Jelsacs are small, crab-like creatures that appear in Metroid Prime 3: Corruption. Encountered on the living planet Phaaze, these creatures are highly aggressive and toxic, just like every other organism on Phaaze. It has a dark-purple carapace, two large foreclaws, and a gelatinous sac on its back.
When disturbed, the Jelsac will approach Samus Aran and the sac on its back will inflate as it fills with a corrosive toxin. After a few seconds, the sac will explode and spray its corrosive contents all over the place, causing significant damage to Samus if she is caught in the blast. Fortunately, this self-destruct reaction can be triggered from a distance by any of Samus' weapons, so firing on Jelsacs from far away is a good way to clear away a potential obstacle without suffering damage.