cnidocil

Cnidocil is a slender, hairlike projection on the surface of cnidocytes, the specialized stinging cells of cnidarians. It acts as the sensory trigger for the discharge of the nematocyst, a venom-containing capsule that enables prey capture and defense.

Cnidocytes are distributed across the epidermis and gastrodermis of cnidarians. The cnidocil typically arises from the cnidocyte near the area that covers the nematocyst, and it is often described as a modified cilium. As a mechanosensor, the cnidocil responds to tactile contact and, in some species, chemical cues in the surrounding water.

Discharge occurs when the cnidocil is deflected by mechanical stimulation. This stimulus causes a rapid cascade inside the cnidocyte, leading to the abrupt release of the nematocyst. The capsule everts a long, barbed tubule that can penetrate the target, delivering venom. The process is extremely fast and can immobilize prey or deter potential threats.

Ecology and evolution: Cnidocils are found across the major cnidarian groups, including Hydrozoa, Scyphozoa, and Anthozoa. The cnidocil-nematocyst system is central to prey capture and defense, representing a highly specialized mechanism for rapid response to contact. Although the general function is well established, the precise molecular details of the trigger cascade continue to be studied.