Metroids in Prime will grow larger if they latch onto Samus and are removed or if they absorb a large amount of damage. You can also release Metroids from their cages, and they will attack any Space Pirates in the area before turning their attentions on Samus. Apparently, the Pirates had a problem with personnel taunting the Metroids in their holding pens... Metroids will also be destroyed automatically if they somehow exit the room in which they appear, such as by latching onto Samus and then leaving the room.
Longtime Metroid fans have also noted that in all previous games, the only way to defeat a Metroid was by freezing it and shattering it with Missiles or destroying it with Power Bombs. Also, due to the game having a much slower rate of fire for Missiles, it only takes a single Missile to kill a Metroid rather than the usual five. These changes were made to better facilitate the gameplay structure of Metroid Prime, even if they are somewhat inconsistent with traditional Metroid lore, leading some fans to classify them as a particular breed of "Tallon Metroids" mutated by Phazon.
The Space Pirates found Phazon strain Vertigo to be particularly effective on the Metroids. They also noted that Metroids would not feed on any of the synthetic energy they created and would only feed on "active bio-energy sources".
