In order for your Snainle to hatch and come out of its shell, set a green leaf outside of it. It will stimulate digestion for the snail and create hunger pangs. Then, they'll come out and eat!
Snainles love eating greenery in and around the fresh waters of the Ark River. They eat so much, however, that they can negatively impact the ecosystem and foliage around them and strip it bare. If you ever come across a brown patch near the Ark River, there were and are Snainles nearby.
Thankfully, the Snainle gestation periods are extremely long so their population is relatively well controlled. It is carefully monitored by the Science and Research Center in Ark City.