An unusual interlocked biomineral interface in bivalve shells

Bivalve ligaments are unusual biominerals that have evolved to facilitate the opening of the two shell valves when required. Over a long period of time, their microstructures have been well investigated and are generally considered to consist of an organic lamellar layer and an aragonitic fibrous layer (FL), where the fibers are mutually parallel with a uniform diameter of ca. 100 nm. However, the microstructure of the ligament-valve interface (LVI) has received little attention. Here, using optical microscopy, FE-SEM, powder XRD, and FTIR, we focus on the LVI's microstructure as well as its chemical composition in the clam Cyclina sinensis. We find that: (1) the LVI of this clam consists of discrete bands of coarse aragonite fibers with a variable diameter of 0.4-7.6 μm surrounded by fine aragonite fibers with a uniform diameter of 106.5 ± 1.6 nm; (2) these fine aragonite fibers are continuous with and similar in diameter to the FL fibers (116.8 ± 1.7 nm); and (3) the bands of coarse aragonite fibers penetrate the normal FL and gradually taper, bifurcate, or bend toward its interior, resulting in the LVI showing an unusual finger-joint-like microstructure. We propose that the LVI serves as a transition between the rigid valves and flexible normal FL and primarily functions to prevent the detachment of the FL from the valves during their movement, which could be caused by modulus mismatch between the flexible FL and the rigid valves. In short, this work provides a new model of biomineral interfaces, which may inspire the design and fabrication of advanced material interfaces.