statolith

A statolith is a dense mineral particle that acts as a gravity-sensing element within the statocyst of many invertebrates. It interacts with sensory hair cells to provide information about orientation and acceleration, enabling balance and posture during movement.

In crustaceans, statoliths are calcified concretions formed from calcium carbonate, often aragonite or calcite, located in the statocyst chamber and free to move with changes in orientation. Their weight and geometry influence how the sensory epithelium is stimulated as the animal tilts or accelerates.

Mechanism-wise, when the animal reorients or experiences acceleration, the statolith shifts relative to the hair cell array, bending the cilia and triggering neural signals that are interpreted as tilt or movement. This provides a neural map of the animal’s orientation in space.

Statoliths occur in various groups with statocysts, especially crustaceans such as crabs, lobsters, and shrimp, and in mollusks including some gastropods and cephalopods. The exact composition, size, and shape vary by species, but calcium carbonate remains a common material.

Significance-wise, statolith-based gravity sensing is an ancient and widely studied sensory mechanism, informing our understanding of balance, proprioception, and the evolution of vestibular systems in invertebrates. See also statocyst and gravity sensing.