Metroid (species)

Metroids are the titular creatures of the Metroid series. Originating from the planet SR388, they first became known to the universe at large when Space Pirates stole a batch of specimens from a Galactic Federation research vessel and attempted to exploit them as bioweapons. Since then, the species was spread out on numerous planets, endangering all life present due to the Metroids living off on the mysterious "life energy" of their prey.

The Metroids were long rumored to be created by an ancient race, though their motive for doing so remained a mystery to the rest of the universe. Eventually, it came to light that the Chozo were responsible for genetically engineering the species, in order to prevent the spread of the more dangerous X Parasites. Appropriately, the word "Metroid" in the Chozo language roughly translates to "Ultimate Warrior".

The species became extinct in the wild at the end of Super Metroid; all Metroids encountered after this event in the timeline have been created in laboratories using the DNA of the last and now-deceased Metroid from SR388.

Biology and characteristics
The Metroids on SR388 possess a multitude of different body structures and sizes, a result of their complex life cycle that drastically alters their physiology as they grow. They begin life as jellyfish-like creatures before molting into a form akin to an arthropod's, until eventually reaching adulthood with the features of a theropod.

Metroids are highly aggressive creatures with no natural predators and are thus at the top of the food chain on SR388. The organisms feed on life energy of their victims in a manner similar to the way a leech drains a host organism's blood. Unlike leeches, Metroids leave all of their prey's blood and internal organs intact, yet lacking the energy necessary to sustain life. It is unknown exactly how the Metroid extracts this life energy or what, in fact, the latter consists of. Metroids can also feed on energy used by both artificially-created beings and technology such as Samus Aran's Power Suit.

All Metroids on the planet are sterile with the exception of a single individual whose sole purpose is to propagate the population of the species: the Queen. While this may insinuate at first that the creatures follow a society devoted to their progenitor in a similar fashion to an ant or bee colony, this is not the case as older members of the species are found scattered across SR388 living independently in order to grow. In fact, only Larva Metroids are seen in groups near their birthing site. Interestingly, fully grown adults are found close to the Queen's location, but still at a sufficient distance to suggest an independent life style.

Despite their various forms there is a specific, physical feature shared by all Metroids inhabiting SR388: the presence of a translucent membrane enclosing a red nucleus. This nucleus has numerous neuron-like connections extending from it which connect to the inner surface of the membrane. Its function is unknown, but it is of vital importance to the Metroid, as destroying the membrane kills the organism. Strangely, Metroids in the larval phase possess numerous nuclei within their membranes and are invulnerable to damage unless frozen, while those in more advanced stages possess a single nucleus and can be readily wounded without the need to freeze them first. This suggests that the defensive strength of a membrane is determined by the number of nuclei present inside. An even greater mystery of their biology is the Metroids' ability to perpetually float in midair, despite lacking any visible means of propulsion.

It has been stated in numerous in-game sources, in particular Metroid Fusion, that the Metroids' true weakness is cold temperatures, yet a strange contradiction is found in older individuals inhabiting SR388 whose carapaces protect them from the effects of the Ice Beam. In any case, their homeworld possesses no sub-zero climate, thus the species are capable of roaming every corner of the planet virtually unchallenged. It is unknown if this vulnerability was intentionally implemented by their Chozo creators.

The Metroids were designed with the intention to control, perhaps even as far as to render extinct the extremely lethal, rampant and once-dominant X Parasite population. As such, the Metroids are immune to the X's infection, allowing them to feed directly on the parasites' pure gelatinous forms. In addition, the Chozo implemented a metamorphosis that allows Metroids to acquire more strength and new abilities as they mature, such as generating electricity as well as launching toxic and fire-based projectiles. These powers are undoubtedly used in their battles against X Parasite hosts and their mimicries which viciously defend themselves using the abilities of prior infected victims. Indeed, the Metroids were successful in lowering their population to the point that Samus was able to safely explore SR388 without encountering a single X. Even in the lack of their main diet, Metroids are more than capable of feeding on other lifeforms as substitutes.

Metroids were initially theorized to have caused the destruction of the Chozo civilization on SR388. This theory, however, has been put into doubt following the discovery of the X Parasites.

Life Cycle
The first encounter between the Galactic Federation and the Metroid species consisted of a horde of Metroid larvae attacking the crew of a research vessel visiting SR388. This incident prevented adequate data to be collected on the planet's ecosystem and consequently, it was erroneously believed for years that the Larva Metroid was the standard form of the entire species. Supporting this false belief was the fact that Metroids cannot metamorphose into their natural advanced stages seen in Metroid II, Samus Returns and Fusion unless they are exposed to SR388's atmosphere and environmental stimuli.

It is only during Samus' mission on SR388 to exterminate the species that the full life cycle seen below would become known.

Metroid Egg
Eggs are laid by the Queen Metroid. They are usually oval in shape and are covered in a secretion that seemingly hardens into root-like structures that keep the eggs firmly connected to the ground. Inside the egg is a developing Infant Metroid that eventually bursts violently from it.

Infant Metroid
The infant has a simple body shape similar to a jellyfish with no visible sensory organs. More than half its mass is comprised of the species' characteristic membrane and four red nuclei. Underneath these is a mass of flesh and/or muscle with four tiny fangs. The organism appears and behaves relatively docile, though it is known to be highly unpredictable at times and can attack without warning.

Larva Metroid
Commonly seen as the face of the entire species due to it being the first Metroid form discovered by the Federation. Its physiology is nearly identical to an Infant's, except every aspect is much larger and more vicious. They can only be harmed with concussive weaponry after freezing it with the Ice Beam. Power Bombs can also harm them, though their level of effectiveness varies greatly. Like a moth or butterfly's caterpillar form, the Larva Metroid has a voracious appetite. Seemingly unique to this stage and the previous Infant form are their responsiveness to Beta-Rays, which cause them to split and multiply within 24 hours. This trait, along with its hunger and its high susceptibility to being frozen, make the larva a preferable choice for controlled bioweapon research. The Larva Metroid and Infant Metroid can be grouped up as the larval phase of the Metroid species.

Alpha Metroid
An Alpha Metroid molts out of a larva's massive membrane. The creature has developed a carapace, four minuscule claw-tipped legs, a mouth with three tusks and a pair of compound eyes, altogether giving it an arthropod-like appearance. The membrane has been relocated to the underside of its body and encloses only a single nucleus; although never confirmed, it would seem that the other nuclei were re-purposed into the creation of the Metroid's new body. Starting at this stage, the Metroid organism is no longer susceptible to the effects of the Ice Beam and will carry its only remaining nucleus for the rest of its life cycle.

Gamma Metroid
This stage is an expansion to the prior form. The four legs have reached their full length, the carapace has developed further, four additional eyes are present and most notably, the tusks have not only enlarged but can now generate electricity. The Metroid can use these medium-ranged projectiles as both an offensive and defensive option. The Gamma and Alpha stages can be categorized as a Metroid's pupal phase although mobile similar to a mosquito's pupa.

Zeta Metroid
The Zeta Metroid molts from the back of the Gamma Metroid. The Zeta Metroid's body shape shares many traits with that of a reptilian, bipedal theropod dinosaur. The creature has grown two arms and two legs (each equipped with two claws), a tail, a head possessing a leech-like mouth with eight individual eyes. The membrane is now seen on its torso, and its back is fully covered with a carapace. The organism can now spit globs of hazardous liquid.

Omega Metroid
The final and most powerful stage of a standard Metroid. The creature is taller and more muscular than the previous stage, and it now unleashes fireballs from its mouth. The membrane is now covered in a rib cage and is extremely resilient to damage. The Omegas and Zetas are together the adult phase of the species.

Queen Metroid
The only natural reproductive stage of the entire species. Only Metroids with specific genes can become a Queen, however it is unknown if they follow the same life cycle seen above. The Queen is quadrupedal and is the largest Metroid on SR388, surpassing the size and strength of even Omega Metroids. Her membrane and nucleus have vastly expanded to cover her entire underside. She is highly protective of her eggs and will defend them to the death.

Unnatural Metroid strains
Throughout the years of extensive research and experiments by Space Pirates, it was discovered that not only are Larva Metroids highly adaptive to alien planets, their biology is also easily influenced by radiations. As a result, the larvae are far more promising and exploitable as bioweapons than the other stages in the species' life cycle. This led to the creation of many unnatural breeds of Metroid, and while there are some that are weaker than their natural counterpart, many others became powerful aberrations. Interesting to note is that most of these unnatural strains were discovered long before the SR388 life cycle itself became known.

The list below covers every single breed of Metroid in the entire series which possess sufficiently distinctive traits that set them apart from the original SR388 strain.

Tallon Metroid
Larva Metroid that adapted to planet Tallon IV. Although its resilience was lowered significantly, it had near limitless potential for additional mutations.

Hunter Metroid
A Tallon Metroid that has developed a distorted body and tentacles through unverified means.

Fission Metroid
A mutated Larva Metroid that multiplies when damage is inflicted onto it.

Metroid Prime (Dark Samus)
A Metroid heavily mutated by a Phazon Core organism. Fully capable of reproduction. It later stole Samus Aran's Phazon Suit which contained her DNA and used it to become her doppelganger, granting it high sentient intelligence and the ability to use Chozo technology. The creature is likely to have initially been a Tallon Metroid.

Infant Tallon Metroid (Metroid Cocoon)
An artificial strain created by Space Pirates. Intended to be used as organic portable batteries, they can also quickly grow into Tallon Metroids. Their cocoons are an artificial creation as well, used to sustain and carry large stocks of these infants.

Dark Tallon Metroid
A Tallon Metroid whose body is invaded and controlled by an Ing.

Phazon Metroid
The result of a Tallon Metroid heavily exposed to energized Phazon. This strain possesses a great number of unique abilities.

Miniroid
A mutated Infant Metroid born from an egg infused with Phazon. Miniroids can grow into Phazon Metroids.

Hopping Metroid
A further mutation of the Phazon Metroid that has acquired a physiology akin to a Gamma Metroid's.

Metroid Hatcher
A tentacled, armored Metroid mutant that branches from the Phazon Metroid strain. It is also capable of reproduction.

Mochtroid
Failed clones of the last surviving Metroid in the galaxy. They lack the physical strength and powerful defenses of a standard larva.

Big Metroid
An anomalous, powerful stage reached by the last Metroid from SR388 when it was brought to planet Zebes. The cause for the abnormal growth is unknown, though it is likely a side effect of excessive Beta-Ray exposure. Giant Metroid larvae can also be created when using the Amplification device.

Unfreezable Metroid
Larva Metroids that have been genetically modified to be immune to cold temperatures.

Samus Aran (Fusion Suit)
The heroine of the series becomes a Metroid hybrid on a genetic level after being injected with an experimental vaccine. The altered properties and appearance of her suit reflect the fusion of her DNA with a Metroid's.

Additional strains
There are a few Metroid breeds which possess very little difference from the strain they originate from. Several of the breeds observed throughout the series reveal new information about the species as a whole. For instance, in addition to draining energy, Metroids can also heal other organisms by injecting them with life energy taken from prey as demonstrated in Super Metroid by the giant anomaly. The scan of a Tallon Metroid in Metroid Prime 2: Echoes states that the stored energy in a Metroid's body can be forcefully drained from it and used for various purposes, allowing the bioform to be used as a living rechargeable power cell. However, the same scan reveals that cellular breakdown can occur in the Metroid if its body is repeatedly drained. It is also revealed through the Pirate Homeworld scans in Metroid Prime 3: Corruption that Metroids have a sensitivity to certain sonic frequencies, which the Pirates exploited with tank barriers to subdue several Phazon Metroids in the Processing chamber.
 * Zebesian Metroid: Larvae that have adapted to planet Zebes. They are identical to the Larva Metroids from SR388 in terms of attack and resilience to damage. A scan in Metroid Prime 3 suggests that the Zebesian breed is potentially more aggressive.
 * Aetherian Metroid: Tallon Metroids imported to planet Aether acquire red translucent membranes instead of the common green. No other difference is observed.
 * Talvanian Metroid: Although all Metroid Eggs and Larvae on planet Talvania have been artificially enlarged with the Amplification Beam by the Space Pirates, they do not possess any note worthy biological changes otherwise.

One of the most interesting discoveries concerns a power rarely demonstrated by the species: their capacity to rob a prey's natural ability via energy draining and, potentially, using the stolen ability afterwards. The only confirmed instance of this power is when the Big Metroid removed the Mother Brain's Laser Brain Attack, though the creature did not use the stolen ability and instead transferred it to Samus Aran. There are three other events in which this ability-stealing power was possibly used by Metroid organisms but cannot be verified. The first involves the Metroid Prime being able to produce Phazon after supposedly devouring a Phazon Core, and the second concerns the BOTTLE SHIP's Metroid Queen being able to breathe fire after absorbing Ridley's clone. The third is Samus herself potentially demonstrating this power after becoming a Metroid hybrid, as she gains several abilities that naturally belonged to animals after she absorbs Core-X Parasites that were either mimicking or inhabiting those very same animals. An example is the Serris mimic's natural high speed becoming Samus' Speed Booster following her absorption of the creature.

Metroid manual
"This protoplasm in suspended animation was discovered on the planet SR388. It clings onto Samus' body and sucks his energy. It can't be destroyed directly with the normal beam. Freeze it with the ice beam, and then fire 5 missile blasts at it."

Virtual Console retranslation
"This protoplasm, discovered on the planet SR388, clings to Samus and drains her energy. It can't be defeated with a normal beam, just repelled. Use a combination of the Ice Beam and missiles to defeat it."

Official Nintendo Player's Guide
"This is a mysterious life form which was discovered on the planet SR388. It sticks to Samus’ body and sucks out her energy."

Metroid II manual
"This is their first shape after hatching from eggs. They will cling to any creature that they can find, drawing its victim's life energy away."

Super Metroid manual
FLOATING LIFE FORMS - METROIDS

"It is said that Metroids are life forms created by an ancient civilization. Metroids engulf living creatures and absorb their energy. They are very intelligent and quick to reproduce."

Super Metroid Nintendo Player's Guide
"Freeze them, then blast them with Super Missiles."

Official Metroid Fusion website
"These energy-based creatures have the ability to absorb the life force from any living thing. As the natural predator of X-Parasites, however, the Metroid may now be the key to Samus's survival."

''An energy-based life-form with a translucent, spherical upper body. Claw-like spikes protrude from the lower-half of the creature, which it uses to latch onto its prey."''

Metroid: Zero Mission manual
"Metroids are the floating organisms discovered on planet SR388. These fearsome predators latch onto other organisms and leech the life energy out of their prey."

Official Metroid: Zero Mission website
"SPECIMEN ID# M-1A

"Extremely dangerous! Gelatinous exterior of Metroid makes it impervious to beam weapons. Metroids can only be destroyed by Missiles while frozen. Large claws grip prey while smaller front fangs drain its life force. Transform to Morph Ball and use Bombs to escape grip. DANGER LEVEL: HIGH"
 * "Gelatinous exterior"
 * "Multiple brain stems"
 * "Gripping claws" (Outer)
 * "Energy-sapping fangs" (Inner)

Official Metroid Prime Website
"Metroids are highly dangerous parasites averse to cold temperatures. The dominant species of planet SR388, Metroids can suck the life force out of living things. A Metroid will latch onto its prey and drain energy, growing larger as it does.

Metroid Prime Pinball manual
"An energy based, highly dangerous parasite."

Smash Tips
"These alien creatures float around and then try to clamp down on you with their sharp fangs. Quickly struggle to break free from their grasp."

Smash Tour (SSB4 Official Game Guide)
"Lasts 5 turns or until a player bumps into it two times"

"Bumping into this enemy costs you several stat boosts! Reclaim lost stat boosts by bumping ino it a second time."

Appearances in other media



 * Metroids appear in the Kid Icarus franchise, under the name Komayto, which is stated in the instruction manual to have come from another planet. This creature can be found flying around Skyworld in packs. Kid Icarus was also created using the Metroid game engine. During the game Kid Icarus: Uprising, the main character Pit states himself that "Komaytos kind of look like little Metroids" before being hushed by Viridi.
 * A Metroid is a collectible treasure in Wario Land II. They are also often in 9-Volt's microgames in the WarioWare series, such as WarioWare, Inc.: Mega Microgame$!, WarioWare, Inc.: Mega Party Game$!, WarioWare: Touched! and WarioWare: Twisted!.
 * Metroids made an appearance in Kirby's Dream Land 3 for the SNES. If Kirby freezes all the Metroids in the level, Samus removes her helmet and gives Kirby a Heart Star.
 * Metroids wander onto stages randomly in Metroid Prime Pinball and also have their own minigame, Metroid Mania.
 * In Metroid Prime 3: Corruption, a Metroid is used on the tab for the creatures section of the Logbook. Interesting to note is that the Metroid depicted here has six nuclei, five encircling the top of a larger, centralized nuclei.
 * The Baby from Metroid II: Return of Samus and Super Metroid appears as an item in the Nintendo DS game Animal Crossing: Wild World. It appears in its signature capsule, and if tapped will play a few seconds of the Super Metroid title screen theme. It is a rare item that is acquired randomly by shooting down Gulliver. The item also appears in Animal Crossing: City Folk, but cannot be transferred through the ingame catalogue due to the item's rarity. It acts the same way as before and is obtained in the same manner as well; it will play the entire Super Metroid theme this time however. The same item appears once again in Animal Crossing: New Leaf, but it is not obtained through the same method as the two previous games. Instead, it is randomly acquired through fortune cookies, which the player can buy using two of the 3DS's Play Coins. This version also plays only a few seconds of the title theme.
 * In Tetris DS, there is a single player game mode called Catch Mode where players catch Tetraminos (blocks) in order to create a 4X4 square and detonating it to make the cluster smaller. The Brinstar theme plays during this and encapsulated Metroids (in capsules similar to the Metroid hatchling's) fall down. If touched, the energy meter will go down and the cluster becomes smaller.
 * A Metroid can be seen on Nintendo Monopoly representing St. James Place for $180.
 * A Metroid is shown in a capsule in Singularity along with the message "Mother my brain hurts".
 * A Metroid is seen in a tank in Starcraft 2: Wings of Liberty in the secret Mission "Piercing the Shroud." It is very similar to that of the baby.

Super Smash Bros series

 * Metroids appear in a minor reference in Super Smash Bros., in Super Smash Bros. Melee as a trophy as well as the baby being seen in the opening cutscene, and in Super Smash Bros. Brawl and Super Smash Bros. for Nintendo 3DS and Wii U as an Assist Trophy that will latch onto opponents and "drain" them of energy (what they actually do is increase the victim's damage, but can still sap health if a stamina match is being played). It can be shaken off, which will cause it to try and seek another victim. It can phase through platforms similar to a Phazon Metroid. Using Up-Special moves while the Metroid is latched on to a character cancels the move after the few starting frames; this can be done multiple times in a row. This allows some characters to stay in the air for quite a while. Interestingly, the model of a larval Metroid used in-game for the Nintendo 3DS and Wii U games exhibits unnaturally large inner-fangs; in fact, they appear to be larger than the outer ones. In Melee, the intro scene from Super Metroid is reflected off of the trophy, with the infant missing from the scene.

Super Smash Bros.
Metroids are briefly mentioned in Samus's "Characters" bio in the "Data" section of SSB. Her bio says that Samus "...pursues the airborne life form, Metroid, throughout the whole universe."

Super Smash Bros. Melee
"A parasitic life form that can absorb all types of energy, Metroids have strong resistance to most conventional weaponry. To defeat them, Samus Aran had to freeze them with Ice Beam shots and then blast them with missiles. Mochtroids, which look like Metroids, are weaker, with fewer internal nuclei."

(Metroid 08/89).

Super Smash Bros. Brawl
"A bioengineered life-form found on planet SR388. Metroids attach to organisms and drain life energy. The Galactic Federation commissions Samus to eliminate them, but Space Pirates try to harness their power. One of the few ways Samus can kill Metroids is by shooting them with the Ice Beam and then shattering them with missiles. The Metroid's cry is chilling and indescribable."

Metroid (1987)

Metroid II: Return of Samus (1991)

Smash Bros. DOJO!! data on the Assist Trophy
"Like the name says, this Assist Trophy is from the game Metroid. It's an artificial life-form that absorbs all kinds of energy.

''In our game, it also attaches to the head and starts to drain its victim. Since it will increase your damage, try to shake it loose."''

Stickers

 * Metroid Metroid Prime Pinball - [Freezing] Attack +18 (All)
 * Metroid Metroid: Zero Mission - [Electric] Attack +20 (Samus, Zero Suit Samus)

Super Smash Bros. for Nintendo 3DS and Wii U Trophy

 * NTSC
 * "First discovered on planet SR388, this floating life-form grows by absorbing the energy around it. It is incredibly durable but extremely weak to cold. In Smash Bros., a Metroid will grab a rival by the head and drain energy. If you get grabbed, shake it off by pressing left and right!"


 * PAL
 * "Mysterious floating life-forms, discovered on the planet SR388, that absorb energy from other living beings. They're tough, except when they're cold. In this game, they'll try to attach themselves to fighters' heads and drain their energy. Press all the buttons you can to shake them off!"

Nintendo Land
Metroids appear as robots. They can suck out the Mii's energy but they also like to drop Miis in the Lava on the volcano map. Metroids must be frozen with the Ice Charge before shooting their core to kill them, reminiscent of how they are killed in the official Metroid games. This game also implies that the Chozo went extinct because they lost control of the Metroids.

Etymology
Kiyotake: ''There was one more staff member, and the two of us decided on it. We attached "android" to the "metro subway" and that's how we got "Metroid".''

Trivia

 * Samus Aran eventually becomes a Metroid hybrid akin to her doppelganger Dark Samus who also possessed both human and Metroid DNA.
 * Chronologically, Samus' first encounter with Metroids occurred in Tourian; ironically, the last of the species would later become extinct there.
 * While the Metroids' purpose was stated in the manual for Metroid Fusion to be the eradication of the X Parasites, the backstory in the PAL Super Metroid guide tells a different story. It states that the Metroids may have been given their ability to transfer energy into organisms by their creators with the hope that the creatures could use it to benefit humanity.
 * The eggs which can be found on the Pirate Homeworld in the room Metroid Processing are in a tank with large metal 'hands' clasping them. This is similar to the scene in Jurassic Park, where dinosaur eggs are being produced and hatched with a similar metal claw.
 * Bioforms that are killed by Metroids turn into husks with characteristics that, for unknown reasons, vary during time. Some turn into an extremely fragile brown or colorless husk that crumbles into dust on contact, while others become hardened corpses that cannot crumble.
 * Metroids have never been successfully contained without them breaking free sooner or later, either through their own means or through the help of another being, intentional or not.
 * Additionally, it is arguable that Mother Brain's telepathic powers were unable to maintain complete dominance over Metroids, as a few varied incidents in the past have provided some skepticism to the level of control Mother Brain has had on the Metroids.
 * There have been no known cases of intraspecific competition between Metroids for right to feed. In occasions where several Metroids are in the presence of a potential prey, they will take turns attacking the victim (in the Prime series and Zero Mission gameplay), or will group together to feed on a victim simultaneously (as seen in Metroid II, Super Metroid and during a cutscene in Zero Mission).
 * Dark Tallon Metroids were seen piling Pirate Troopers together on Dark Aether, seemingly conserving their food for later group feedings. However, this behavior is most likely non-existent in other Metroid variants, as these Dark Tallon Metroids are actually controlled by the sentient race known as the Ing. Interestingly, this also means that the Ing were capable of overcoming their possessed victims' insatiable hunger and no longer devour their prey quickly and completely.


 * A Metroid project "Dread" is mentioned in a scan in Metroid Processing detailing failed Pirate plans to use Metroids as a battery source, although many fans assumed it was a reference to Metroid Dread, a supposedly lost Metroid game.
 * The full limitations of Metroid behavior has not fully been explored. However, it may extend beyond the simple feeding instinct, as demonstrated by the baby in Super Metroid. Technically, it had grown beyond the imprinting of Samus as its mother, yet it still recognized her and restrained itself, even going so far as to save her from Mother Brain and transfer energy and a new weapon back to her.
 * Ironically, the Galactic Federation has been successful in many elements concerning the Metroids, many of which the Space Pirates ultimately failed to achieve despite years of trial and error. The ringleaders had found an extremely reliable way of controlling Metroids in the form of the humanoid MB. They had also successfully rid the creatures of their greatest vulnerability: cold. Finally, the Federation was able to give the Metroids access to their natural mutations, despite the creatures being located outside of their homeworld, SR388.
 * The Metroids have actually been able to nullify their weakness to cold on one occasion, long before the Federation did. This was seen in specific Fission Metroid variants and Metroid Prime's core form during Samus's mission on Tallon IV. However, this invulnerability was unintentional, as these two Metroid species had achieved this ability through an unpredictable series of mutations caused by Phazon.


 * During the events of Super Metroid, the Space Pirates allegedly tried different ways to clone Metroids other than using Beta-Rays, resulting in the weaker variant seen in Maridia, Mochtroids.
 * Metroids seem to not have the need to breathe oxygen. This is seen when entering the G.F.S. Valhalla; while exploring the destroyed ship, Metroids are seen outside of the ship and wandering about in space. Also, an Omega Metroid was in the hanger bay of the Biologic Space Laboratories research station, as the door was broken showing the vacuum of space, possibly having the room decompressing.
 * In Metroid Prime and Metroid Prime 2: Echoes, Metroids who have broken free from their containment tubes will engage in combat with the Pirates and vice versa. However, on the Pirate Homeworld in Metroid Prime 3: Corruption, both sides will ignore each other and assault Samus in tandem. A logical explanation would be that both sides, being corrupted by Phazon, no longer see each other as adversaries (this has been noted in Pirate research experiment reports found by Samus in Metroid Prime) or that both species are forced by Dark Samus to work as a group. This could also be due to a programming error, as both creatures never appear in the same room outside of containment unless Samus trips the alarm and enters a Metroid-infested room, forcing the Pirates to appear in it.
 * In Metroid Prime and Metroid Prime 2: Echoes, when a Metroid collides with Phazon or an open door, it will explode.
 * Ironically, the Blue X of Metroid Fusion attempted to counteract against Samus due to her Metroid cells and her weakness to cold. It is doubtful Cold X existed back on SR388 due to the planet's lack of a cold climate. If they had existed, it is possible the Blue X would have had an advantage over the Metroids on SR388.
 * In early encounters with the SA-X, the imitation would attempt to kill Samus by using the traditional tactic used to kill larval Metroids (freezing her in place with the Ice Beam, then shooting a Super Missile at her).
 * Some Space Pirates are known to have treated Metroids as pets. Although High Command strictly forbids this, some Metroids can be scanned with evidence of being recently fed with Space Pirate rations and pet treats. This caused the Metroids in question to suffer food poisoning, and occasionally became frenzied creatures.
 * Metroids are the Metroid series equivalent to the titular Aliens of the Alien series, featuring a life cycle, a Queen producing eggs and being a menace to the female protagonist.
 * Coincidentally, the spinoff prequel film Prometheus, created well after the Metroid franchise, implies that the Xenomorphs were created by an advanced alien race, the Engineers, for use as a bioweapon, similar to the Chozo developing the Metroids to stop the spread of the X Parasites. Similarly, the film Alien: Covenant reveals that the Xenomorphs were created by the android David 8 by merging Neomorph DNA with that of a wasp.
 * A Metroid appears in the Piercing the Shroud level of Starcraft II: Wings of Liberty, in a capsule similar to the baby's.
 * Metroid Other M Metroid Art 94.jpg
 * The background of Metroid Prime Pinball 's Wireless Mission mode features a Metroid with many more, smaller nuclei and a jagged shell.
 * When reading the temporary scan of a creature in Corruption, a pair of Metroids are seen to either side of the scan text. The nuclei of the two Metroids are placed side to side and are connected by lines to the bottom of their membrane.
 * NES Remix includes a Metroid Miiverse stamp.
 * Phazon was originally explained as the remaining energy from the Metroids Samus exterminated on Zebes, undergoing fractal cloning.
 * Despite their weakness to cold temperatures, Metroids can survive in space, which constantly fluctuates between extreme heat and extreme cold.

Gallery
For official artwork, see Metroids' Gallery.

Notes and References
Метроид (вид) Metroid Estándar Metroid (specie)