the zoology and biochemistry of xenomorphs from the Alien franchise – Journal of Geek Studies
Luca Tonietti1–3 & Guillermo Climent Gargallo4
1Department of Science and Technology, University Parthenope, Naples, Italy.
2International PhD Programme/UNESCO Chair “Environment, Resources and Sustainable Development”, Naples, Italy.
3INAF-OAC, Osservatorio Astronomico di Capodimonte, Naples, Italy.
4Department of Biology, University Federico II, Naples, Italy.
Emails: luca.tonietti001 (at) studenti.uniparthenope (dot) it; guillermo.climent.gargallo (at) gmail (dot) com
In science fiction, the xenomorph emerges as a creature that transcends the boundaries of traditional extraterrestrial movie knowledge (Fordham, 2023). From the corridors of the spaceship USCSS Nostromo to the haunting silence of the primordial exoplanet Acheron LV-426 (Flowers, 2020), the creature created by Ridley Scott and H.R. Giger has become the symbol of alien terror in movies and in collective imagination (Domino, 2019).
Beyond the movies, xenomorphs can also be seen as a zoological and biochemical scientific challenge (Chemist, 2017). The cinematic universe of Alien introduces the xenomorph as an alien force, both literally and metaphorically (Feel No Pain, 2023). Our exploration begins with an examination of their hypothetical taxonomic classification, an attempt to place these creatures within the framework of known biological diversity (Pratt, 1972).
TAXONOMY
Taxonomic levels presented in this paragraph are based only on morphological features not considering genomic analysis due to the absence of DNA sequences for fictional creatures. Some characteristics are shared between the various classification levels of living organisms and can be used as comparative parameters for our speculation.
The xenomorph’s multicellular structure serves as a cornerstone for its classification within Animalia (Ros-Rocher et al., 2021). Its intricate organization of cells, tissues, and organs reflects a level of biological complexity commonly associated with animals. Xenomorphs can be included into the Arthropoda phylum due to the morphological similarities shared with certain terrestrial arthropods such as an exoskeletal structure, a segmented body plan, the presence of hemolymph, etc. (Akam, 2000). Xenomorphs and chelicerates can be morphologically connected, justifying its classification within the class Chelicerata (Sharma, 2018). Similarities are shared in limb structure and mouthparts, aligning the xenomorph with arachnids. The chelicerate-like tail, i.e., the telson, is a distinctive hallmark of the xenomorph, further reinforcing this classification (Howard et al., 2019). In recognition of the xenomorph’s unparalleled morphology and extraterrestrial origin, a novel taxonomic order, Xenomorpha, is proposed. This order serves as a specialized category crafted to accommodate the unique characteristics exhibited by the xenomorph. The family Xenomorphidae emerges as the categorization that embraces the diverse variations among different xenomorph species depicted in cinematic portrayals. This taxonomic family acknowledges the variability within the xenomorph lineage, providing a framework to encapsulate the differences observed in various cinematic depictions such as the dog-xenomorph in Alien 3 or the neomorph in the new franchise. The family is also based on the original host of the facehugger. At the genus level, the overarching features shared among diverse xenomorph variants coalesce under the name Xenomorphus. This taxonomic designation emphasizes the exoskeleton, the scorpion-like tail, the acidic blood, and all the characteristics of some arthropods, as a central unifying trait among different xenomorph manifestations. The species name, Xenomorphus extraterrestris, encapsulates the xenomorph’s extraterrestrial origin. While maintaining a taxonomically specific identity, this nomenclature acknowledges the unique evolutionary path and biological distinctiveness of the xenomorph within the proposed taxonomic framework.
MORPHOLOGICAL FEATURES
Xenomorphs are extraordinary animals with unique characteristics that let them be able to colonize extreme environments, occupying every single niche. As for some living organisms on our planet, xenomorphs can be considered extremophiles; particularly, they can be defined as polyextremophiles as shown in the different movies. They are able to survive in many different environments spanning from the cold interstellar space to the extremely hot furnace in Alien 3 (1992). Their ability to thrive and hunt in extremely harsh conditions could be due to their morphological features such as the robust exoskeleton, the scorpion-like tail and some biochemical adaptations, e.g., the acidic blood.
Exoskeleton. The xenomorph’s exoskeleton is a structure composed probably of a unique amalgamation of chitin and other resistant organic compounds, and stands as the main characteristic of resilience. As depicted in Aliens (1986), it not only serves as a protective barrier against flamethrowers and bullets but also showcases an adaptability akin to the exoskeletons found in Earth’s terrestrial arthropods (Giribet & Edgecombe, 2019). The cinematic narrative mimics the real-world resilience observed in beetles, renowned for their robust exoskeletons that withstand different adversities (Nagasawa, 2012; Stamm et al., 2021).
Cephalothorax. A defining feature inspired by arachnid anatomy is the xenomorph’s cephalothorax that takes a focal point in its anatomical design, as observed in Alien: Resurrection (1997). The emphasis on the xenomorph’s cephalothorax reveals a carapace that not only shields vital sensory organs but also integrates with the creature’s sensory systems as shown by the tube-like structures on the back that are probably chemo-sensors or the breathing system, similar to the book-lungs of arachnids (Machalowski et al., 2020).
Chelicerae. The xenomorph’s chelicerae are a captivating feature, as shown in Alien (1979), offering a glimpse into their predatory arsenal. Chelicerae are probably reminiscent of the efficient mouthparts seen in arachnids, horseshoe crabs, and sea spiders, unfolding as multifunctional tools that play a pivotal role in the xenomorph’s hunting and defensive strategies (Sharma, 2017). In the whole series of movies, it is evident that the fangs, pincers, or jaws of the xenomorphs are transparent, maybe suggesting that they are hollow and they can contain some venomous glands. In addition to the arachnid-like chelicerae, xenomorphs also present a predator inner jaw that mimics the one found in moray eels (Johnson, 2019).
Limbs. Prominently featured in Alien 3, the xenomorph’s segmented limbs can be seen as scythe-like talons contribute to a predatory adaptation (Panganiban et al., 1995). The segmented structure of the limbs offers both flexibility and strength (Gao et al., 2012), allowing the xenomorph to transition between stalking its prey and executing lethal strikes. A behavior that we can also find in some animals such as mantis shrimps (Stomatopoda). The dynamic mobility enables the xenomorph to move in different environments with agility, from the confined spaces of spacecraft to the corridors of alien worlds.
Scorpion-like tail. The xenomorph’s elongated segmented tail, featured in all the franchise, emerges as one of the most fascinating morphological characteristics with a multifaceted purpose. This tail, complete with a stinger or a telson that serves as offensive and defensive roles, aligns with the multifunctional tails observed in scorpions (Lourenço, 2018; Carmichael, 2022). Differently from them, the tail of xenomorphs does not seems to contain a venomous substance.
Extraterrestrial ovipositor. The extraterrestrial ovipositor (seen only for xenomorph queens) facilitate the precise implantation of embryos into host organisms. The xenomorph’s ovipositor showcases reproductive adaptations reminiscent of certain wasps (Quicke & Fitton, 1995). In the movies, the massive tube-jelly-like organ that composes the whole ovipositor is evident. It seems that the queen is not able to move freely when connected to this structure, which can be removed when necessary.
XENOMORPH BIOCHEMISTRY
Acidic hemolymph
The acidic hemolymph of xenomorphs is a hallmark of their unique biochemistry. Hypotheses surrounding the origins of acidity point towards an unconventional enzymatic cascade within the xenomorph’s hemolymph, potentially involving hyper-reactive acid-base equilibria (Grifoni et al., 2019). We can hypothesize the presence of specialized enzymes capable of generating highly acidic intermediates during metabolic processes.
Thus, we could hypothesize, for instance, the presence of organic acids such as thioacids and haloacids in xenomorph hemolymph, which would raise questions about the biochemical pathways responsible for their synthesis and their specific roles in maintaining the acidic milieu (Dong et al., 2018). Thioacids, known for their strong acidic properties, present a fascinating topic for exploration (Ulrich & Jakob, 2019; Wang et al., 2022). The reactivity of these thioacids could serve dual purposes, acting both as defensive agents against external threats and as key components in the xenomorph’s metabolic processes.
The possible presence also of haloacids, featuring halogen atoms such as chlorine, would introduce another layer of complexity to xenomorph biochemistry (Su et al., 2016). The incorporation of haloacids into the acidic hemolymph could enhance corrosive properties, potentially explaining the rapid degradation observed in xenomorph blood interactions with various materials (e.g., HF if able to corrode glass). Drawing inspiration from terrestrial haloacid-tolerating organisms (Wang et al., 2021), the xenomorph’s ability to synthesize and deploy these acids unveils a biochemical strategy that extends beyond conventional defensive mechanisms.
Determining the acidity of the xenomorph hemolymph involves making several assumptions due to the speculative nature of the creature and its fictional biochemistry. However, we can discuss a hypothetical scenario. Sulfuric acid is a strong acid, and its derivatives could contribute to the xenomorph hemolymph acidity. If we consider a concentration of 1 M sulfuric acid, the pH would be approximately 0 (all the protons of the acid are released into the solution). The xenomorph’s hemolymph may contain a mixture of acids, such as thioacids and haloacids. We can assume a collective concentration of these acids equivalent to a 1 M solution of hydrochloric acid. Using the Henderson-Hasselbalch Equation:
pH = pKa + log([A–]/[HA]),
we can assume a pKa of -log(1), and a 1:1 ratio of dissociated (A-) to undissociated (HA) acid, giving a pH result of approximately 0.
Chemist (2017) suggested that the acidic composition of the hemolymph could be the superacid HF⋅SbF5 or a mixture between minor components, such as hydrofluoric acid (HF), sulfuric acid (H2SO4), hydrochloric acid (HCl), and nitric acid (HNO3).
The purpose of xenomorph blood is also explored through comic book lore, particularly from Lasalle Bionational’s research in the Aliens vs Predator series (Xenopedia, 2024). According to this lore, xenomorph blood serves as a biological battery, generating a powerful bio-electric charge through a chemical reaction. This unique energy source replaces the need for traditional respiration and digestion, which raises questions about how xenomorphs might recharge this energy source, leading to the eventual death of the creatures in case it is not renewable (Chemist, 2017).
The concept introduces an intriguing idea that xenomorphs have evolved a more efficient energy source than humans. While humans rely on consuming food for energy through oxidation/reduction reactions, xenomorphs use a stream of electrons as a power source, similar to recently discovered organisms on Earth, e.g., chemolithophilic organisms. This alternative energy pathway suggests a unique evolutionary path for the xenomorphs, making them “cosmically tenacious”.
Exoskeletal resilience
Unlike typical corrosive substances, the xenomorph’s acidic hemolymph displays a specificity in its corrosive action. Hypotheses propose the presence of molecular inhibitors within the exoskeleton, forming a protective barrier against the acid’s corrosive effects (e.g., Wang et al., 2018). One potential candidate is a class of chelating agents that selectively bind to metal ions, preventing them from participating in the acid-base reactions responsible for corrosion (Gulcin & Alwasel, 2022). To resist the extremely acidic conditions of the hemolymph, the exoskeleton could be composed by anti-corrosive substances such as fluoropolymers. Polytetrafluoroethylene (PTFE) stands out as one of the best molecular candidates for composing the exoskeleton of xenomorphs, owing to several distinctive characteristics. Firstly, it is renowned for its extreme chemical inertness and high-temperature resistance. Being a fluorine-based compound, it does not react with hydrofluoric acid, showcasing resistance to chemical agents. It is well-documented that xenomorph chitin-like structure (Elieh-Ali-Komi & Hamblin, 2016) retains its resistance to acid even after the creature’s demise or the removal of its exoskeleton as shown in Alien vs Predator (2004). The chemical inertness of PTFE would be crucial in ensuring the durability and resilience of the xenomorph exoskeleton. The high-temperature rating of PTFE would also allow xenomorphs to operate in extremely hot environments without compromising their physical abilities as shown in Alien 3 when molten metallic substances are poured onto xenomorph’s body.
The fluorescent color
The vivid fluorescent green/yellow shade of xenomorph hemolymph introduces another layer of mystery to its biochemistry. Fluorescence in biological systems often stems from the presence of specific molecules, and in the case of xenomorphs, hypothetical bio-fluorophores would contribute to this glow. One plausible candidate is a family of polyaromatic hydrocarbons (PAHs) with extended conjugated systems (Zhang et al., 2020). These molecules, absorbing light at one wavelength and re-emitting it at a longer wavelength, could be responsible for the xenomorph’s fluorescent coloration. PAHs are generally known for their stability and resistance to acidic conditions, particularly in comparison to more reactive compounds (Abdel-Shafy & Mansour, 2016). The aromatic nature of PAHs, characterized by a stable ring structure, contributes to their overall resistance to chemical degradation under acidic conditions (Patel et al., 2020). This structural robustness is a key factor in explaining how the xenomorph’s fluorescent biochemistry maintains its luminosity even within the context of its acidic hemolymph.
On the other hand, one typical solution in terrestrial organisms for bioluminescence is the luciferin pathway. A phenomenon that occurs in species belonging to very distant phyla, the emission of light plays an important role in the life cycle of the involved organisms (Syed & Anderson, 2021; Ke & Tsai, 2022). By oxidizing luciferins, the enzyme luciferase is able to produce an intermediate excited state oxyluciferin that will later decay to a ground energy state by emitting photons. Given such an ominous name, it is not unlikely that the creators of the xenomorph could have engineered a similar mechanism to that of the luciferase pathway, thanks to their natural prowess in genetic engineering as featured in both Prometheus (2012) and Alien: Covenant (2017). Special attention must have been paid to stabilize the tertiary structure under such acidic conditions, notably the binding site and the corresponding luciferin substrate, a feat that is more than extraordinary given our current means and knowledge in bioengineering.
PARASITOID BEHAVIOR
It is evident from the movies that there are many different stages in the organism’s growth. Beginning with the parasitoid implantation of embryos, a biological relationship is established as the xenomorph utilizes host organisms as incubators for its progeny. This parasitoid interplay is exemplified in Alien, where a facehugger marks the initiation of the xenomorph’s life cycle (Kuris & Luo, 2023).
As the xenomorph’s life cycle progresses, the chestburster emerges. The xenomorph’s ability to change different host species emphasizes its parasitoid versatility (e.g., Manwell, 1957). This flexibility is exemplified in Alien 3, which showcased the infection of the dog in the facility by a facehugger, and in Alien: Covenant, which introduced the neomorph, a variant of the xenomorph species displaying distinct parasitic characteristics. Thus, the seed of the xenomorph, the “black goo” (Chemical A0-3959X.91–15), serves to illustrate the skills of the extraterrestrial creators of the xenomorph. Not only they engineered a lethal weapon, but managed to bypass all the extant species barriers successfully targeting all animal-like life-forms.
Giving some real-world analogs, the xenomorph’s tactics aligns with those of parasitic wasp, such as species of Glyptapanteles, which lays eggs inside caterpillars; the chestburster’s emergence mimics the invasive nature of parasitoids in terrestrial environments (Surridge, 2008). Species of the ichneumonid wasp genus Megarhyssa use its long ovipositor to deposit eggs deep within wood-boring insects, which is also a striking parallel to the xenomorph’s ovipositor and its precise implantation of embryos into host organisms (Crankshaw & Matthews, 1981). In Alien: Covenant it is evident the common features between the neomorphs and the parasitic fungi of the genus Ophiocordyceps, which manipulate the behavior of their insect hosts (Araújo et al., 2021).
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Acknowledgements
We would like to thank all the people involved in these kinds of scientific speculation and we are grateful to the ExoBioNapoli group, the ExoPlaNats group, and the GiovannelliLab for their support. We thank MUR (Ministero dell’Università e della Ricerca) for the PhD program PON “Ricerca e Innovazione” 2014–2020, DM n.1061 (10/Aug/2021) and n. 1233 (30/Jul/2020). We would like to thank Ridley Scott for the big inspiration in this work. Lastly, we would also like to add this last comment — “Sometimes science inspires science fiction. Sometimes it does not.”
An AI tool (ChatGPT, GPT-4, OpenAI) was used to refine the writing style of this article. The authors reviewed, revised, and edited the AI-generated text to their own liking, taking full responsibility for the final content, which reflects a collaboration between humans and machines. To L.T. integrating AI into creative processes is not only a step forward in efficiency but also a glimpse into a future where full AI assistance becomes a reality.
Nomenclatural disclaimer
The taxonomy in this paper is entirely fictional and does not adhere to the rules of the International Code of Zoological Nomenclature (ICZN). The names and classifications used are created for narrative and entertainment purposes only and have no scientific or official value.
About the authors
Luca Tonietti is a PhD student in Astrobiology at the Parthenope University of Naples, Italy in collaboration with the Federico II, University of Naples, Italy, the Italian National Astrophysics Institute INAF-OAC, Italy. Luca is also a visiting PhD student at the UK Centre for Astrobiology, University of Edinburgh, Scotland, UK, and a visiting scientist at the Bicocca University of Milan, Italy, and at the National Research Council CNR-IRSA, Verbania, Italy. Luca’s main project is involved in the microbial applications in space exploration.
Guillermo Climent Gargallo is a PhD student in Biotechnology at the Federico II University of Naples, Italy. Guillermo’s project is centered on the impact of subsurface microbiology in potential underground hydrogen storage sites, with a special focus on the production of H2S and consumption of the stored hydrogen.
