pink rods when they are shed, but these rods break up upon sinking to the bottom. The eggs are fertilized by sperm released into the water.
Free-swimming ciliated embryos emerge after about 5 hours. The shell appears towards the end of the second day. At this stage, the larvae feed and propel themselves by means of ciliated lobes projecting from their shells. The free larval stage lasts about a month and then the mussel settles to the bottom. Once settled, young mussels may glide about rapidly on their ciliated feet or may simply anchor themselves with the byssus threads. Movement becomes much more restricted once the mussel reaches about 1/6 inch in length.
Starfish are the mussel's principal enemy, aside from man. Dog whelks, herring gulls, oystercatchers, ducks, walruses, and some fish also prey on mussels.
Edible mussels are considered large enough to eat when just over 2 inches long and two or more years old. They are generally collected by dredging or raking, but in some parts of the world they are "farmed."
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This page was last updated on October 30, 2017.