I'm gonna go with alyssia's theory and ask if they appear to be under intelligent control. Did you ever sit and watch them and see if something is reeling the net back in? I had ones that were just like a single strand of fishing line. It would shoot out of the rock, inch across the bottom picking up little bits, then reel back in. I never did figure out what was on the other end. The "what the heck is that thing" factor is why I love reef tanks. You can spend $1000s on coral and stuff but end up totally mystified by something like that. :notsure: