From Halopedia, the Halo wiki
Merse are a species of carnivorous aquatic creature. They resemble fleshy lily pads ringed with long black teeth, floating beneath the surface of the water and rooted to the lakebed by thick stalks. They are highly territorial towards intruders in their waters.
Merse are sensitive to electromagnetic fields, and respond violently when a strong source of them (such as a Forerunner suited in armor) approaches. They are also irritated by excess noise and rough weather. They communicate by "singing" to one another, and by using instruments to replicate their songs, one can sail through them safely. Merse reproduce asexually, and their songs change in tone and composition with each generation. However, the new generations rarely invent new songs, and instead often recycle older ones in a cyclic fashion.
Their home planet is unknown, but it is known that a population of merse were transplanted into Djamonkin Crater on Earth to guard the Didact's Cryptum. Additionally, the Librarian raised merse in the garden of her and the Didact's domicile.
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