title for mussel section of A SNAL'S ODYSSEY
title for learn-about section of A SNAIL'S ODYSSEY

photograph of the inner part of a mussel bed Mytilus californianusCommon features of mussels are an ability to inhabit even the most wave-exposed coastal areas and a tendency to aggregate into beds, both dependent on anchoring by byssus threads. 

NOTE lit. “fine thread” G.  Ancient Mediterranean weavers spun the fine golden threads of pen shells Pinna spp. and weaved them into a highly sought-after cloth known as “sea silk”.  However, there appears to be no record of weavers using the coarser threads of Mytilus in the same way






View inside a bed of mussels Mytilus californianus
showing a tangled array of byssus threads 1X

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Mytilus edulis

  As more research has been done on the closely related mussels Mytilus edulis than on local species, some of this material is included here as introduction and as a source of research ideas.  Studies on the west-coast species MYTILUS CALIFORNIANUS & OTHER MYTILIDS are presented in another section.
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Research study 1

graph showing secretory glands in the foot of a mussel Mytilus for making byssus threadsMussels anchor to the substratum by means of byssus threads produced in a byssus gland located in the base of the foot.  A groove in the ventral part of the foot carries secretions from the byssus gland along the length of the foot to near its tip.  The gland is multiple and produces 2 types of secretion that flow, injection-mold fashion, along the groove (see drawing upper Right).  While the thread is still liquid the tip of the foot presses onto the substratum and forms an adhesive pad.  A protective protein varnish is applied to the thread just before it is released from the foot groove.  It takes just a few minutes to manufacture each thread, and 40-50 threads may be formed in a day.  For a 6-cm shell-length Mytilus edulis, thread thickness ranges from 0.1-0.2mm along its length. The byssus threads can be broken by the mussel pulling on them, or the entire byssus complex may be shed and later regenerated.  This allows the animal to change its position, a behaviour that is more prevalent in juveniles. 

drawing showing processes leading to disc detachment in byssus threads of a musselMost day-to-day stress-failures occur at the attachment disc.  These include photograph of mussel Mytilus trossulus with byssus threads attachedpeeling, tearing within the pad, and loss of adhesion (see drawing lower Right). Allen et al. 1976 J Mollusc Stud 42: 279; Crisp et al. 1985 J Colloid Interface Sci 104: 40.


Mussel Mytilus trossulus with
5 threads attached 1.2X

Research study 2

drawing of stem and thread of byssus in a musselThe threads actually attach to a stem of the gland which projects into the seawater.  Each thread originates from a ring on the stem, and it is at this point that threads can be broken off.  Breakage is said to be by contraction of special byssus retractor muscles that connect to the shell.  However, this does not explain how individual threads can be broken, and for this the foot may be involved.  Each thread consists of 2 parts, a highly extensible wrinkled drawing of disc size of a mussel's byssus thread on different-sized substrataproximal half and a less extensible smooth distal half (see drawing on Right).  An adhesive disc attaches the thread to the substratum.  Smeathers & Vincent 1979 J Mollusc Stud 45: 219 .

NOTE the shape of the terminal pad of a Mytilus thread varies with the nature of the substratum.  On rock and coarse sediment, for example, the pad is flat and expansive; on fine sediment it adopts a more 3-dimensional form for greater adhesion (see drawings on Left).  Meadows & Shand 1989 Mar Biol 101: 219.

Research study 3

photograph of mussel Mytilus californianus, close view of threadsDigestion of threads of Mytilus with pepsin enzyme, followed by electrophoretic separation and identification of the protein and amino acid residues, confirms that the threads are composed of 2 different types of collagen.  The inextensible distal part of the thread is made up of what the authors call Col-D (Collagen-Distal) and the extensible proximal part is made up of Col-P, each with typical collagen amino-acid compositions.  Each molecular fragment comprises 3 identical µ-like chains joined by cross-links.  In Col-D, the stiffer collagen component of the thread, the ends of the chains are comprised of silk-like domains, while in Col-P, the more extensible collagen component, the ends of the chains bear elastic domains.  The 2 collagens grade into one another in the central part of the thread but, while Col-P stops mid-way down the thread, Col-D extends and inter-grades with Col-P in the proximal, extensible part of the thread.  The authors note that a thread functions under the expectation that it be strong under tension as well as shock-absorbing.  They note that collagen is as strong as silk, but the latter is up to 40 times tougher; that is, it has a higher breaking-strain coefficient.  Thus, the presence of silk in Col-D adds toughness to the thread, while its partial presence in Col-P may limit the extensibility of the thread.  Qin & Waite 1995 J Exp Biol 198: 633.

NOTE specimens are M. edulis collected and studied in Delaware

NOTE 35% glycine and 20% proline plus 4-trans-hydroxyproline

Research study 4

schematic drawing of thread and adhesive disc of mussel Mytilus edulis, with the 5 proteins making up the disc shownAs described in Research Study 3 above, the thread is made up of 2 main proteins, each a type of collagen, joined lengthwise.  The proximal end of the thread, that nearest the gland, is the more elastic of the 2 protein types,while the distal end of the thread is a more rigid, stiffer, and stronger protein type.  This structural differentiation is ideal for absorbing energies of waves and water movement, and providing strong anchoring.  The terminal pad is more chemically complex still, and is made up of several different types of protein produced by glands within the distal part of the foot-groove.  Five have been identified in the adhesive pad of the Atlantic mussel Mytilus edulis (see accompanying figure).  The first is a cuticular protein that forms a protective outer coating on both the thread and pad, another 2 are matrix proteins that form a solid foam in the interior of the pad, and the remaining 2 are adhesive proteins that connect the pad to the substratum.  The pad adheres to any solid surface, wet or dry.  Wiegemann 2005 Aquat Sci 67: 166.

NOTE the mechanical properties of byssus threads are similar to those of the elastic collagen components of human tendons and skin

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