The byssus is the set of proteinaceous threads widely used by bivalves to attach themselves to the substrate. Previous researchers have focused on a single byssate family, the Mytilidae. However, the properties of byssal threads from species outside this family are of interest – first,because evolutionary patterns are only detectable if species from a range of taxa are examined, and second, because recent biomimetic research efforts would benefit from a wider range of mussel glue' exemplars. In the present study, we measured the mechanical properties of the byssal threads of two species outside the Mytilidae, the pen shell Atrina rigida Lightfoot and the flame scallop' Ctenoides mitis Lamarck. The mechanical properties of their byssal threads were significantly different from those of mytilids. For instance, the byssal threads of both species were significantly weaker than mytilid threads. Atrina rigida threads were significantly less extensible than mytilid threads, while C. mitis threads exhibited the highest extensibility ever recorded for the distal region of byssal threads. However, there were also interesting similarities in material properties across taxonomic groups. For instance, the threads of A. rigida and Modiolus modiolus Linnaeus both exhibited a prominent double-yield behavior, high stiffness combined with low extensibility, and similar correlations between stiffness and other thread properties. These similarities suggest that the thread properties of some semi-infaunal species may have evolved convergently. Further research on these patterns, along with biochemical analysis of threads which exhibit unusual properties like double-yield behavior, promises to contribute to both evolutionary biology and materials engineering.

According to Tertullian (ca. 155–230 CE), it was not always possible to find sufficient earthly textiles; luckily, it also proved possible to fish for clothes'. He goes on to say that fleeces also come from the sea' (Tertullian, 2005). Though he may have had his tongue held firmly in cheek, Tertullian's fleeces of the sea' were the byssal threads of Pinna nobilis Linnaeus which,at least by the 18th century, actually were sometimes woven into hats and gloves (Maeder, 2002).

Despite such interesting historical uses, no work in the last 50 years has investigated the mechanical properties of pinnid byssal threads [the most recent study is by Lucas and colleagues(Lucas et al., 1955)]. Instead, most byssal research to date has focused on the byssal threads of mytilids (mussels and their near relatives). This disproportionate emphasis on the Mytilidae is likely due to the presence of mytilids in easily accessible intertidal areas, but it fails to capture the widespread occurrence of byssal attachment among the Bivalvia and the potential diversity in byssal function and properties.

Although the byssus first evolved to aid in post-larval dispersal and settlement (Yonge, 1962; Stanley, 1972; Sigurdsson et al., 1976; De Blok and Tan-Maas, 1977; Lane et al., 1985), a recent catalogue of tropical marine bivalves revealed that about a quarter of the genera surveyed are byssally attached as adults(Todd, 2001). In fact, the only pteriomorph superfamilies without byssate adult representatives are characterized by a different attachment strategy – cementation(Márquez-Aliaga et al.,2005; Bieler and Mikkelsen,2006). Although there has been some research into non-mytilid byssal thread chemical composition, Dreissena polymorpha Pallas is the only bivalve from outside the Mytilidae whose threads have been the subject of biomechanical investigation(Jackson et al., 1953; Pujol, 1967; Pujol et al., 1970; Mascolo and Waite, 1986; Brazee and Carrington,2006).

Biomechanics researchers have thus restricted their study of an attachment structure that appears in every pteriomorph order to only a single family. This narrow focus is a problem that should be remedied, for two reasons:first, because of the phenomenon of phylogenetic non-independence, many interesting evolutionary questions can only be answered through comparative work across a wide range of taxa; and second, as researchers have pointed out the possible engineering applications of simulated mussel glue', knowledge of a wider range of byssal thread compositions and properties is likely to yield rich insights into potential technological applications(Waite et al., 2005; Waite, 2008).

With more biomechanical data on the threads of both epifaunal and semi-infaunal bivalves from a variety of pteriomorph orders, one would be able to sort out whether life habits are correlated with the mechanical properties of byssal threads. This is an especially interesting question, as both endobyssate (infaunal or semi-infaunal with byssal attachment) and epibyssate(epifaunal with byssal attachment) groups declined during the Paleozoic and Mesozoic, perhaps due to increased predation pressure(Stanley, 1972; Stanley, 1977; Vermeij, 1983; Skelton et al., 1990; Aberhan et al., 2006; Harper, 2006). The surviving byssate groups live in a variety of environments, and their survival probably involved adjustments of their thread mechanics. There are certainly interesting chemical differences between the threads of different bivalve groups, which may translate into differences in mechanical properties. For example, mytilid threads are collagenous, whereas the threads of pinnids,anomiids and dreissenids are not (Jackson et al., 1953; Pujol,1967; Pujol et al.,1970; Mascolo and Waite,1986; Brazee and Carrington,2006).

We investigated the mechanical properties of the byssal threads of two bivalve species from two orders outside the Mytiloida: the pen shell Atrina rigida Lightfoot (Pterioida: Pinnidae) and the flame scallop'Ctenoides mitis Lamarck (Limoida: Limidae). There is some debate in the literature about the breakdown of pteriomorph orders; we have adopted the classifications of Bieler and Mikkelsen rather than those of Matsumoto but these authors all agree that limids and pinnids belong to different orders(Matsumoto, 2003; Bieler and Mikkelsen, 2006). Thus, although A. rigida and C. mitis are more closely related to one another than to mytilid species, they are still only distantly related. This study does not seek to compare mytilid threads withnon-mytilid' threads in general but instead compares mytilid threads with the threads of two unrelated species from outside the Mytilidae.

The two species under investigation have quite different life habits– A. rigida is semi-infaunal, and usually lives in protected subtidal or low intertidal areas with most of its shell buried in muddy or sandy sediment; C. mitis, in contrast, is epifaunal, and normally lives byssally attached in crevices of reefs and ledges, swimming only if disturbed (Stanley, 1970; Mikkelsen and Bieler, 2003). 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 californianusConrad and Mytilus edulis Linnaeus(Pearce and LaBarbera,2009).

Atrina rigida Lightfoot specimens were ordered from Gulf Specimen Marine Laboratories (Panacea, FL, USA), and kept in a tank at room temperature(approximately 18°C). We buried them as deeply as possible (∼5 cm) in the calcareous gravel in the aquarium, and lightly supported the exposed shell to prevent toppling. Ctenoides mitis Lamarck specimens were ordered from Ward's Natural Science (Rochester, NY, USA) [the Lima scabra'specimens obtained from Ward's were identified as C. mitis rather than Ctenoides scaber Born, following Mikkelsen and Bieler(Mikkelsen and Bieler, 2003),on the basis of their white tentacles and greater number of radial ribs in the shell]. The C. mitis were kept in individual enclosures (polyethylene freezer containers with sections of the walls replaced with plastic mesh) in the same tank as the A. rigida specimens. Tank salinity was maintained at approximately 31–32 p.p.t. by adding either tap water or Instant Ocean® (Aquarium Systems, Inc., Mentor, OH, USA) sea salt mixture as necessary. Animals were fed daily on an artificial phytoplankton substitute(Kent Marine®, PhytoPlex™, Franklin, WI, USA), producing byssal threads and surviving without obvious ill effects for over 2 months.

We measured the shell length of all animals using digital calipers. To harvest threads from the C. mitis specimens, we opened each enclosure and disturbed the animal inside, causing it to release its threads and swim away. We then lifted the enclosure out of the tank and removed the thread plaques from the plastic walls using a razor blade. To harvest the A. rigida threads, we carefully dug out each animal and transferred it underwater into a smaller tray, which was then lifted out of the tank. We snipped each thread at the proximal end using iris scissors; the plaques usually remained attached to a small piece of gravel. All samples were stored in salt water (31–32 p.p.t.) at 5°C until testing.

Thread mechanical properties were measured using a custom-built tensile tester. The apparatus consisted of a lower grip at the bottom of a Plexiglas tank and an upper grip that could be displaced by turning a crank on a dovetail slider (Velmex, Bloomfield, NY, USA; Model A6027K1M-S6). The upper grip was attached to a 10 lb (∼45 N full scale) force transducer(OmegaDyne®, Sunbury, OH, USA; Model LC703-10). The four strain gauges in the transducer were set up as a full Wheatstone bridge supplied with a constant 5V excitation; the excitation and amplification of the voltage output of the bridge circuit were supplied by a bridge amplifier (Vishay®Micro-Measurements, Shelton, CT, USA; Model 2120A). We calibrated the voltage output of the amplifier to yield a voltage-to-force conversion factor. A linear variable differential transformer (Pickering Controls, Plainview, NY,USA; Model 7308-X2-A0) powered by a constant 5V DC from an external power supply converted the displacement of the upper grip into a voltage, which could then be converted back into a displacement value following calibration. The voltage was digitized using a GW Instruments (Somerville, MA, USA) Model 100B analog-to-digital converter.

We limited each testing run to 10–15 byssal thread samples to minimize drying during preparation. Between one and six byssal threads from each individual were tested, with a total sample of about 20–25 threads per species. To ensure proper gripping, we sandwiched each end of each thread between two small squares of 100% rag paper using a drop of cyanoacrylate adhesive (Loctite® Gel Control' super glue; Henkel Consumer Adhesives,Inc., Avon, OH, USA) to maximize adhesion. Before testing, we measured the length of each byssal thread sample with digital calipers.

Prior to each test, we secured one end of the thread in the upper grip of the tester and the other end in the lower grip at the base of the tank; the entire thread was immersed in sea water for the duration of the test. The tank was filled with salt water from the 5°C tank (salinity 31–32 p.p.t.)during all tests. Once the thread was secured, we initiated data capture in the application instruNet World Mac (GW Instruments) and displaced the upper grip at approximately 0.5 mm s–1 until thread failure. At the outset of the test, the samples were slack; the beginning of the tensile test was taken to be the point at which there was a non-negligible force on the sample.

Following testing, we inspected the broken ends of each byssal thread under a dissecting microscope to assess the failure mode (e.g. smooth break,fraying, etc.). We took digital photographs (Nikon D100 camera back) of each broken end through the dissecting microscope at approximately ×100, and measured thread diameter using ImageJ (NIH). Following previous work,cross-sections were assumed to be circular even though byssal threads are often elliptical in cross-section (Brazee and Carrington, 2006). Initially we measured the minimum thread diameter before testing, but discovered that the samples invariably broke at a different (and wider) location, presumably a cryptic weak point in the structure. Thus the diameter at failure was used in all calculations of strain to ensure consistency, although this does result in an underestimate of the inherent strength of byssal thread material.

The stress (force per unit area) and strain (displacement per unit length)for each test were plotted in Microsoft® Excel® to produce a stress–strain curve. Because strains were always in excess of 50%, it was clear that byssal thread cross-sectional area and length changed significantly during the test. Thus instead of engineering' strain(ϵEL/L0, where Lis length and subscript 0 indicates initial) we used true' or logarithmic'strain [ϵT=ln(L/L0)], which does not assume constant length or constant volume. Stress is always calculated assuming a certain value for Poisson's ratio (ν), which is defined in this case as the negative of the ratio of transverse to axial strain. The instantaneous diameter of the thread is given by d=d0exp(–νϵT). There are two possible approaches. (1) Engineering' stress (σE)assumes constant area: ν=0, thus d=d0 andσ E=F/A0 (where F is force and A is cross-sectional area). (2) True' stress(σT) assumes constant volume: ν=0.5, andσ TEexp(ϵT). 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. In almost all cases, there was a sharp drop in stiffness at a characteristic stress level – the yield stress. The slope of the stress–strain curve represents the stiffness of the material;thread stiffness was determined both for the initial loading of the thread and at thread failure. The stress and strain at failure are termed strength and extensibility, respectively. Finally, by fitting a polynomial to the stress–strain curve and integrating over the total strain, the area under the curve was determined; this area is the energy absorbed per unit volume, or the toughness of the material.

A small percentage of the byssal thread stress–strain curves for each species differed dramatically from the characteristic shape of the curve for that species. In almost all cases, the discrepancy appeared to result from splitting and fraying of the thread prior to failure; we did not include the data from these samples in the analysis.

We analyzed the data using StatView 5.0 (SAS Institute, Cary, NC, USA). First, we conducted an ANOVA on the threads of each individual, followed by an ANOVA of all threads of each species, split by individual. Because no significant differences were detected, we then pooled the individuals within each species and ran an overall ANOVA, split by species. We performed post-hoc Scheffe tests to determine the specific differences detected by the ANOVA. We also ran a Kruskal–Wallis test (a non-parametric version of a standard ANOVA), as a normal distribution of the data could not be assumed. Finally, we produced a partial correlation matrix for each species to determine whether any two of the measured variables were significantly correlated when all other variables were held constant.

For all measured variables, ANOVA revealed no significant differences between threads of a given individual or between individuals of a given species; thus the threads for each species were pooled in the overall analysis.

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. However, the threads of the epifaunal C. mitis were significantly thinner than those of one of the three other epifaunal species, M. californianus (Scheffe test: P=0.007). While the shells of the mytilid species fell into a similar size range (60–70 mm on average), those of C. mitis were somewhat smaller and those of A. rigida were much larger.

Table 1.

Byssal thread diameter and shell length

SpeciesThread diameter (μm)Range in shell length (mm)
Geukensia demissa 37.6±2.3 (32)A 62.2–76.9 [69.1±1.6] (11)
Modiolus modiolus 46.3±2.4 (28)A 49.2–72.3 [57.9±2.0] (12)
Atrina rigida* 54.2±3.3 (20)A 119, 131 (2)
Ctenoides mitis* 103.5±13.2 (30)B 43.3–59.3 [49.4±1.7] (10)
Perna canaliculus 129.7±8.2 (34)B,C 50.0–70.0 [62.3±2.7] (7)
Mytilus edulis 132.3±6.0 (55)B,C 58.6–88.4 [72.6±3.4] (10)
Mytilus californianus 149.6±6.6 (30)C 49.4–91.4 [70.0±6.1] (7)
SpeciesThread diameter (μm)Range in shell length (mm)
Geukensia demissa 37.6±2.3 (32)A 62.2–76.9 [69.1±1.6] (11)
Modiolus modiolus 46.3±2.4 (28)A 49.2–72.3 [57.9±2.0] (12)
Atrina rigida* 54.2±3.3 (20)A 119, 131 (2)
Ctenoides mitis* 103.5±13.2 (30)B 43.3–59.3 [49.4±1.7] (10)
Perna canaliculus 129.7±8.2 (34)B,C 50.0–70.0 [62.3±2.7] (7)
Mytilus edulis 132.3±6.0 (55)B,C 58.6–88.4 [72.6±3.4] (10)
Mytilus californianus 149.6±6.6 (30)C 49.4–91.4 [70.0±6.1] (7)

Values given are means ± s.e.m., followed by the sample size(N). Data for species marked with an asterisk are from this study. All other data are from our previous study(Pearce and LaBarbera, 2009). There were significant differences in thread thickness between species (ANOVA: P<0.0001; Kruskal–Wallis: P<0.0001). Values marked with the same superscript letter are not significantly different from one another (Scheffe test). Each of the semi-infaunal species – first three rows – had significantly thinner threads than each of the epifaunal species – last four rows (Scheffe test: P<0.0120)

True' stress and strain were used to construct the stress–strain curves for all of the byssal thread samples. The curve of a representative byssal thread from each species is given in Fig. 1. Mytilid stress–strain curves from a previous study(Pearce and LaBarbera, 2009)have been included for comparison. The curve for C. mitis (green)exhibits a dramatically different shape from those of other threads examined to date – it has an early yield point and then a very long region of relatively uniform, low stiffness, finally stiffening slightly and breaking at an extremely high strain.

Strikingly, the A. rigida curve(Fig. 1, yellow) exhibits two distinct yield points. Thus A. rigida threads have a stress–strain curve similar to those of M. modiolus(Fig. 1, light blue), which display the same double-yield behavior(Fig. 2). Atrina rigida and M. modiolus, both semi-infaunal species, have threads that yield twice before failure, while the threads of all tested epifaunal species exhibit only a single distinct yield point.

In terms of byssal thread mechanical properties, C. mitis and A. rigida differed significantly from each other as well as from species within the Mytilidae. As Fig. 1 suggests, the threads of C. mitis were consistently weaker and less stiff than those of other species, often significantly so(Table 2). In addition, C. mitis threads yielded at a significantly lower stress than all other threads tested. However, despite their low strength and stiffness, C. mitis threads proved highly extensible, with an average final strain of 81% (Table 2). This extensibility was significantly greater than that of all other byssal threads tested, which ranged between 44% and 67%. Nevertheless, even with this higher strain to failure, C. mitis threads were significantly less tough than those of most mytilid species due to their low strength(Table 2).

Table 2.

Species (N)Yield stress (MPa)Strength (MPa)Initial stiffness (MPa)Final stiffness (MPa)ExtensibilityToughness (Jm–3)
Atrina rigida (13) 24.5±2.8A,B 90.2±12.8A 609.2±86.0A,B 167.8±33.7A 0.444±0.033A 24.1±4.3A,B
Ctenoides mitis (18) 5.2±0.7C 55.1±06.0A 101.6±18.4C 210.0±30.3A 0.805±0.031B 15.9±2.0B
Geukensia demissa (19) 23.9±4.2B 140.8±18.7A,B 324.7±60.8A,B,C 319.1±45.2A 0.637±0.018C 43.3±5.6A,B,C
Modiolus modiolus (20) 35.5±5.8A,B 287.8±35.6C 593.3±94.6B 1039.6±129.0B 0.571±0.024C 67.4±8.5C
Mytilus californianus (21) 33.0±2.9A,B 215.3±25.3B,C 432.3±45.5A,B 810.0±93.9B 0.640±0.016C 51.7±5.9A,C
Mytilus edulis (25) 44.4±6.6A 216.9±18.8B,C 328.6±30.8A,C 784.4±62.7B 0.669±0.017C 56.9±6.3C
Scheffe test, P-values <0.0327 <0.0369 <0.0475 <0.0034 <0.0302 <0.0347
Species (N)Yield stress (MPa)Strength (MPa)Initial stiffness (MPa)Final stiffness (MPa)ExtensibilityToughness (Jm–3)
Atrina rigida (13) 24.5±2.8A,B 90.2±12.8A 609.2±86.0A,B 167.8±33.7A 0.444±0.033A 24.1±4.3A,B
Ctenoides mitis (18) 5.2±0.7C 55.1±06.0A 101.6±18.4C 210.0±30.3A 0.805±0.031B 15.9±2.0B
Geukensia demissa (19) 23.9±4.2B 140.8±18.7A,B 324.7±60.8A,B,C 319.1±45.2A 0.637±0.018C 43.3±5.6A,B,C
Modiolus modiolus (20) 35.5±5.8A,B 287.8±35.6C 593.3±94.6B 1039.6±129.0B 0.571±0.024C 67.4±8.5C
Mytilus californianus (21) 33.0±2.9A,B 215.3±25.3B,C 432.3±45.5A,B 810.0±93.9B 0.640±0.016C 51.7±5.9A,C
Mytilus edulis (25) 44.4±6.6A 216.9±18.8B,C 328.6±30.8A,C 784.4±62.7B 0.669±0.017C 56.9±6.3C
Scheffe test, P-values <0.0327 <0.0369 <0.0475 <0.0034 <0.0302 <0.0347

Values given are means ± s.e.m. Data for species in the last four rows are from our previous study (Pearce and LaBarbera, 2009). For each material property listed, the null hypothesis of similar values across species was robustly rejected (ANOVA: P<0.0001; Kruskal–Wallis: P<0.0001). In each column, values marked with the same superscript letter are not significantly different from one another (Scheffe test). Because the yield point was not obvious in all tests, only 12 M. edulis, 13 M. modiolus and 20 M. californianus data points were used in the analysis for yield stress

Fig. 1.

Byssal thread stress–strain curves. A typical stress–strain curve was chosen for each species. Curves from a previous study have been included for comparison (Pearce and LaBarbera, 2009). Note that the Atrina rigida curve(yellow), like that of Modiolus modiolus (light blue), has two distinct yield points. The Ctenoides mitis curve (green) shows that these threads had extremely low strength and stiffness but were very extensible.

Fig. 1.

Byssal thread stress–strain curves. A typical stress–strain curve was chosen for each species. Curves from a previous study have been included for comparison (Pearce and LaBarbera, 2009). Note that the Atrina rigida curve(yellow), like that of Modiolus modiolus (light blue), has two distinct yield points. The Ctenoides mitis curve (green) shows that these threads had extremely low strength and stiffness but were very extensible.

Table 3.

Selected coefficients from partial correlation matrices

Species (N)Strength–toughnessYield stress–initial stiffnessStrength–final stiffnessToughness–final stiffnessInitial stiffness–final stiffness
Atrina rigida (13) 0.793 0.649 –0.068 0.311 0.054
Ctenoides mitis (18) 0.758 0.126 0.883 – –
Geukensia demissa (19) 0.955 0.410 0.706 –0.551 0.514
Modiolus modiolus (13) 0.855 0.620 –0.159 0.549 0.091
Mytilus californianus (21) 0.982 0.579 0.756 –0.641 –
Mytilus edulis (12) 0.952 – 0.769 –0.603 0.563
P-values <0.0001 <0.0001 <0.0025 <0.0004 <0.0446
Species (N)Strength–toughnessYield stress–initial stiffnessStrength–final stiffnessToughness–final stiffnessInitial stiffness–final stiffness
Atrina rigida (13) 0.793 0.649 –0.068 0.311 0.054
Ctenoides mitis (18) 0.758 0.126 0.883 – –
Geukensia demissa (19) 0.955 0.410 0.706 –0.551 0.514
Modiolus modiolus (13) 0.855 0.620 –0.159 0.549 0.091
Mytilus californianus (21) 0.982 0.579 0.756 –0.641 –
Mytilus edulis (12) 0.952 – 0.769 –0.603 0.563
P-values <0.0001 <0.0001 <0.0025 <0.0004 <0.0446

Byssal thread strength and toughness were highly correlated(R>0.75) for all species. Yield stress was well correlated with initial stiffness in most cases. The final stiffness of A. rigida and M. modiolus threads tended not to share the correlations seen in threads of other species (entries in bold). Non-significant correlations are not shown

The correlation found here between life habit (epifaunal vssemi-infaunal) and byssal thread diameter mirrors a similar relationship among mytilids [see accompanying paper (Pearce and LaBarbera, 2009)]. The association between semi-infaunal life habits and small thread diameters found within the Mytilidae might have been due to the fact that tested mytilid species with similar life habits were closely related to one another – although Geukensia is not a sister taxon of Modiolus, the two epifaunal mytilids tested are both in the genus Mytilus (Distel,2000). However, the data on byssal thread diameter presented here for three different pteriomorph orders strengthens considerably the connection between life habit and byssal thread size. And although shell length (i.e. size) is likely involved in determining thread diameter within species(Brazee and Carrington, 2006),it is striking that threads produced by the A. rigida specimens,whose shells measured over 100 mm and were the largest in our study, were significantly thinner than those produced by the much smaller mytilids.

Fig. 2.

Double-yield behavior. These curves illustrate the double-yield behavior of A. rigida and M. modiolus threads. Each curve has two distinct yield points, each represented by a relatively sudden drop in stiffness (the slope of the curve). Following each yield point, the thread becomes stiffer prior to either once again yielding or failing.

Fig. 2.

Double-yield behavior. These curves illustrate the double-yield behavior of A. rigida and M. modiolus threads. Each curve has two distinct yield points, each represented by a relatively sudden drop in stiffness (the slope of the curve). Following each yield point, the thread becomes stiffer prior to either once again yielding or failing.

Lucas and colleagues (Lucas et al.,1955) performed a tensile test on a single P. nobilisbyssal thread submerged in (presumably distilled) water at 20°C and generated a stress–strain curve. They report stress' as force per linear density (grams per denier), a variable commonly used in the textile literature which unfortunately confounds volumetric density and cross-sectional area. Thus, without knowing the volumetric density of the P. nobilis thread tested, it is impossible to calculate its breaking stress as defined in the engineering and biomechanics literature. (A similar problem applies to their reported value for strain rate.) Nonetheless, a comparison of strain values is possible: the P. nobilis thread broke at an engineering' strain of about 56%, a value close to the averageengineering' extensibility of A. rigida threads, 57%; moreover, the yield strain of the P. nobilis thread was in the same range as that of A. rigida threads, although the former yielded at only a single point whereas the latter exhibited a second yield point at a higher strain. It would be unwise to place too much weight on this comparison, however, given that it is based on a single P. nobilis thread that was likely dried– and shipped from Milan to Manchester – before being re-hydrated and strained at an unknown rate.

Interestingly, the threads of A. rigida seem to share certain properties with those of each of the other two semi-infaunal species tested. Like those of G. demissa, its threads have a low yield stress, low strength and low toughness, but like those of M. modiolus, they have a high initial stiffness and a low extensibility(Table 2). Commonalities such as these are easy to explain away as being related to functional requirements:with a large number of threads and external support from the substrate,strength and toughness may be less important; and if the threads have very little give (high stiffness, low extensibility), that could stop predators from easily manipulating the animal. However, post-hocexplanations such as these do not solve the problem of why the three semi-infaunal species diverge in certain of their properties. This problem is impossible to fully address without research into the threads of other semi-infaunal mytilids and pinnids, as well as unrelated species with similar life habits.

As the data presented here demonstrate, pinnid and limid byssal threads have mechanical properties that often differ significantly from those of mytilid threads. Despite these differences, however, our data suggest a connection between the semi-infaunal life habit and certain thread properties,e.g. small diameter and double-yield behavior. A wider survey of bivalve byssal thread properties, both within and beyond the orders examined to date,would provide a wealth of information about connections between thread properties and evolutionary patterns within the Bivalvia. Moreover, with further work on byssal thread composition outside the Mytilidae, connections between microscopic molecular structures and macroscopic material properties might suggest new avenues for ongoing biomimetic research(Yu and Deming, 1998; Yamada et al., 2000; Tonegawa et al., 2004; Waite et al., 2005; Lee et al., 2006; Lee et al., 2007; Waite, 2008). Thus the comparative biomechanics of bivalve byssal threads has much to offer to both evolutionary biology and materials engineering.

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