Zooplankton
Shrimp and prawns are able to filter food through their gills. They feed on the suspended zooplankton that make up a large percentage of the biomass in the ocean. These often microscopic creatures form the base of the food web. One of their most important functions is carrying carbon from shallower waters to the sea floor in the form of detritus. This, along with phytoplankton, comprises the majority of the shrimp diet.
Algae
Decapods also feed upon algae during both larval and adult stages. Algae grows in a variety of forms, from unicellular organisms, often referred to as phytoplankton, up to large multicellular plants. Kelp is a form of algae that can grow to be more than 200 feet long. Algae is eaten by a large variety of sea life and are responsible for oxygen levels in water as a byproduct of photosynthesis. Prawns can filter feed on phytoplankton or actively feed on more macroscopic algae.
Carrion
As prawns and shrimp tend to live on the sea floor, their diet consists of a variety of detritus including that of phyto- and zooplankton. Prawns and shrimp also will feed upon dead fish, crustaceans or other sea life they find on the sea floor. When this food is found, the crustaceans actively feed using their moving mouths and their limbs to grasp food.
Small Fish
Prawns and shrimp are not just passive feeders. They can be predatory and are known to capture and feed upon small fish and fish larva.
Other Shrimp
Prawns and shrimp also will eat other smaller shrimp or even shrimp that have recently molted, leaving their soft flesh exposed to predators. For this reason, they have received the nickname "Cleaners of the Sea."