Ligament (bivalve)

In bivalve mollusks, the ligament is a crucial elastic structure that connects the two valves (shells) of the animal. It is primarily responsible for opening the shell. The ligament is located at the hinge area, typically dorsally along the umbo (the oldest part of the shell).

The ligament's composition and structure can vary between different bivalve species. It can be either internal (resilium), external, or both.

• External Ligament: This type lies on the exterior surface of the shell near the hinge. It is generally composed of layers of tanned protein, a hardened form of conchiolin. When the adductor muscles close the shell, compressing the ligament, elastic energy is stored. When the adductor muscles relax, this stored energy is released, causing the ligament to spring back and force the valves apart.

• Internal Ligament (Resilium): This type is located internally, often within a depression or pit in the hinge called a chondrophore. It's composed of a different material than the external ligament, often being more fibrous and containing calcite or aragonite. The resilium functions similarly to the external ligament, storing and releasing energy to open the shell.

The ligament's effectiveness relies on its elasticity and its ability to withstand repeated cycles of compression and release. Damage to the ligament can impair a bivalve's ability to open and close its shell, affecting its feeding and defense mechanisms.