Thanks for all of your game play questions!

This particular crab is just a cute pet your villagers can play with, though another crab will appear later in the game with a very specific purpose!

The Lemur will automatically explore volcanic cracks whenever they appear and anything it finds will be awarded to your villagers.

Regarding the disappearance of your Lemur, we unfortunately can't review your game for you, as you're playing one of the paid versions; we can only support the free-to-play versions of Virtual Villagers: Origins 2.