Their most well-known and identifiable form is that of an infant Metroid, which consists of a jellyfish-like membrane encasing three or more raspberry-like nuclei, and two sets of mandibles: the larger outer set for gripping onto victims, and the smaller set which extracts the victims essence. The kinetic energy generated by their bodies allows them to defy gravity and float above ground. At very early stages in a Metroids lifecycle, they will only be able to ram creatures as their mandibles have not yet developed enough to draw energy. Before long, though, they grow in size and are able to latch themselves onto creatures.
Tallon Metroids, a strain that were created by the Space Pirates to be more docile and used as portable energy batteries, can pass to their developed stage simply by absorbing some Phazon. Infant Metroids can only be dislodged if Samus can roll into Morph Ball mode and drop Morph Ball Bombs, which will cause the Metroid to let go of her. Metroids are notoriously resilient to conventional weaponry, the only exception being sub-zero temperature weapons like the Ice Beam and Ice Missiles, which will freeze their bodies, allowing them to be shattered by concussive blasts. If left to grow naturally, the infant Metroid will enter larval states and begin to evolve. Metroids exposed to Phazon, however, follow a different evolution pattern. They appear in almost every Metroid game to date.
After entering a larval state, an Infant Metroid will hatch into an Alpha Metroid, revealing its membrane growing a small insect-like creature on top. They are slower than infant Metroids and somewhat weaker in this evolution, only able to ram into Samus as they float around the screen. Firing missiles into its exposed membrane will destroy it. Alpha Metroids only appear on SR388 in Metroid 2: Return of Samus.
Gamma Metroids are the next stage after Alpha Metroid. The chitinous shell has grown larger and tougher in this evolution, as well as sprouting spider-like legs, and the Metroid is able to attack and absorb energy at range with a lashing whip of energy. Again, attacking its exposed membrane with missiles is key to victory. Gamma Metroids only appear on SR388 in Metroid 2: Return of Samus.
Zeta Metroids are Gamma Metroids that have further developed to have arms and legs, their membrane almost fully covered by their shell. Their reptilian heads can spit fireballs, and they are quite agile, floating around and attempting get behind Samus in an effort to avoid her shots. Missiles, again, will spell the end to a Zeta Metroid if fired at its membrane, which is only exposed at the front of the creature. Zeta Metroids only appear on SR388 in Metroid 2: Return of Samus.
The largest of the Metroid strains, Omega Metroids are huge reptilian creatures, a fully developed shell covering most of their weak membrane. Although slower than Zeta Metroids, the Omega Metroids are much stronger and more resilient. In Metroid 2: Return Of Samus, missiles are the only thing required to destroy the Omega Metroid. She also encounters one in the closing stages of Metroid: Fusion as she is escaping to her ship. The Omega will slash at Samus, nearly killing her, and paralyzing her, leaving her at its mercy.
The SA-X will then arrive and take on the Omega in a battle of the races, the Omega coming out the quick victor, due to the Metroid's natural disposition towards eliminating X parasites. The SA-X's death allows Samus to absorb it, however, and she is powered up with the Omega Suit and Ice Beam, which the Omega Metroid is weak to. She must fire charged shots at the Omega while avoiding its shots to destroy it.
The Queen Metroid is the highest form of natural Metroid evolution. The Metroid Queens produce infant Metroids, and their membrane is completely covered by a tough layer of chitinous armor. It appears in Metroid 2: Return of Samus as the final boss. The Queen will attack Samus by extending her crocodile-like head in attempt to bite her in its mighty jaws. While its mouth is open, Samus can fire missiles to stun the Queen. Samus is able to roll into Morph Ball mode and enter the Queens mouth while it is stunned before it retracts its head. This will damage Samus, but allow her to roll into the Queen's stomach in order to bomb the membrane inside her, after which the Queen will spit her out. Continuing in this way will destroy the Queen, allowing you access to the final room of the game containing the last infant Metroid.
Metroids that were exposed to Phazon evolved in a different manner to Metroids that were allowed to evolve naturally. Their main form is still basically the same apart from their membrane being split down the middle into to hemispheres, however, the corrupting power of Phazon could still be testing on even a Metroids body, meaning very few managed to evolve above their infant stage. Phazon Metroids are much the same as their normal infant cousins, however, as well as siphoning life force from their target, they also deal damage through the raw Phazon coursing through their bodies.
They also have the ability to phase out of reality in order to avoid Ice Missile shots, which are best used when they are otherwise distracted, such as when they are attacking. The X-Ray visor also reveals a small area that is their weakpoint, which can be pierced by the Nova Beam, killing them in a single shot. Metroids on the planet Phaaze also had a prior evolution, tiny versions of infant Metroids known as Minitroids, that floated around in large swarms. Phazon Metroids appear in Metroid Prime 3: Corruption.
Fission Metroids are Metroids that, through Phazon exposure, evolved into being able to split themselves in two, becoming completely impervious to most attacks. The colour of the Metroid's membrane signifies what beam weapon it is vulnerable to, and only attacks from that beam or beam charge combo can harm the Fission Metroid; yellow being Power Beam, purple being Wave , blue being Ice Beam and red being Plasma Beam. They only appear in Metroid Prime.
Hopping Metroids are curious in that they share very few traits with other Metroid species. Their appearance is closest in form to a Gamma Metroid and the shell of the Metroid Prime, spider-like and armored, although their nucleus is entirely encased within this armor. They are possibly an evolved form of Metroids that have had large exposure to Phazon. Hopping Metroids are unable to fly, scuttling around on their spider-like legs. They are more resilient than their flying brethren however, and are able to deal large amounts of damage with their Phazon talons and Phazon venom that they can spit when Samus is at range. They can also enter Hypermode at will, raising their strength and toughness considerably.
The Metroid Hatcher is a further evolved form of Metroids that have been mutated by Phazon. The Metroid Hatcher is completely covered in a protective shell, and resembles the Core Essence of Metroid Prime, although encased in heavy armor. Samus faces several of these creatures in Metroid Prime 3: Corruption. To defeat them, Samus must first fire at its tentacles, which will retract when fired at. Once they are all retracted, it will start to generate Phazon Metroids from its mouth, during which time it is vulnerable to attack. Enough shots to the mouth will stun the Hatcher, allowing Samus to latch onto one of its tentacles with the Grapple Beam and rip it off. Once all four tentacles are removed in this way, the Hatcher will be defeated. Once Samus has the Nova Beam and X-Ray Visor, however, she will be able to penetrate the Hatchers Phazite shell and fire at its weak point, which will kill it in a single shot.
The Metroid Prime is a powerful Metroid that was mutated by the corruptive power of Phazon. Prolonged exposure and absorption of the substance caused it to evolve beyond a typical Metroid, its limitless hunger for Phazon causing it to continue to grow in size and power. Its insatiable appetite continued even after its defeat at the hands of Samus Aran, ripping away her Phazon Suit enhancement and being reborn as the Phazon entity known as Dark Samus. Samus encounters the Metroid Prime as the final boss of the game of the same name.
The Metroid Prime has two forms. The first is the Metroid encased in spider-like chitinous armor, completely impervious to all attacks, except for at its weak point, the eyes. The Metroid Prime's armor also had a power-shift property, its current color depicting what beam weapon is was currently vulnerable to: yellow being Power Beam, purple being Wave Beam, white being Ice Beam and red being Plasma Beam. Only attacks made from corresponding weapon or the weapons' charge combo would harm the Metroid Prime. Its current color also effected the property of its attacks, namely the beam of energy it shot from the mouth in its underbelly, which could freeze Samus if the Metroid Prime was white, or disrupt her visors when it was purple. As well as these attacks, the Metroid Prime also fired slow-moving projectiles that could be destroyed for pickups, lasso Samus with a beam of energy and drag her into its jaws, and rear up and charge at her, which could be avoided by rolling into Morph Ball mode and rolling underneath the Metroid Prime in one of the grooves in the floor. Once Samus defeated the spider form, it retreated deeper into the Leviathan with Samus in pursuit. At the very heart of the Leviathan, she found the Metroid Prime had cast off its armor, revealing its core essence.
Metroid Prime's core will lash at Samus with its tentacles, as well as creating shockwaves that can be jumped. As well as these direct forms of attacks, the Metroid Prime will create pools of Phazon which will spawn Phazon Metroids and Fission Metroids, and then turn invisible. Samus can deal with the Metroids in the typical fashion by freezing them with the Ice Beam and then shattering them with a missile, or by laying a Power Bomb, which will decimate them. The only way to harm the Metroid Prime is to put on the X-Ray Visor to locate the creature, and stand in the pools of Phazon it generates, which will put Samus in Hypermode, allowing her to fire Phazon blasts. Once Samus does enough damage to the Metroid Prime, it will destabilize, and in a last ditch effort it will latch onto Samus, pulling the Phazon Suit upgrade from her. Samus flees as the Metroid Prime and the Phazon inside the Leviathan becomes unstable, escaping to her gunship in time to see the artifact temple collapse from the resulting explosion.