Complete the following food chain in the pond with a suitable organism.

Grass <- Worms <- Fish <- ?

The series of organisms taking part at various biotic levels form a food chain. The grass is an autotroph that is eaten by worms. As worms are herbivores that feed on grass, is usually eaten by fish, fishes are the smaller carnivores that are eaten by organisms like cranes of higher trophic levels in the food chain.