Fig. 1: Overview of byssus cuticle formation and structure.

a Marine mussels (Mytilus edulis) synthesize byssal threads using an organ known as the foot. b CT image of the distal region of a mussel foot highlighting the foot groove, in which the thread forms. c Schematic of a foot transverse cross-section from a region of the foot indicated by white dashed line in c showing location of specific glands in which thread-forming proteins are stockpiled. d Trichrome stained transverse section of foot gland tissue showing the core (blue) and cuticle (red) secretory vesicles. Scale bar = 10 µm. e Trichrome stained longitudinal thread section captured during induced formation showing the core and formation of the cuticle. Clusters of cuticle secretory vesicles coalesce and are partially spread over the core surface creating the cuticle. Scale bar = 4 µm. f Trichrome stained longitudinal section of a native distal byssal thread fixed on a glass slide. Scale bar = 4 µm. g SEM image of a native distal thread surface with false coloring to differentiate the cuticle (red) and exposed core (blue). Scale bar = 1 µm. h TEM image of a thin osmium stained transverse cross-section of a native distal byssal thread with false coloring to indicate the cuticle and fibrous core. Scale bar = 500 nm. i The cuticle is known to be partially comprised of a protein called mefp-1, with an extended domain made of decapeptide repeats containing 3,4-dihydroxyphenylalanine (DOPA), which is believed to be coordinated to metal ions including vanadium and iron. Panels b, d–f are adapted from ref. ¹³ under the Creative Commons License.