History of the Metroids

The Metroids were a creation of the Chozo, created with the intention of combating the X Parasite. Their name in Chozo literally translates to "Ultimate Warrior". The Metroids soon became berserk, and were shown to prey on any life form. They have been supposedly exterminated repeatedly by Samus Aran, but they always return, usually through cloning.

Metroid/Metroid: Zero Mission
In Metroid and its remake, Metroids were said to have been originally discovered by the Galactic Federation while researching SR388. Not long after that, the Space Pirates boarded the Space Research Vessel Marina in which some Metroids were being transported and stole them, 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 in her Arm Cannon, though they 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 dangerous tactic as it requires considerable amount of energy.

After the Space Pirates discovered the mutagen Phazon on Tallon IV, 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,  a Phazon-mutated creature with an unclear relation to Metroids. All the Metroid variants encountered on Tallon IV, save for the Metroid Prime, can be dispatched with a single Power Bomb, contrary to other variants seen elsewhere. Danny Richardson designed several Metroid forms that were not included in Prime.

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 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 eventually possessed several of them, creating Dark Metroids. Dark Metroids cannot be seen through, unlike the non-possessed varieties. Also seen are Metroid Cocoons, which release Infant Tallon Metroids that require physical contact with Phazon in order to grow into the red-membraned Tallon 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. These Infants are much smaller and fragile than those seen on SR388. Being based off the strain from Tallon IV, all the Metroids encountered on Aether are highly vulnerable to Power Bombs.

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 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 or Dark Samus 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 mutations 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 on board. 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. The Metroids found in the earlier rooms in Tourian could only be dispatched quickly by freezing them then followed with blasts from concussive weaponry. Power Bombs could be used to harm them without the use of the Ice Beam, but did very little damage; in addition to having a slow rate of drops between each Power Bomb, they were far less reliable against Metroids this time around.

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 and had actually grown bigger than the Queen Metroid. 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, or the Space Pirates tried re-engineener it to be immune to its weakness or it's life-cycle in order to continue cloning infant metroids. 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 afterward, MB, posing as Madeline Bergman, tells Samus about "Project Metroid Warriors", in which Metroids were cloned from the Baby's particles that landed on Samus's Power 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 that have reached the mature larval stage are likely to 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.

Adam shot Samus earlier to prevent her from interfering with his next course of action: because the Metroids are immune to Samus' weaponry, Adam goes to activate a self-destruct protocol on Sector Zero by causing severe damage from within, which detaches the sector from the station before the explosion. Seeing this as a suicide mission, 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 disable the Metroids' natural mutations when located outside of their homeworld. However, it is currently unknown how MB succeeded in making an Infant Metroid reach the Queen stage.

Metroid Fusion
Metroid Fusion takes place shortly after Other M and it is "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 of it size maybe that the Infant Metroid was supposed to grow into a Queen Metroid as stated in the prequel and different theory maybe that the DNA of the Infant was still holding the re-engineered growth from the space pirates. At this point, the only known Metroid DNA remaining in the universe resided within Samus Aran, but any negative effects were likely removed during her absorption of the SA-X's "pure" genetic code.