baby shrimp with curled tails?

[rye]

I have a colony of RCS that are doing well, reproducing etc. in the last generation they produced, a lot of the small ones keep their tails curled under at all times, they seem to not be able to swim that well. They can swim quickly enough, but they cant seem to steer that well, I guess the tail is a sort of rudder. None of them are full grown yet, but they dont seem to be growing out of it. They would probably be about 1-2 cm right now if they had their tails out. The adult shrimp and some of the young seem just fine, no deaths or anything. Just wondering if anyone has seen this behaviour before, and what does it mean?