antennule

An antennule is the paired, typically smaller first antenna on the head of a crustacean. Located in front of the larger second antennae, the antennule is usually a multi-segmented appendage consisting of a proximal peduncle and a distal flagellum. The surface bears numerous sensory hairs, including specialized olfactory sensilla called aesthetascs, which house olfactory receptor neurons in many species. In addition, the antennule often carries mechanosensory setae that detect water movement and touch. In some crustaceans, a statocyst, a gravity- or balance-sensing organ, is located in or near the base of the antennule.

Functions of the antennule are primarily sensory. It plays a central role in chemical detection, helping individuals locate food, detect pheromones, and navigate their environment. It also contributes to tactile and hydrodynamic sensing, enabling the animal to sense current, obstacles, and other nearby organisms. The antennule’s sensory input is integrated with information from other sensory organs to guide behavior such as foraging, sheltering, and social interactions.

Variation among crustaceans is substantial. Antennules differ in size, segmentation, and ornamentation across groups such as copepods, Amphipoda, and Decapoda, reflecting ecological and evolutionary differences. In taxonomy and ecological studies, the structure and composition of the antennule and its sensilla are important characters for identifying species and understanding sensory ecology.