The lazy way out (heh) is just to spell SHELL and sign FISH.
But suppose your conversation partner doesn't know what that means? Well then you get to do a very quick listing of CLAM, CRAB, and LOBSTER, followed by the sign for "so forth" (which is done using the sign VARIOUS).
SHELL-FISH = "CLAM/OYSTER-CRAB-LOBSTER-VARIOUS"
[Yes indeed, I've seen the other version of lobster that uses a "bent hand" and the thumb as pinchers on each hand. It is a good version that I need to add to my site.]
The sign for "VARIOUS" flutters the index fingers up and down as you move the hands out to the side.
Also see: FISH
Notes: