Judging from the
Google results I just got, these are apparently a
mythical foodstuff among SF fans. No one's quite sure what they are, but most agree that they're
decidedly unpleasant.
However, I know what they are, because I've read (or at least started to read)
The Gripping Hand, by
Larry Niven and
Jerry Pournelle. Greeps are an alien species vaguely like
crabs, but less aquatic. They are
served live in
local restaurants in a large shallow bowl with edges just steep enough to prevent them from escaping. You stab one in the neck with a
crottling fork, expose the meat, and
eat it while it's still moving. The scene in the book where the protaganist orders them is quite memorable, and culminates with the line,
"So, wretched sand dweller! Now you will tell me of your troop movements!"
It's also my
band name. So don't steal it.