These little beasties are new to the game and with the clues mentioned so far in game they seem to appear of their own accord. Yet to work out whether it's set to a regular time schedule or whether it's a randomised event.
But anyway, I thought I would kick start a thread solely dedicated to what a treasure chest can give.
First treasure chest. Opening cost: Free
Reward: 1 pearl
This was tied to a quest which gave a reward of 450 experience, 50 algae and 50 SD.