Anyone preparing mussels for moules marinières  is familiar with the beards that need to be pulled free of the closed shell. These beards are more properly termed byssus threads and they are the means by which mussels attach to substrata, allowing them to withstand the effects of waves and water currents. Unlike snails, bivalves do not use their foot for gliding locomotion and, instead, it provides an effective burrowing organ or, in mussels and their relatives, a means of secreting threads and then holding them in tension. The threads are produced from a gland on the foot and pass along a groove, being attached to the substratum and the foot then withdrawn, each proteinaceous thread hardening very rapidly and thus ensuring secure attachment. The bivalve shells are opened by the elastic hinge when the muscles that hold the valves tightly together are relaxed and the mussels then feed on the good supply of suspended food particles brought by tidal flows.
Pinna nobilis, commonly called the pen shell, is a large bivalve that can grow to 1 metre in length and it requires strong attachment by byssus to avoid being moved by currents and to hold the animal upright to allow efficient feeding (an example of a pen shell is shown in the video clip above). Pen shells are found on soft bottoms but are most commonly associated with beds of seagrasses, underwater flowering plants that are anchored into sediments by means of rhizomes . In a study in the Ebro Delta in Spain, Prado et al  concluded that:
Seagrass beds have been considered to be the most suitable substrate for P. nobilis, since their rhizomes allow a complex connection between byssus filaments and the sediments. In fact, although individuals were detected in both vegetated and unvegetated areas, higher abundances (by ca. 40%) were observed in areas with 80%-100% cover, thus suggesting that dense meadows may favour the highest abundances of individuals.
In addition to providing ideal locations for attachment, seagrass beds are also highly productive marine habitats and it is likely that these provide an abundance of food for the growing bivalves. The large number of shells also provides good conditions for the colonisation by plants and animals; shells from dead bivalves transplanted experimentally into areas of bare soft sediment readily became colonised by a diverse community of marine creatures .
The natural community of Pinna nobilis, and its associated plants and animals, could not develop if it were not for byssus, recognised increasingly as an important biomaterial . Byssus consists mainly of collagen, a fibrous protein that shares many characteristics with fibroin, the main constituent of insect silk. The most well known insect silk is that produced by silkworms to form a cocoon in which to pupate. The pupae and cocoons are harvested and it is important that these are placed into boiling water to kill the pupae before the adult insects emerge to cut through the threads. The long fibres are unwound and, as they are produced by being exuded through an aperture, they are uniform in both diameter and consistency. Byssus threads, on the other hand, vary in diameter and they are usually elliptical in cross section, a result of their method of production from the byssal gland and byssal groove on the bivalve foot.
It may come as a surprise to know that the byssus of Pinna nobilis is collected, carded and spun to produce a thread used in weaving, knitting and embroidery. Recently, this use was highlighted in an article on the BBC web site  that included a portrait of Chiara Vigo, a Sardinian collector, weaver and embroiderer and the strong feeling and respect that she has for byssus. The same attitude is shown by Felicitas Maeder who has a wonderful web site that is packed with historical and scientific information, as well as having an illustrated inventory of many items made from sea silk. If you are interested in the Natural History of this unusual material, an historian of textiles, or an enthusiast for weaving, knitting and embroidery please visit http://www.muschelseide.ch/en/projekt.html. It is a fascinating site and makes one wonder about the evolution of byssus and the ingenuity of humans who saw it as a useful fibre, both for clothing and ornament.
 Patricia Prado, Nuno Caiola and Carles Ibáñez (2014) Habitat use by a large population of Pinna nobilis in shallow waters. Scientia Marina 78:555-565
 Lotfi Rabaoui, Walid Belgacem, Dorsaf Ben Ismail, Lamjed Mansour and Sabiha Tlig-Zouari (2015) Engineering effects of Pinna nobilis shells on benthic communities. Oceanologia 57:271-279.
 J. Herbert Waite and Christopher C. Broomell (2012) Changing environments and structure-property relationships in marine biomaterials. The Journal of Experimental Biology 215:873-883.