Swim Bladder & Circulatory System
Buoyancy: fish may float in water despite higher weight of bony skeleton by help of swim bladder (sharks have to move or sink)
Buoyancy regulation: fill/drain gas (N2, O2) through gas gland
Missing in fish that swim fast or change depth rapidly (tuna)
Circulatory system: closed, two-chamber heart (pumps only deoxygenated blood)