From Halopedia, the Halo wiki
A no-shell snail is a species of snail native to Gao, a human Outer Colony. Apparently shell-less, the animals are a troglobitic species that are blind and albino, thriving in the depths of the Montero Cave System near the Forerunner Covert Support Base 4276. No-shell snails are one of the few species that live near the support base, sharing their environment with fellow troglobites such as the ghost scorpions and chirping spiders. Some no-shell snails were cared for by Lifeworker Huragok Roams Alone near the Covert Support Base.