Thread-like organs that emerge from the back of a Kelpien's head when danger is near, a sign of the species' status as prey, and which fall away after the change called vahar'ai.
Threat ganglia are the instinctive alarm system of the Kelpiens of Kaminar. When a Kelpien senses the approach of death or a predator, pale filaments spread out from behind the ears, often before the conscious mind has registered anything wrong.
Discovery introduces them with Saru in The Vulcan Hello, in which his ganglia unfurl on the bridge of the USS Shenzhou as the Klingons gather at the binary stars. In the first two seasons his crewmates treat them as a reliable, if unnerving, warning.
The episode An Obol for Charon reveals that the ganglia are not permanent. Saru undergoes vahar'ai, the illness his people believe must end in death at the hands of the Ba'ul, but survives it; the ganglia drop off and his constant fear goes with them. In their place Kelpiens grow sharp spines that can be fired at an enemy, which shows that the species was once the predator on Kaminar.