Pszemol,
Yes, I have actually witnessed one capturing a crab. The worm was about
2.5-3" long and 1/4" or so in diameter (the visible part at least).
Yesterday it was "siffing" the san and came acrost a crab that fell on
it's back. Fro what I saw it grabbed the crab and retracted to its
hole.
On previous ocasion I just was one in the shell, not the actuala
attach, so I assumed taht crab had just died from natural causes.

Thank you
Yuriy