The creature's origin is a mystery, but it bears some resemblance to the Leviathan and the Slug, consisting of a rather formless main body surrounded by multiple tentacles. Specifically, the Urchin has four large tentacle "arms" tipped with yellow, eye-like orbs that can be protected by a tripartite 'eyelid', a triad of smaller, rear-mounted tentacle-mouths that can disgorge explosive sacs (akin to those produced by the female Slashers), and a frontal tentacle tipped with a knot of faintly luminescent, swollen, gnarled, yellowish cartilage. As with The Leviathan, The Slug, and Drag Tentacles, this last tendril is its vulnerable spot.
The Urchin is discovered growing in one of the Water Purification facilities in the Ishimura's vast Sewer System, below Hydroponics, which may mean that the Urchin is an offshoot of The Leviathan. It spends most of the battle submerged, attacking by battering the player character with its primary arms.
The player can disrupt these attacks by shooting at the yellow orbs in either tentacle, a task best suited for rapid-firing, instant-hit weapons such as the P-Sec Pistol or the Pulse Rifle, After a certain number of swings, the player will then move to a raised ventilation shaft on either side of the Urchin and enter free-look mode; when this happens, the player needs to use Stasis on the fan, whereupon they will head into the tunnel and the Urchin will try to grab them, only to have its tentacle severed when the fan starts up again, destroying that fan.
After its first set of primary tentacles has been severed, its secondary set will be deployed. Explosive barrels will now begin to emerge from under the water. The player must use Kinesis to grab these and hurl them at the arms when the yellow orbs are exposed, blowing the tentacles apart. Only a single hit with a barrel is needed to destroy an arm, and it is best to wait until an arm is lunging in to grab the player. If the player is grabbed, though, they will be drawn to a now-exposed mouth, which will repeatedly bite the player until they either die or shoot the mouth sufficiently to persuade the Urchin to release them.
When the secondary set of tentacles has been destroyed, the main body of the Urchin will surface and it will begin disgorging explosive pods, either singularly or in batteries of three. The player may use Kinesis to grab these pods and throw them at the knot of yellow cartilage rapidly swinging back and forth in front of the Urchin's body, or simply open fire with their weapons. Approximately three or more direct hits will destroy the Urchin.
- The Urchin is the first Boss fought in Extraction.
- The Urchin is one of the two aquatic Necromorphs encountered in Extraction; the other is the Grabber.