Xen

The Xen asteroids are home to a wide variety of flora and fauna, forming a complex ecosystem. Although residents of Xen at the time of the Black Mesa Incident, no living organisms are native to Xen, so all species present must have originated somewhere else. Water tends to be reddish or green in color. Some species, such as Barnacles, Bullsquid, Headcrabs, and Houndeyes appear to occupy niches similar to various carnivorous terrestrial animals (albeit with unusual anatomical or physiological features). However, even these creatures are dwarfed by the towering six meters high, four-legged invertebrate known as the Gonarch, being somehow connected with Headcrabs. Various harmless creatures also exist in Xen. Seen frequently in Xen (and occasionally in Black Mesa, where they transport Alien Grunts), Alien Aircraft glide seemingly without effort through the void. It is uncertain whether these ray-like beings are living creatures or examples of war machines built around advanced biotechnology (such as those used by the Combine). A large floating harmless amoeba-like organism which "sings" like a bird may be seen all throughout Xen. In the closing sequence of Half-Life, and in several areas of Blue Shift, Boids can be seen flying in flocks, emitting resonant calls. The intelligent species of Xen, such as the Alien Controllers and Vortigaunts, form part of an advanced alien civilization, with species like the Alien Grunts and Gargantua composing their military. As the player progresses through Half-Life, it becomes clear that this civilization is controlled by a central intelligence, the Nihilanth. Although differing in many ways, the Controllers, Vortigaunts, Grunts, Gargantuas, and the Nihilanth share a common morphology that includes a vestigial third limb in the middle of the thorax, suggesting that they are all closely related. A few creatures that appear in Xen are difficult to categorize as either fauna or flora due to their unusual appearance. Stationary bio-luminescent stalks appear in large numbers throughout Xen and will retract when approached. The much larger and extremely aggressive (but rarely encountered) Tentacles pose a major hazard to those who wander near their pits. There are also bizarre tree-like structures dotted around the landscape, which may be juvenile or even separate species of Tentacle. The "trees" sway gently and appear harmless when observed from a distance, but stab viciously at any creature that enters their immediate vicinity. There are also strange trampoline-like plants seen in Black Mesa and Xen that release a blast of air when stepped on. A similar kind was seen on Xen which opens and closes, releasing a gust of air like a vent.