Diatoms are autotrophs which prepare their own food. Crustaceans are herbivorous animals which feed on diatoms. Herrings are carnivorous animals which feed on Crustaceans. This completes the food chain. Diatoms are autotrophs, prepare their own food. Crustaceans eats diatoms. Crustaceons members of zooplants and are eaten by Herring fish when small.