Metroid Husks are the organic materials discarded by Metroids, usually when the creatures have undergone a metamorphosis on SR388 or a replica of its environment. Various types of husks, shed skin and membranes are seen throughout the Metroid series.
On planet SR388, a Metroid will shed twice in its lifetime, each time when it drastically alters its physiology. The first husk is the entire membrane of its larval form. Prior to its metamorphosis, a Larva will cease to levitate and lay on the ground, flashing brightly until the Alpha Metroid emerges from the topside, leaving an empty and shattered membrane. These discarded husks are present in nearly every area of the planet and their presence means there are Metroids in close proximity; more precisely, every single shed membrane discovered belongs to a different individual. They are seemingly soft and viscous on the inside, as Samus' body will slowly sink through it if she stands on top of the membrane's torn opening.
The second husk consists of the body of a Gamma Metroid. It is discarded in its entirety when a Zeta Metroid breaks out from the former's backside, akin to an insect emerging from its pupa. Like the previous shedding, the Metroid is initially flashing brightly prior to the transformation as well. The process is done in midair, which causes the empty body of the Gamma Metroid to fall to the ground off-screen. Contrary to the membrane of the Larva Metroid, this second husk does not remain in the area and vanishes after metamorphosis.
Gamma and Omega Metroids did not discard any material from their Alpha and Zeta Metroid stages, respectively; instead, their growth occurs in the form of rapidly changing back and forth between the two forms for a few seconds before attacking Samus.
In the remake of Metroid II, the husks are quite different and more varied. They are named in the Map key. A Metroid will now leave a husk behind for nearly every stage of its life cycle, and all shed material features an overall dark yellow color.
When the Metroid larva begins to transform, it lays on the ground as its membrane and mandibles harden and turn into a dull color. The Alpha Metroid within then struggles to break free, briefly levitating the larva's entire body before it finally shatters the topside and shakes off the husk. This is the only metamorphosis witnessed by Samus, due to all the others occurring off-screen.
As the discarded material is left on the ground, a bright, opaque, yellow fluid leaks and spreads out from it. Interestingly, the Alpha Metroid discards not only the prior form's membrane, but also the flesh and fangs underneath it, contrary to Metroid II where only the membrane was shed.
If Samus obtains Metroid DNA and inserts it into the Chozo Seal without having exterminated all Metroids present in the area, the location of a husk corresponding to any surviving individual will be marked on her map; curiously, the discarded membrane emits energized steam but only if it has been marked on the Map by a Chozo Seal.
Every Metroid stage following the larval phase will also shed their entire skin and/or body in order to reach the next form, all the way to the fully grown Omega Metroid. However, only the larva's husk is seemingly durable, as all others' will crumble and disintegrate either at the slightest touch or on their own during their respective cutscenes. Due to this, along with the larvae's husks being located in the background, none of the Metroids' shed bodies can be interacted with during gameplay.
The Queen Metroid and the last surviving Infant Metroid are the only stages to not undergo a metamorphosis during the course of the game (from a prior or advanced form, respectively) nor in any other title in the series. Consequently, it is unknown if they produce husks, but based on the fact that the other stages in the species' life cycle shed organic material, the Queen can be assumed to have shed her's long ago. The recently-hatched Infant may be too young to molt yet.
In the first game of the series, the highly mutated member of the species known as the Metroid Prime discards its first form's exoskeleton during its confrontation against Samus on Tallon IV. However, this shedding is not the result of a metamorphosis, but rather a consequence of being heavily injured by her.
At the end of their first battle, the Metroid Prime, exhausted, falls into a deep pit. Samus jumps down and discovers the creature's body at the bottom where it is leaking out copious amounts of Phazon from all sides. Both its helm-like structure and face are absent; in their place is instead a black, gaping hole. Soon after, the organism's Core Essence, featuring the aforementioned missing face, flies out of the hole and the second battle begins. The now empty black shell of the Metroid Prime remains in the room and can be climbed on by Samus.
In Metroid Prime 3: Corruption, numerous husks identical to the Metroid Prime's exoskeleton are found embedded into the floors and walls of the Genesis Chamber on Phaaze. Though never confirmed, it is likely they belong to other Metroid Primes that have evolved from the lesser mutants present on the planet. Unlike the exoskeleton shed by the creature on Tallon IV, these appear to be dried and maintain the helm-like structures, which implies that Phaaze's Metroid Primes molted out of their husks in an entirely dissimilar process as well as under different conditions.
- "These husks are identical to the bioform you encountered within the crater on Tallon IV."
After the Restricted Laboratory containing cloned Metroids was detached from the Biologic Space Laboratories research station and destroyed, Samus comes across several empty yellow membranes resembling a larval Metroid's, littered throughout Sector 1. There are five in total, each one bigger than the last and they all feature an opening that was seemingly caused by a creature bursting out from the inside. Eventually, she finds the final and largest membrane in the Docking Bays before she is confronted by the husk's former occupant: an Omega Metroid. This sequence of events reveal that this Metroid grew from an unseen Infant that escaped the Restricted Laboratory. The massive husk is crushed by the Omega Metroid jumping into view. Contrary to the discarded husks in Metroid II and Return of Samus, all of the shed membranes in Metroid Fusion belong to a single individual. This discrepancy may be linked to the rapid growth technique implemented on the organism by the B.S.L.'s scientists which, as its description states, also caused the Metroid to grow rapidly.
- As previously mentioned, the original Metroid II features only two detailed metamorphoses that result in the creation of a husk, specifically in the larva-Alpha Metroid and Gamma-Zeta Metroid transitions. This served to emphasize on the drastic changes in physiology between the three main Metroid body types: the jellyfish, arthropod, and theropod forms. All other transitions that do not leave a husk are relatively simple in comparison, which reflect the core form they keep (example: Alpha and Gamma Metroid possess arthropod-like bodies).
- The remake Samus Returns removes this emphasis altogether by making the Metroid forms mentioned above discard a husk from the previous stage, regardless of how complex or minor the changes are.
- In Metroid Fusion, the Omega Metroid shed five husks as it grew from its infant form. Strangely, all husks are in the shape of Metroid larva membranes.
- The husks' identical shapes suggest that the Omega Metroid somehow skipped the Alpha, Gamma, and Zeta stages of its life cycle, instead remaining in its larval form and growing to massive proportions until it molted directly into a full grown adult.
- The number of larva husks coincides with the number of metamorphosis a Metroid undergoes from its infant stage to the Omega form. Though the discarded membranes in the B.S.L. do not match the expected shapes seen on SR388, it is currently the only instance that implies an Infant Metroid sheds organic material when it becomes a Larva.
- Three out of the four nuclei present inside the Metroid larva's membrane seemingly behave like stem cells during the creature's transformation into an Alpha Metroid. This comes from the fact that only a single nucleus remains after the metamorphosis, as the other three were undoubtedly used to create the Metroid's new body.