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

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.