Shark

The Shark is an unlockable fish with 2 evolutions

Stealthy Hunter
The Shark becomes invisible and can't be eaten for a short time. Bonus for enemies eaten while invisible. After eating a fish the shark becomes visible again and the effect is over

Stealthy Gladiator
Longer duration, bigger bonus

Stealthy Champion
Longer duration, bigger bonus

Strategy
Surprise Fish while being invisible. The invincibiliy while being invisible might help you to escape from big fish