Mussels Mytilis edulis spend their infancy in the plankton, as swimming veliger larvae (see http://oceanexplorer.noaa.gov/explorations/02mexico/background/mussels/media/bivalve_veliger.html), but then they settle on a substrate and begin a more sedentary life. This (above, x40) is a minute juvenile mussel that anchored itself to a green seaweed frond in a rockpool on a Northumberland beach (Warkworth).
In this slightly older example the tiny shell it developed as a planktonic larva is at its base (pale brown) and since it settled it has produced the vestiges of its future shell, but it has yet to develop much pigmentation, so at this stage the shell is still transparent, creating some interesting possibilities for examining its internal structure under the microscope.... and here (above, x100) you can just make out the comb-like gills inside the pair of shells - they are the row of downward-pointing teeth running along the length of the shell, from bottom left to top right. Take a look at the two videos at the bottom of this post and you'll see how these gills work - they're lined with tiny beating hairs (cilia) that create a powerful current of water over the gills, that extract oxygen and also capture tiny food particles that are wafted into the animal's digestive tract. Somehow (and no one yet knows how) the animal can separate organic food particles from indigestible inorganic grit and debris that is expelled. Even a tiny mussel like this can process a large volume of seawater, thanks to these frantically beating rows of cilia on the gills, here shown in the videos at the bottom of this post at magnifications of x100 and x200.
This still image (above) shows a mussel at a slightly later stage (about 3mm. long), when the shell valves have become pigmented and have lost their transparency. Between the gaping shell valves you can just make out the inhalent and exhalent ports where water is wafted in and squirted out by the ciliary current.
In this side view of the same juvenile mussel (above), the original transparent shell valves of the infant mussel are visible, attached to the pigmented shell that has subsequently developed. They mark the point where the two shell vales are hinged together.
Mussels often settle at very high densities - like these, several months older than the microscopic examples depicted above, packed shoulder-to-shoulder on a rocky outcrop on the shore at Warkworth in Northumberland. Mussels attach themselves to their substrate with a protein glue that sets underwater, to form extremely strong byssus threats that prevent the animal being dislodged, even when pounded by breaking waves in the full fury of a storm. There is a lot of scientific research going on into this protein, for potential medical use – as a glue for repairing broken human bones or in dentistry (see http://www.asknature.org/strategy/4f16bf8321224ea8b146277ccdace9690). For more on the marine biology of mussels, see http://www.marlin.ac.uk/speciesfullreview.php?speciesID=3848
