Dorsal tubes, also called protuberances or spines, are structures that occur dorsally on most variations of the Xenomorph and are arguably the most mysterious piece of external anatomy on the creature.
OverviewAccording to the "Book of Alien: Augmented Reality Survival Manual", the Xenomorph is said to lack lungs which allows them to survive the oxygen-less vacuum of space. In their place, they have Dorsal Spines, which are believed to function as "air filters". This suggest that the Xenomorph home place, Xenomorph Prime, may be amphibious in nature, perhaps even the alien itself.
As evident in the AVP game released in 2010, the spines are filled with blood, which can be observed when the Xenomorph is shot or stabbed.
Queens and PraetoriansQueens have a variation of the dorsal tubes. Instead of actual, tube-like structures, spines run down its back, numbering at six instead of four. Praetorians have a similar set-up along their spines, being genetically similar to a Queen Alien.
PredaliensSome Predaliens shown also lack dorsal tubes, similar to the runner. Interestingly, the back spines of a Predalien resemble that of a Praetorian and are the only ones to do so. It is unknown if this is just an aesthetic similarity or if there is anything more behind it.
Runner aliens completely lack spines, instead having a smooth back. This is likely so that the creature, more tuned into its speed than its strength, is more aerodynamic, making it faster. However, this could be a side effect of a non-humanoid host.
CarriersCarrier Xenomorphs have specialized back spines, which have evolved to carry around the otherwise vulnerable Facehugger. Their spines shed nutritious material that Facehuggers consume while aboard them.
- The Xenomorph's creator, H. R. Giger, originally included the tubes in the creature's design to balance out its elongated head.
- Alien writer Dan O'Bannon originally wanted the tubes to extend from the Alien's chest, but director Ridley Scott ultimately chose to have them on the creature's back.
- In Alien: Isolation, the Alien's dorsal tubes can be seen vibrating from time to time when it stands still and is searching for the player. This may indicate the tubes are a sensory organ of some sort.
- Xenomorphs have been seen swimming in sand and water with their dorsal tubes protruding from the surface, not unlike a shark. It is unclear why they do this.