From Black Mesa: Wiki

The Barnacle from Episode 2, also featured in Black Mesa.

Overview

The Barnacle is a stationary sessile organism, encountered throughout the Black Mesa Research Facility after the Resonance Cascade incident. It generally presents a brownish-red outer skin layer, and is radially symmetrical in four quarters. This ceiling dweller sports four protruding teeth on the outside, several inner teeth, and a sticky tongue/tentacle-like appendage that paralyzes and reels in prey. Based on the animals appearance, it is assumed that it's features present four stomachs and two different types of organs surrounding the top (or base) of the Barnacle. An eye shaped organ, possibly means of reproduction or respiratory purposes, and possible anuses at the corners of the creatures body. It is unknown for sure what the purposes of these features are though. It can be noted that Barnacles are abundant on earth, whereas on Xen they can only be found in caves and grottoes, usually hanging over water. This suggests that while the Barnacles have teleported to earth and reproduced, their natural predators have not, unbalancing Earth's ecosystem.

Behaviour

Barnacles attach themselves to ceilings, and never move from the original point of attachment. They can be found in a variety of environs, but seem to prefer dark, damp areas. Barnacles have a very simple predatory strategy: they wait for prey to become stuck on their tongues, then hoist them up to eat them.

Mechanics

Bodies that hit the tongue instantly ragdoll, because the tongue is designed to work with physics objects rather than animated NPCs (which is why boxes and barrels and things can be dragged up). A possible in-universe explanation for this phenomenon is that the barnacle coats its tongue in paralyzing agents, rendering its prey motionless. The developers have stated repeatedly that it is beyond the scope of Black Mesa to change this behaviour.[1]

Tactics

Barnacles are relatively weak enemies. A single crowbar hit will kill the barnacle which has dragged the unfortunate player up; otherwise, from a distance, several 9mm shots or s shotgun blast can kill them easily.

Media

Barnacles concealed on a dimly lit ceiling in Unforeseen Consequences.

Barnacles' exposed tongues near the rails in On a Rail.

Trivia

• Early versions of Half-Life's barnacle had it taking a role as a more active enemy, with its tongue retracted and poised to snatch at passing prey.[2] It was eventually reduced to its current, passive role, serving more as a gameplay element to restrict player movement in cramped areas than as an aggressive enemy.
• The barnacle is one of three enemies - the others being the Headcrab and the Leech- that is being ported directly to Black Mesa from Half-Life 2 with no changes.