n large animal aquatic swarm 

Perception +9; darkvision, wavesense (imprecise) 60 feet

Skills Athletics +8, Stealth +9, Survival +7

Str +1 Dex +4 Con +3 Int -5 Wis +2 Cha +0

AC  18; Fort +10, Ref +9, Will +7

HP 30; Immunities precision, swarm mind; Weaknesses area damage 5, splash damage 5

Speed swim 25 feet

Clinging Bites [>] The trilobites in the swarm latch onto creatures and gnaw at them. Each enemy in the swarm's space takes 2d6 slashing damage (DC 18 basic Reflex save).

Bestiary 3, page 275.