Monstrous Crab (CR 3)

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

AC: 19 (-1 size, +2 Dex, +8 natural), touch 11, flat-footed 17
Hit Dice: 7d8+35 (66 hp)
Fort +10, Ref +4, Will +2
Speed: 40 ft., swim 30 ft.
Space: 10 ft./10 ft.
Base Attack +5; Grapple +19
Attack: Claw +10 melee
Full Attack: 2 claws +10 melee
Damage: Claw 1d8+9
Special Attacks/Actions: Constrict 1d8+9, improved grab, powerful claws
Abilities: Str 22, Dex 14, Con 21, Int -, Wis 10, Cha 2
Special Qualities: Amphibious, vermin traits
Feats: -
Skills: -
Advancement: 8-10 HD (Large), 11-21 HD (Huge)
Climate/Terrain: Temperate coastal
Organization: Solitary, pair, or swarm (6-10)
Treasure/Possessions: None

Source: Web

Constrict (Ex): With a successful grapple check, a monstrous crab can crush a grabbed opponent, dealing 1d8+9 points of bludgeoning damage.

Improved Grab (Ex): If a monstrous crab hits an opponent that is at least one size category smaller than itself with a claw attack, it deals normal damage and attempts to start a grapple as a free action without provoking an attack of opportunity. If it gets a hold, it also constricts on the same round.

Powerful Claws (Ex): A monstrous crab always applies 1.5 times its Strength modifier to damage inflicted with its claws. Additionally, it gains a +4 racial bonus on grapple checks.

A monstrous crab is straightforward in combat. It lumbers forth toward the nearest target and attacks with its claws. Once a monstrous crab has a morsel or creature in each claw, it retreats into the water to feed. Creatures held in its claws when it does so soon drown if they can't breathe water.

Monstrous crabs are scavengers, and they feed upon the reeking bodies of dead sea creatures that wash up on shore. Despite this, they aren't above attacking still-living creatures upon sight, since monstrous crabs are always hungry.

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.