Experience
When animals are eating food (excluding bubbles and seagull poops), they are gaining XP.
To evolve from one animal to another animal, you need some XP. For example, clownfish has to collect 4,000 XP to evolve into crab.
XP bar is displayed as a gray line with information about animal and remainind XP. Collected XP is green.
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Score
Every particle of food, that was eaten by an animal, adds to its total score. Tier 10 animals don't gaining XP, but they are still gaining score.
Top 10 players of high score are on leaderboard on the left of the screen.