Mutagen

The mutagen that was present in the Pit's waters mutated almost any being that stayed submerged in it long enough into a water-breather that resembled a sea Rahi. The mutation only took minutes of exposure to take its effect, and it was impossible to reverse without the Mask of Life.

Known Beings Mutated

 * Barraki
 * Matoran of Mahri Nui (though not to the point of becoming water-breathers)
 * Brutaka (slightly)
 * Toa Lesovikk
 * Spinax
 * Piraka (very slightly at this point)
 * Most of the Pit's prisoners

Known Beings Immune to the Mutagen

 * Nocturn
 * Hydraxon
 * Zyglak
 * Hydruka
 * Organic Rahi

Trivia

 * The Kanohi Ignika was being dissolved by the Pit's waters during the 2007 storyline, and as it did so it let off small amounts of its power.
 * On the way to the Chamber of Life, there was a large pool of water that the Piraka fell in that contained the mutagen.
 * Greg Farshtey has  revealed  that the mutagen was caused  by  the  Great  Cataclysm.