Metroid (species)

Metroids are artificial organisms in the video game series of the same name. Discovered on the planet SR388 years after they were created by the Chozo, they were promptly used as a bioweapon by the Space Pirates. Metroids feed on the "life energy" of their prey 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.

According to the Super Metroid instruction manual, the Metroids were rumored to be created by an ancient race. In Metroid Fusion's manual, it is revealed that the Chozo were the ones who genetically engineered the Metroids in order to prevent the spread of the more dangerous X Parasites. This is expanded upon in chapter four of the Metroid manga. Interestingly, the name "Metroid" in the Chozo language means "Ultimate Warrior".

Physiology and morphology
The body of a "typical" Metroid consists of a thick gelatinous membrane (resembling that of a jellyfish) and contains a set of three (in Phazon Metroids) to four (in Tallon Metroids) red, raspberry-shaped, quadripartite/tripartite nuclei. They have two pairs of mandibles, one for gripping their prey and the other for extracting its life energy. Metroids appear to be able to directly sense the life energy of their prey, as they have no visible sensory organs. In the early stages of their development, Metroids hover in the air. Metroid Prime 2: Echoes explains that they defy gravity by using kinetic energy that is stored in their bodies. These mature Larval Metroids are the ones most often encountered in the Metroid games and are in the third natural stage of their life-cycle (including the egg stage). Traditionally, Larval Metroids are transparent or translucent with hints of certain colors (green, red, blue, etc)

Metroids are vicious creatures with no natural predators. They appear to be quite intelligent -- for example, when a Metroid is released into a room containing both Space Pirates and Samus Aran, it will attack the Space Pirates first as they are easier prey. Once attached to their prey, Metroids are very difficult to dislodge and will quickly drain their victim's life force completely, often killing it in seconds. Using Morph Ball Bombs is the only known way of dislodging an attached Metroid; consequently, Samus is one of the only two beings known to have survived a Metroid attack, the other survivor being Mother Brain by using an unknown 'resurrection' ability.

Metroids have an endless appetite and will feed on any and all living creatures they come across with the exception of their own species. This immense threat is compounded not only by their invulnerability to most forms of weaponry, but also because exposure to extremely high amounts of beta radiation causes them to asexually reproduce via division much like a cell undergoing mitosis; the latter ability is seemingly restricted to the Infant and Larval stages of a Metroid. Metroids have only one real weakness: an extreme sensitivity to cold temperatures. Once their membrane is frozen, it can be shattered by concussive weaponry (such as Missiles and Super Missiles), killing the Metroid. In Corruption, Samus was able to use the X-Ray Visor to lock onto a Metroid's nuclei, allowing her to use the high-frequency Nova Beam to penetrate the protective membrane and strike the nuclei directly, instantly dispatching the creature. While most Metroid variants seen in the Prime games still retain their weakness to cold, some specific mutated strains had rid themselves of this vulnerability.

In their natural life cycle (as seen in Metroid II: Return of Samus and near the end of Metroid Fusion), Metroids gradually metamorphose into a creature resembling an insect or a crustacean before finally attaining a somewhat reptilian form. As they change, they eventually replace their gelatinous membrane with a hard organic shell and grow a head and a set of limbs. The shell removes their weakness to cold temperatures, but all forms retain a section of weak, uncovered membrane containing a single nucleus which is vulnerable to attack. The only known form with a complete shell is the mutated Metroid Prime's first form. The egg-laying Queen Metroid appears to be the only form in which Metroids can naturally reproduce. As they grow, Metroids also gain the ability to project bolts of static electricity or spit toxic saliva at their prey.

Space Pirate Data found in Metroid Prime 3: Corruption states that Metroids mutate differently depending on what planet they are exposed to, such as the Hunter Metroids encountered on Tallon IV in Metroid Prime. Metroids cannot mutate into their natural advanced stages seen in Metroid 2 and Fusion unless they are on SR388 or a perfect replica. When mutated by Phazon, Metroids will also gain various odd qualities such as the ability to phase through walls. (See Phazon for notes on Phazon-mutated Metroids.)

The Metroids live on a mysterious energy that they drain from their victims. What this energy is remains unknown; the victim loses no bodily fluids but perishes nonetheless. As of Metroid Prime, this energy is referred to as "life energy". Metroids are stated to be energy-based organisms and thus increase in mass as they drain energy. Metroids can also feed off the energy used by Samus's Power Suit, which will protect Samus herself as long as it is active. However, were her suit to lose all of its energy, Samus would be rendered vulnerable to the Metroids.

In addition to draining life energy from victims, Metroids can heal other life forms by transferring stored life energy into them as demonstrated by the Infant Metroid in Super Metroid. This stored energy can also be drained from the Metroid, allowing it to be used as a living rechargeable power cell.

It is also revealed through Pirate Homeworld scans in Corruption that Metroids have a sensitivity to certain sonic frequencies, a weakness that the Pirates exploited to subdue several Phazon Metroids in the Metroid Processing chamber.

Metroid
In Metroid, Metroids were said to have been originally discovered by the Galactic Federation while researching SR388. Not long after that, the Space Pirates boarded a ship in which some Metroids were being transported and stole the specimens, taking them to their base deep within the planet Zebes. Shortly after that incident, Bounty Hunter Samus Aran was sent to Zebes to destroy the Space Pirates and their stolen Metroids.

Metroid Prime
A variation known as Tallon Metroids appear in Metroid Prime. These Metroids are smaller and less durable than the Metroids seen on Zebes; while they retain the weakness to the Ice Beam, Samus is able to kill them with any of her other weapons, though it will take longer for her to do so. Samus can even let them drain her Power Suit's energy until they explode, though this is a somewhat risky tactic.

After the Space Pirates discovered the mutagen Phazon, they exposed the Metroids to it along with many other life-forms, including themselves. This revealed two new stages in Metroid development that contradict the natural life cycle seen in Metroid II: 'Hunter' Metroids, which have orange pigmentation and two energy-extracting tentacles, and 'Fission' Metroids, which split into two Metroids when attacked. Also making an appearance is a creature called Metroid Prime, which is a Phazon-mutated creature with an unclear relation to Metroids.

In Metroid Prime 3: Corruption, various scan data on the Pirate Homeworld explains the contradiction in Metroid physiology as the result of being exposed to different atmospheres and other stimuli such as Phazon. This may also serve to explain why the Tallon Metroids in the first two Prime games were not as resilient as those seen in other games.

Metroid Prime: Hunters
Metroid Prime Hunters is the only game in the Metroid series that does not feature Metroids nor make any references to them (aside from the game's title). In the demo version, Metroid Prime Hunters: First Hunt, there are Metroids, but these are said to be holograms. Metroid Prime Hunters does, however, feature a new species: Quadtroid, a genetically engineered species that is similar in appearance and function to Metroids.

Metroid Prime 2: Echoes
In Metroid Prime 2: Echoes, the Space Pirates bring Metroids with them to Aether. Metroids in this game appeared with red membranes. The Ing later possessed some Metroids, creating Dark Metroids. Also seen are Metroid cocoons, which release very small Infant Metroids which need to come in contact with Phazon in order to grow to the size of normal Metroids. These Infant Metroids cannot yet absorb energy, and instead use ramming attacks to damage their foes. The logbook scan for these Metroids suggests that the Space Pirates may have somehow created this Metroid strain to be less dangerous and easier to transport, supported by the fact that in the end of Metroid II: Return of Samus, the Metroid hatchling is already in its Larval form when it hatches, though it is much smaller than the other Larval Metroids seen on SR388.

Metroid Prime 3: Corruption
In Metroid Prime 3: Corruption, four new varieties of Metroids are introduced. They came from Tallon Metroids that were raised on the Pirate Homeworld after their eggs were infused with Phazon and are known collectively as Phazon Metroids. These variations are the Miniroid, the adult Phazon Metroid, the Hopping Metroid, and the Metroid Hatcher.

Many Metroids are found on Phaaze at the end of the game, raising the question of how they got there. While husks of a creature almost identical to the Metroid Prime appear in one chamber, implying that the Metroids are somehow native to the sentient planet, Phaaze's great distance from all other locations in the Metroid series makes it more likely that the Space Pirates brought the Metroids along upon their arrival.

Interesting to note, in a room on the Pirate Homeworld in Metroid Prime 3: Corruption, there are three empty tanks. When scanned, the first reveals that the Metroid sample taken from Zebes has proven highly aggressive, even for a Metroid. The second tank reveals that a particular strain of Metroid from Tallon IV has been moved for further examination. The final tank reveals that the SR388 Metroid has incredible DNA-altering abilities and is being taken to a Research Station to be studied, referencing its evolutionary forms in Metroid II: Return of Samus and foreshadowing Samus's transformation in Metroid Fusion by the vaccine made of Metroid cells.

Metroid II: Return of Samus
In Metroid II: Return of Samus, seeing the dangers the Metroids posed to the galaxy, the Galactic Federation attempted to destroy them. After two failed attempts by the Federation's army to eradicate the Metroids on their home planet of SR388, they called on Samus Aran to deal with them as she was the only person able to survive several encounters with the creatures. It was in this game that the natural Metroid life-cycle was first revealed (both chronologically in the time-line of the series and the order in which the Metroid games were released).

The game begins with a total global Metroid population of 47, though the number displayed is 39 at first -- the last 8 Metroids hatch before Samus engages the Queen Metroid. Samus' goal was to explore SR388 and hunt the Metroids down one by one. As she progresses further into the planet, the Metroids continue along their natural life-cycle, eventually culminating in the Omega Metroids. Samus eventually makes her way into the Metroid nest at the center of the planet, destroying several Larval Metroids before engaging in a final battle with the Queen Metroid.

After the Queen was destroyed, an infant Metroid hatched and imprinted on Samus as its mother (thus raising questions as to the true instinctual intelligence of the average Metroid). Because of Samus's efforts, this infant was the last surviving Metroid in the galaxy. It helped Samus escape from SR388, and she took it back to the Federation Academy for further research.

Super Metroid

 * "The last Metroid is in captivity. The galaxy is at peace..."

"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."

In Super Metroid, the Metroid larva that had imprinted on Samus was studied by scientists on Ceres Station; it was found that the Metroids' potential as beneficial creatures was as great as their destructive powers. Right after Samus left Ceres Station, the Space Pirates besieged it, killing everyone onboard. Ridley stole the Metroid larva, taking it back to the Space Pirates' rebuilt base on Zebes.

The Space Pirates quickly used beta rays to create a new army of feral Metroids from the Metroid larva. In Maridia, Samus also found Mochtroids, early Space Pirate attempts at cloning Metroids. While they could leech life-energy, they were very weak and could not latch onto their prey.

At the end of the game, the Metroid larva sacrificed itself to save Samus from Mother Brain. Before dying, the Metroid demonstrated the life-saving properties that could have benefited galactic civilization (what the scientists revealed at the beginning the game): transferring energy to living beings instead of simply absorbing it. Its demise meant the apparent extinction of the entire Metroid species; it also demonstrated the Metroids' ability to show compassion.

One noteworthy fact: the Larval Metroid grew to a tremendous size without moving on to the next stage of a Metroid's natural life-cycle (by the end of the game, it is actually bigger than the Queen Metroid was). It could be that prolonged exposure to beta-rays caused this unnatural growth; another possibility is that the Metroid had been removed from its natural environment (thus missing some sort of compound/chemical only present on SR-388) and was therefore restricted to the larval stage. A similar anomaly in size is found in the Omega Metroid from Metroid Fusion, where it has grown 3 times taller than Samus unlike the Omega Metroids from SR388 that are about as tall as Samus.

Metroid: Other M
After Samus Aran completed her second mission on Zebes, she spends a great deal of time in thought, reliving the death of the Baby at the hands of Mother Brain. Adrift in space and in memories, she receives a message codenamed "Baby's Cry", reminding her of her lost "child". Following the signal, Samus arrives at the Bottle Ship.

In the Cryosphere, Samus finds a deceased Gigafraug showing signs of Metroid predation. Samus has doubts since not only would Metroids perish in the Cryosphere's cold temperatures, they were extinct thanks to the efforts of both the Space Pirates and herself.

Soon afterwards, MB, posing as Madeline Bergman, tells Samus about "Project Metroid Warriors", in which Metroids were cloned from the Baby's particles that landed on her suit and are located in Sector Zero. She also explains that they made an AI of Mother Brain's intelligence to control them. Samus heads to Sector Zero but stops short as an Infant Metroid pops up in front of her, reminding her of the Baby. When Samus decides to kill the Metroid, Adam shoots her in the back; the Infant sees her as food, but is also shot.

Adam explains that the cloned Metroids might have been re-engineered to remove their vulnerability to cold, making them virtually unstoppable; this also confirms that the dead Gigafraug Samus found earlier was indeed the subject of Metroid predation. He also states that the breeding project was named after him, though he was not actually involved in the project. He was petitioned for the authority to create the project, but he refused. However, scientists co-signed it to get the project running.

Because the Metroids are immune to Samus' weaponry, Adam activates a self-destruct protocol on Sector Zero that detaches it from the station before the explosion. Samus attempts to convince him to let her destroy the Metroids, but Adam ignores her, saying that she has work left on the Bottle Ship. Adam succeeds in his mission to destroy the Metroids at the cost of his own life.

However, a Queen Metroid was kept in another part of the Bottle Ship. It not only attacks Samus, but launches several other Metroids. Ultimately, Samus is forced to resort to an old tactic and uses a Power Bomb in the belly of the beast. The presence of a Queen Metroid in an environment different from SR388's may refute the theory of a lack of specific chemicals which disabled the Baby's evolution on Zebes (see upper) while it could be assumed that metroids become queens whatever the environment is, if it is not too cold and if they have the necessary genetic program (unlike 'classic' metroids, which need an environment similar to what can be found on SR388 to mutate, according to Metroid Fusion).

Metroid Fusion
Metroid Fusion takes place shortly after Other M and is considered "Metroid IV" in the timeline. On planet SR388, the Metroids had been the top predators, and with them gone the ecosystem was thrown completely out of balance. Samus was hired to protect some Federation scientists who were studying the recovery of the SR388 ecosystem. While they were on the planet, Samus was infected with an X Parasite. She was rushed back to Federation Headquarters with a minimal chance of survival. It had recently been discovered that the Chozo had created the Metroids to prey upon the X Parasites, which were dominating the ecosystem of SR388. Using this information, the Federation saved Samus by injecting her with a Metroid vaccine made from a DNA sample of the deceased hatchling.

Later in Metroid Fusion, it was revealed that the Federation had a secret Metroid breeding program going on (see Restricted Lab), presumably based on clones of the last Metroid. The program was destroyed by a powerful X Parasite mimicking Samus known as the SA-X. The entire laboratory was detached from the station and ejected into space just after Samus escaped, completely destroying the Federation's research. However, one Metroid had escaped and advanced to Omega level, and Samus had to defeat it before evacuating. Worthy of note is the difference in size between this Metroid and the Omegas found on SR388, the latter which were only slightly taller than Samus; it is a possibility that the Federation had exposed the Metroid to an unknown radiation that caused its irregular growth, much like the theory for the Infant Metroid in the prequel. At this point, the only known Metroid DNA remaining in the universe now resides within Samus Aran.

Logbook entries
Metroid Prime: "Metroids are energy-based parasitic predators. The dominant species of planet SR388, Metroids can suck the life force out of living things. A Metroid latches onto its prey and drain energy, growing larger as it does. The only way to shake an attached Metroid is to enter Morph Ball mode and lay a Bomb."

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."

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 his energy."

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."

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."''

Appearances in other media

 * Metroids appeared in Kid Icarus and its sequel, Kid Icarus: Of Myths and Monsters, 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.


 * 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.


 * 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.


 * 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.

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 as an Assist Trophy that will latch onto opponents and drain them of energy. 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 her "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)

Stickers

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

Real-World Etymology
The word "Metroid" can be broken into two parts: "Metro" (meaning "of the womb", referring to metrology, or a tunnel system) and "oid" (indicating a "similarity, not necessarily exact, to something else" or "having the likeness of"). The word also resembles "meteoroid", which refers to space debris. Its root word meteor comes from the Greek "meteōros", meaning "high in the air".

Trivia

 * The eggs which can be found on the Pirate Homeworld in the room Metroid Processing are in a tank with large metal 'hands' are clasping them. This is similar to the scene in Jurassic Park, where dinosaur eggs are being produced and hatched with a similar metal claw.
 * The first Metroid that Samus encounters in Metroid Prime, titled BR5497, is in a containment tube very similar to the hatchling's in Super Metroid.
 * 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, or (as seen in a cutscene in Zero Mission) a group will feed on a victim together.
 * Dark Tallon Metroids were seen pilling up many weakened Pirate Troopers in a single spot on Dark Aether, seemingly conserving their food for later 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 completely.


 * Strangely, although Infant Tallon Metroids are said to be used as batteries, a scan in the Metroid Processing room on the Pirate Homeworld in Metroid Prime 3: Corruption (next to the controversial Metroid Dread scan) says that a project to use Metroids to power mobile weapons failed. It could be possible that this project was for the use of adult Metroids, instead of the weaker and more manageable Infants.
 * The full limitations of Metroid behavior has not fully been explored. However, it possibly extends beyond the simple feeding instinct, as demonstrated by the infant Metroid 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 former organization 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 temperatures. Finally, the Federation was able to give the Metroids access to their natural evolution twice over, 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 the 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 the intergalactic substance known as Phazon.


 * Metroids seem to not have the need to breathe oxygen. This is seen in Corruption 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 in Fusion, 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. This may shed light on one of the theories behind Metroid Prime's origin, which consisted of a Metroid somehow traveling from SR388 to Phaaze.
 * In Prime and Echoes, Metroids who have broken free from their containment tubes will engage in combat with the Pirates and vice versa. However, in Corruption, both sides will ignore each other and assault Samus in tandem. A logical explanation would be that both sides, containing Phazon within them, show reduced hostility. This has been confirmed in Pirate research experiment reports found by Samus in Metroid Prime. Another possibility is that the two factions were not programmed to battle each other during development.
 * In Echoes, when a Metroid collides with any Door, it will explode.
 * In Metroid Fusion, Samus is saved from X infection by a Metroid vaccine only made from Metroid cells. This suggests that the Chozo engineered even the Metroid's base cells to prey upon those of the X.
 * Ironically, the Cold 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 possible, the Cold X could have slowed down the Metroid's activity on SR388, though it is unlikely they would have been able to kill a single Metroid, as they lacked concussive weaponry to shatter them. On the other hand, they could have used a large, powerful creature to break them (such as Arachnus).
 * 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). It is most likely from Samus's memories that the SA-X learned of this combination of attacks.
 * 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 equal to the Alien Series' Xenomorph.
 * Despite Metroids that cannot be frozen being a major plot device of Other M every Metroid encountered can be frozen except the Queen Metroid who's genetics were not tampered with.