Red Worms Living in Kratky Hydroponics Roots

These bins are about 2 feet off the ground, I didn't put them damn things in there, there was no soil at all used when I planted these plants. They are only in one bin. The fertilizer in this bin is pure "Masterblend" Synthetic. I just don't understand how they got in there but clearly they are living and seem quite healthy and plump. When I harvest this lettuce I will relocate them to a wicking bed.