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fish eat algae and bigger fish like sharks eat the fish
Maybe it's part of their food chain
Sharks eat animals and get eaten by other animals.
If they didn't exist the animals they eat would overpopulate.
No sharks are at the top of the food chain The only thing that kills sharks are humans
To eat the fish then the fish eat the bacteria. Also Dolphins eat the fish then sharks eat the dolphins.
The food chain will begin with light. Algae need the light to make food. Small fish, whales and crustaceans eat the algae. Bigger fish, sharks, dolphin's, eat the fish and crustaceans.
Then there would be a break in the food chain and all the animals that sharks normally eat will become more abundant and the food that they eat will become scarcer and all marine life will suffer.
All types of reef sharks are vulnerable when young. Common predators of juvenile reef sharks include other sharks, groupers and many larger fish species. As adults, reef sharks may be preyed upon by much larger sharks, like Hammerheads, Tiger sharks and Silvertip sharks.
It all depends on the shark. I know most sharks eat birds, and smaller fish, though
Spotted eagle rays eat bivalves and crustaceans, and they are hunted by reef sharks.
If they weren't there then what they ate would over populate. Then those would eat more of their food and deplete them.