Here are the monstrous crab stats from Stormwrack...
Monstrous Crab, Small
Reduce that to Tiny and cut the Toughness feat and it'll do for the component animal of a Crab Swarm...
Monstrous Crab, Tiny
Tiny Vermin (Aquatic)
Hit Dice: ½d8 (2 hp)
Initiative: +0
Speed: 20 ft. (4 squares)
Armor Class: 16 (+2 size, +4 natural), touch 12, flat-footed 16
Base Attack/Grapple: +0/-6
Attack: Claw +2 melee (1d3-2)
Full Attack: 2 claws +2 melee (1d3-2)
Space/Reach: 2½ ft./0 ft.
Special Attacks: Constrict 2d3-2, improved grab
Special Qualities: Amphibious, low-light vision, scent, vermin traits
Saves: Fort +2, Ref +0, Will +0
Abilities: Str 6, Dex 11, Con 10, Int —, Wis 11, Cha 2
Skills: Hide +12, Spot +4
Feats: —
Environment: Temperate aquatic
Organization: Colony (8–16) [
?]
[
I like Solitary, scrabble (2-4 swarms), or horde (5-8 swarms) for the Swarms]
Challenge Rating: 1/4 [
I feel it should be lower, like CR 1/6 or 1/8]
Treasure: None
Alignment: Always neutral
Advancement: —
Level Adjustment: —