From 2speccers2tools wiki

Image note: A colony on a many km long escaped Domesticated Piss Monke.

Some Pee stringer spores land on many Giant Piss Eater descendants and grow very small, clutching onto the surfaces of their new hosts. Eventually, substrate developed so they didn't need to rely solely on clutching, though in order to colonies new areas and other hosts, they still retain the ability. Though they can grow on any large surface, such as piss comets or piss planet corpses, they are more broken up due to being disturbed either by grazing or being trampled, which is why they're the most abundant on Machine Fauna such as descendants of Giant Piss Eaters. When in favorable conditions and reaching their full grown size, they release massive spore clouds of string. The strings have been greatly reduced and feel more like silk or threads of string, being so thin.

A closeup of an individual Ureust.