alrite... those red worms? hmmm... is it those hairy type? those can sting man, and it gonna hurt ur finger. if its those hair worm u are talking about, then its bristle worm. for these worms, i dun have any experince with these worms harming my fish. but from wat i read, these worms can kill fish. But these are very slow creatures... so dun worry too much on it
for crabs, how big is ur crabs? small tiny crabs are harmless to ur fish. but when they grow larger, it might post a threat to ur LS.
for mantis shrimp(stomatopod), do a research on how it look like.
do u hear cracking sounds out from ur tank? "clap!" "clap!" "clap" ?
if so, it might be possible that mantis shrimp is hidden in ur rocks. and it hunts for ur LS, snail, fish, crab, shrimp.
do a search on stomatopod on the link below.
www.blueboard.com/mantis
the same shrimp that will produce the same sound its actually pistol shrimp. again, search the google and see how they look like. i didnt have any experince with this type of shrimp. so do do a search on the google.
for as the LR, of course when u boil it, it will be dead rocks. however if u allow it to cycle and providing optimun suitable condition again, it will be "resurrected".
and of course, u can DIY a trap for these pests. the result might not be instantaneous, it will need some time.
check out the link below for some ideas.
http://saltaquarium.about.com/cs/fishinver...quariumtrap.htm
well... hope my info and experince helps u in a bit.
good luck and cheers,
regards,
LaW