Petrasyls are floating, jellyfish-like creatures that appear in Metroid Prime Hunters. They are usually found in a group of two or more of their own kind and are not overly aggressive. They bear a striking similarity to Metroids and Mochtroids, but upon closer inspection are revealed to be more like Earth's jellyfish, having tentacles rather than talons. In addition, they also lack the ability to drain energy from other organisms, and instead rely on toxins they excrete that can burn through most materials. However, they are passive in nature, and prefer to hover around aimlessly in large groups, attacking only if touched.
Petrasyls are hyperthermophiles and are capable of living within lava flows, as seen in the Thermal Vast of Alinos. They are easy to kill and will perish from as little as two shots from Samus's Power Beam. In some places, such as the aforementioned Thermal Vast, Petrasyls live en masse, and will continue to emerge from the lava pools no matter how many Samus manages to slaughter.