FWIW, this is a conversion of the OD&D giant crab that I came across today as I was digging up the conversion information I had for X4. It's definitely the little brother of the monstrous crab that Shade linked to, but might prove useful.
Crab, Giant
Medium-size Vermin
HD: 3d8 (13 hp)
Initiative: -1 (Dex)
Speed: 20 ft.
AC: 18 (-1 Dex, +9 natural)
Attacks: 2 claws +3 melee
Damage: Claw 1d6+3
Face/Reach: 5 ft. by 5 ft./5 ft.
Special Attacks: Improved grab, squeeze
Special Qualities: Vermin
Saves: Fort +3, Ref +0, Will -1
Abilities: Str 14, Dex 9, Con 11, Int -, Wis 10, Cha 2
Skills: Listen +6, Spot +4
Climate/Terrain: Temperate and warm aquatic
Organization: Solitary or Group (1-6)
Challege Rating: 2
Treasure: None
Alignment: Always neutral
Advancement: 3-4 (Medium-size), 5-9 (Large)
Improved Grab (Ex): To use this ability, the giant crab must hit with a claw attack.
Squeeze (Ex): A giant crab that gets a hold automatically deals claw damage, with an additional 1d3 points of bludgeoning damage from the crushing force, each round the hold is maintained.