A comparative study of the mechanical properties of Mytilid byssal threads

Abstract

SUMMARYMytilid bivalves employ a set of threads (the byssus) to attach themselves to both hard and soft substrates. In this study, we measured the mechanical properties of byssal threads from two semi-infaunal mytilids (Geukensia demissa Dillwyn and Modiolus modiolus Linnaeus) and two epifaunal mytilids (Mytilus californianus Conrad and Mytilus edulis Linnaeus). We compared material properties with and without the assumption that changes of length and area during tensile testing are insignificant, demonstrating that previous researchers have overestimated extensibility values by 30% and may also have underestimated strength values. We detected significant differences in thread properties among tested mytilid species, contrary to previous findings. Threads from semi-infaunal species were significantly thinner than those from epifaunal species, perhaps to allow the production of a greater number of threads, which form a dense network within the substrate. Geukensia demissa threads were weaker than those of the other species, and had a significantly lower stiffness at failure. Modiolus modiolus threads were significantly stiffer than M. edulis threads but also significantly less extensible, suggesting a trade-off between stiffness and extensibility. The only thread property that did not show significant differences across species was toughness -even when byssal threads differ in strength or stiffness, they seem to absorb similar amounts of energy per unit volume prior to failure. This study reveals notable differences between the byssal thread properties of different mytilid bivalves and provides a reliable and thorough methodology for future comparative studies. Supplementary material available online at

Interspecies comparison of the mechanical properties and biochemical composition of byssal threads

“…In addition, I. bicolor sampled in a marine environment in Brazil produces homogenous filaments without distinction between the proximal and distal sections. Accordingly, the structural specialization of the two sections is likely to be specific for marine mytilid species, confirming the observations of Brazee and Carrington (2006) and Pearce and LaBarbera (2009).…”

“…Variations in mechanical performance have also been observed among different mussel species (e.g. Bell and Gosline, 1996;Brazee and Carrington, 2006;Pearce and LaBarbera, 2009), and the biochemical structure of the byssal threads could potentially explain these interspecific differences . In particular, byssal fibres contain non-collagenous amino acid residues Sagert and Waite, 2009) and metal ions (Tsukada et al, 1995) that could differ between species.…”

Biomechanics of byssal threads outside the Mytilidae: Atrina rigida and Ctenoides mitis

“…As shown in Table 1, the overall ANOVA for diameter, which included mytilid species from a previous study (see Pearce and LaBarbera, 2009), revealed a clear division between semi-infaunal and epifaunal species: the threads of all epifaunal species were significantly thicker than those of all semi-infaunal species (Scheffe test: P<0.012). Threads of epifaunal species were 2-4 times the diameter of threads of infaunal species.…”

“…(2) 'True' stress (σ T ) assumes constant volume: ν=0.5, and σ T =σ E exp(ε T ). We conservatively assumed constant volume rather than constant area (see Pearce and LaBarbera, 2009). A number of different mechanical properties can be determined from the stress-strain curve.…”

“…Because A. rigida is semi-infaunal, data on the properties of its threads can be compared with data from our recent study that includes two semi-infaunal mytilids, Modiolus modiolus Linnaeus and Geukensia demissa Dillwyn [see accompanying paper (Pearce and LaBarbera, 2009)]. The properties of epifaunal C. mitis threads can similarly be compared with those of epifaunal mytilids like Mytilus californianus Conrad and Mytilus edulis Linnaeus (Pearce and LaBarbera, 2009). …”