- Main Caverns, Ruins I, Ruins II, Ruins III
After a Metroid’s first molting, it will turn into an Alpha Metroid. Alpha Metroids have a thick shell, as well as rudimentary legs and horns, and retain the distinctive Metroid globule. The Alpha Metroid is more advanced in that its shell protects it from being frozen, but the molting process has also removed the Metroid’s resistance to Missiles, as its shell has not yet fully hardened, nor does it effectively cover the Alpha’s vitals. The Alpha Metroid seems aware of this weakness, so tends to lie in secluded areas, ambushing prey that comes close. Despite its weakness, the Alpha Metroid is incredibly aggressive when threatened, ramming prey with little regard for its own safety. This tactic will normally ensure it will either kill its attackers or frighten them off, but can be exploited by a fearless, disciplined, and well-lead unit of Marines. Even so, Alpha Metroids are considered highly dangerous and best not taken lightly.
Alpha Metroids are the first Metroid mutations Samus encounters. They are the weakest form of Metroid by far and will fly forward quickly, but in short bursts. Because they are quite aggressive and appear in a variety of environments, they can be difficult for first-time players. Whenever an Alpha Metroid is hit with a Missile, it will fly back at an angle relative to the shot, so aim carefully. Since Samus can only fire one Missile at a time, the closer she is to the Alpha Metroid, the faster she can shoot. Since an Alpha is destroyed with only five Missiles, if you get close enough, you can fire all five in a fraction of a second.
You can cheap shot some Alphas (ones that aren't in the process of molting) by firing a Missile from a distance before the Alpha has registered your presence. The Missile will hit the Alpha, and when the Alpha begins to attack, your shot will immediately register as a hit. You cannot hit the Alpha more than once in this fashion.
The first Metroid Samus encounters on SR388 is in the process of molting into an Alpha Metroid. Metroids are presumed to molt after they have consumed enough energy, but only under certain conditions; in most cases, they will simply split into two. In Metroid Fusion, BSL scientists discovered the environment of SR388 specifically causes Metroids to molt. Since the X had not yet been discovered when the scientists were experimenting on the Metroids, their molting is not caused by the X parasite, which would otherwise be the simplest explanation for the molting process. Whether it is the temperature of SR388, native plant life, background radiation, or something else entirely, SR388 was the only planet known to support Metroid mutations.