Anyone preparing mussels for moules marinières [1] 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to the substratum and the foot then withdrawn, each proteinaceous thread hardening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 [2]. In a study in
the Ebro Delta in Spain, Prado et al [3] 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 [4].
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 [5]. 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 [6] 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.
[3] 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
[4] 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.
[5] 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.
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