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Filter feeding

Many bivalve molluscs, for example the mussel Mytilus edulis, feed on particles suspended in the surrounding water. Mussel
They do this by filter feeding.
The mussel lives in the intertidal zone, attached to a solid surface by threads secreted by the muscular foot, so it is not able to move around like some related molluscs. When it is covered by seawater, the two shells open slightly, allowing water to enter the body of the mollusc, which is protected by the shell when the tide is out. From this extend a pair of short tubes, also called siphons. A current of water is drawn in through one opening, then across the gills and out through another opening. This movement is powered by the combined effect of many cilia: hair-like projections from cells lining the spaces inside the mussel's shell.

Mussel gills
The gills hang down like net curtains, trapping small particles as well as extracting dissolved oxygen from the water. Cilia on the filaments send a current of mucus down each strand and then onward towards the mouth. Any particles of a suitable size which are trapped are then digested.
Incidentally, this is not unlike the means by which foreign particles including bacteria are trapped in mucus in the human respiratory system, and then taken into the stomach and digested!

What is likely to be the main type of particles extracted by mussels from the water?
plankton (phytoplankton - especially single-celled algae), also possibly bacteria and sediment (mud?)
Why do mussels need to spend most of the time drawing water over their gills?
1 to extract the (limited) nutrition from the water
2 to take in oxygen

How is the structure of their gills adapted to perform their functions?
large surface area, narrow gaps allowing water to permeate slowly through
Why do mussels close their shells?
1 to prevent drying out when exposed, and
2 to resist predators

Before being offered for sale as human food, it is customary to place mussels in a tank of clean seawater for about a day. Why is this?
to allow any trapped particles - which may include bacteria - to (gradually!) move off the gills and be digested by the mussels - a long drawn out process due to the slow speed of ciliary movement
When mussels are eaten by humans, it is customary to reject any which are still open before cooking, and any which are still closed afterwards. Why is this?
Normally, live mussels draw the two shells shut when handled, but after being cooked their muscles do not hold the shell shut!
The reject mussels inside the shell are likely to be dead (and how long ago?), so they do not react to handling, or to the heat of the cooking process.

Related points of interest:
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