Normally, the species reproduce via a Queen laying eggs, but if they are exposed to extremely high amounts of Beta-Rays, Metroids are capable of asexual reproduction via division, in a process much like a cell undergoing mitosis. This artificial method of propagation seems to be restricted to the Metroid's jellyfish-like phase, specifically the Infant and larval stages; whether those in the later stages of their life cycle can be multiplied in the same manner is never stated upon.
The Space Pirates have used Beta-Rays on the creatures since the time they stole the first discovered specimens from the Galactic Federation's Space Research Vessel Marina. When the Pirates reached Tourian, their headquarters on planet Zebes, they continued to expose them to the radiation in order to greatly increase their initially low number of Metroids. This allowed them to perform any number of research and experiments on the species in the hopes of exploiting their unique powers and ultimately achieve ways in using them as bioweapons against galactic civilizations.
This method of Beta-Ray exposure is presumably what allowed the Pirates to maintain a steady supply of Metroids throughout the years that covered the entirety of the Metroid Prime series following the downfall of Zebes. Interesting to note is that the highly radioactive mutagen known as Phazon has also demonstrated on several occasions the power to multiply unnatural strains of Metroid larvae. Interestingly, some mutants created via Phazon exposure acquired the ability to spawn Metroids.
Following the Space Pirate organization's multiple defeats and the destruction of their research bases at the hands of Samus, their supply of Metroids eventually ran out. To prevent future misuse of the now-dreaded species, the Galactic Federation ordered the extinction of the Metroids on their natural homeworld, SR388; this effectively left the Pirates without a source of specimens until the capture of the last Metroid on Ceres Space Colony. After successfully stealing it and bringing it to their rebuilt base on Zebes, the Pirates were quick to use the tried-and-true Beta-Rays on it and resupplied themselves for the last time before the organization's complete collapse at the end of Super Metroid.
Possible side effectEdit
The Super Metroid comic has suggested that excessive beta-ray exposure can mutate a Metroid and cause it to undergo a monstrous growth in size and strength. This occured to the last living Metroid during the events of both the game and comic. Although the events of the latter barely follows the plot of the game and is therefore non-canon, its suggestion of the side effect may hold some validity within the canon series. Considering the circumstances in which, following the theft of the Metroid from the Ceres Station, the Space Pirates found themselves in a race against time to produce a satisfactory amount of specimens before a quick retaliation occured from Samus and/or the Galactic Federation, it can be assumed that the marauding race did indeed resort to exposing the lone Metroid with an unprecedented amount of Beta-Rays.
As to why this mutation had never transpired with other specimens in the past, several sources of media have stated the space vessel Marina had several Metroids onboard when the Pirates attacked it. Therefore, the organization had the luxury of exposing their numerous specimens to an equal and simultaneous amount of Beta-Rays rather than being forced to use a single individual as the initial sole target of all radiation exposure.
The existence of Mochtroids is an additional implication of the Pirates' overusage of Beta-Rays during Super Metroid. These are failed attempts at cloning the species, demonstrating that other methods were used in desperation to create more specimens instead of solely using the reliable Beta-Rays. While true Metroids were eventually spawned with Beta-Rays which rendered the Mochtroids obsolete, the problem remained of the last Metroid being used as the entire backbone of the species' propagation on Zebes, likely leading to its gigantism.
- It is unknown if the Galactic Federation continued to use Beta-Ray exposure over the course of their Metroid Breeding programs on the BOTTLE SHIP and Biologic Space Laboratories research station after the successful cloning of the last Metroid.
- The Metroid: Zebes Invasion Order and Super Metroid Players' Guide backstories imply that the Federation were the first to use Beta-Rays on Metroids when the species was first discovered. This would thus be followed by the Pirates emulating the work achieved by the Federation.
- Due to the seemingly lack of a Queen Metroid on the B.S.L. station (contrary to the BOTTLE SHIP), it is more likely that Beta-rays were used by the Federation within said station.
- It is unknown if the Beta-rays' effect on Metroids was an intended feature implemented by the Chozo when they created the species. If it is the case, it would lean on the theory that the Metroids' life cycle on SR388, and thus the metamorphosis into an egg-laying Queen, was an unforeseen trait that the creatures developed on their own outside of their creators' intentions.