Slugs or snails. They creep out every night and cause havoc amongst nice fresh young shoots. If you've got any large plasic lemonade or mineral water bottles, cut the bottoms off and remove the lids. Place them over your individual small plants in pots. Press the base of the bottle firmly into the soil and anchor it with a thin stick poked right through the bottle into the soil. Unless it's very hot, leave them under cover like this until they've grown tall enough for the slugs not to easily reach the tips of the new shoots. I use this method regularly to protect runner bean plants which I've newly planted out and it works very well. I usually find a circle of slimy slug trails around the bottom of the bottle where they've tried in vain to get a nice tasty supper !.