Mmmm, mudbugs! I love those tasty bottom feeders. The dragon magazine articles ones were kinda wimpy for their CR. Here is one a just built based off the large scorpion stats.
Lost: 30' of land speed, poison sting DC14 [1d4 Con /1d4 Con]
Gained: 50' Swim, aquatic subtype, amphibious special quality, +1 HD, +2 Str.
Fair trade? Or would an aquatic scorpion be the same CR without it's sting?
Monstrous crayfish, Large [/b]
Size/Type: Large Vermin {aquatic]
Hit Dice: 6d8+10 (39 hp)
Initiative: +0
Speed: 20 ft (4 squares) Swim 50 ft.
Armor Class: 16 (-1 size, +7 natural), touch 9, flat-footed 16
Base Attack/Grapple: +4/+12
Attack: Claw +8 melee (1d6+5)
Full Attack: 2 claws +8 melee (1d6+5)
Space/Reach: 10 ft./5 ft.
Special Attacks: Constrict 1d6+5, improved grab
Special Qualities: Darkvision 60 ft., vermin traits, amphibious
Saves: Fort +6, Ref +1, Will +1
Abilities: Str 21, Dex 10, Con 14, Int Ø, Wis 10, Cha 2
Skills: Climb +8, Hide +0, Spot +4
Feats: —
Environment: Any aquatic
Organization: Solitary or colony (2-5)
Challenge Rating: 3
Treasure: 1/10 coins; 50% goods; 50% items
Advancement: 7-9 HD (Large)
Level Adjustment: —
Monstrous crayfish are normally patient bottom feeders, but are likely to attack any creature that approaches them. Those whose habitats include dry land often lay in wait for creatures near water, charging when prey nears the water and dragging the prey underwater while tearing it apart.
Constrict (Ex) [/B] A Monstrous crayfish deals automatic claw damage on a successful grapple check.
Improved Grab (Ex) To use this ability, a Monstrous crayfish must hit with a claw attack. A Monstrous crayfish can use either its Strength modifier or Dexterity modifier for grapple checks, whichever is better.
Skills A Monstrous crayfish has a +4 racial bonus on Climb, Hide, and Spot checks.