Eyestalk

An eyestalk is a slender, stalk-like projection on the head or cephalothorax of many arthropods, most notably crustaceans, that bears the eye at its distal end. The eye sits at the tip of the stalk, and the stalk can often be moved independently of the body, giving the animal a wide range of visual angles. The appearance and length of eyestalks vary across species; some are long and tapering, others short and stout, and some crustaceans have eyes that can be retracted into protective sockets.

Inside the eyestalks lies part of the central nervous system, including the eyestalk ganglion. In crustaceans this ganglion contains neurosecretory tissues (the X-organ and the sinus glands) that release hormones controlling processes such as molting, reproduction, and osmoregulation. Because of this hormonal role, damage or removal of the eyestalks can disrupt development, growth, and reproductive cycles.

Functionally, eyestalks serve to position the eyes for enhanced vision in dynamic environments and to protect the eyes while allowing flexion or retraction. In aquatic settings, movable eyestalks can help the animal quickly scan for predators or prey while keeping the body relatively still.

Ecology and evolution: Eyestalks are a characteristic feature of many crustaceans (including crabs, lobsters, and shrimp) and are often an adaptation for improved environmental sensing in aquatic habitats. Their presence and shape reflect ecological needs such as habitat complexity and predator pressure.