Ligament (bivalve)

This interior view of the hinge line of a blue mussel, Mytilidae shows the external ligament, which is dried out and cracked in this specimen
This interior view of the hinge line of a scallop shell Pectinidae shows the internal ligament, located in the resilifer.
Interior view of the hinge ligament of Tridacna derasa

A hinge ligament is a crucial part of the anatomical structure of a bivalve shell, i.e. the shell of a bivalve mollusk. The shell of a bivalve has two valves and these are joined by the ligament at the dorsal edge of the shell. The ligament is made of a strong, flexible and elastic, fibrous, proteinaceous material which is usually pale brown, dark brown or black in color.

In life, the shell needs to be able to open a little (to allow the foot and siphons to protrude) and then close again. As well as connecting the two bivalve shells together at the hinge line, the ligament also functions as a spring which automatically opens the valves when the adductor muscle or muscles (that close the valves) relax.