Siege Crab, Greater (CR 18)

Colossal Vermin (Aquatic)
Alignment: Always neutral
Initiative: -2 (Dex); Senses: darkvision 60 ft.

AC: 38 (-8 size, -2 Dex, +28 natural, +10 deflection), touch 10, flat-footed 38
Hit Dice: 32d8+288 (432 hp); DR: 15/adamantine
Fort +35, Ref +16, Will +18
Speed: 60 ft., swim 60 ft.
Space: 30 ft./20 ft.
Base Attack +24; Grapple +54
Attack: Claw +30 melee
Full Attack: 4 claws +30 melee
Damage: Claw 2d10+14
Special Attacks/Actions: Constrict 2d10+14, improved grab, trample 2d8+21
Abilities: Str 38, Dex 6, Con 28, Int -, Wis 10, Cha 2
Special Qualities: Amphibious, force carapace, shielded compartment, vermin traits
Feats: Great Cleave; Power Attack
Skills: Swim +22
Advancement: 33-60 HD (Colossal)
Climate/Terrain: Warm aquatic
Organization: Solitary
Treasure/Possessions: None

Source: Monster Manual III

A greater siege crab is simply a larger version of an ordinary siege crab.

Trample (Ex): Reflex half DC 40. The save DC is Strength-based.

Shielded Compartment (Ex): A greater siege crab's passenger space is about 10 feet by 20 feet by 10 feet; it can hold 4 Large creatures or 16 Medium or Small creatures.

Greater siege crabs are almost 30 feet in diameter, stand 20 feet tall, and weigh over 130 tons.

Aquatic Subtype

Creatures with the aquatic subtype always have swim speeds and thus can move in water without making Swim checks. An aquatic creature can breathe underwater. It cannot also breathe air unless it has the amphibious special quality.