During foraging, shrimp rapidly probe the substratum using the first three pair of chelate pereopods. Once food is located, it is grasped by the pereopods which pass it to the mouthparts. Small particles are placed directly in a pre-oral cavity, while larger items are held to the mouthparts by the third maxillipeds for further manipulation. Sand grains and other inedible items are usually rejected, although some items may be ingested and aid in.food grinding (Bailey-Brock & Moss, 1992).

Preliminary digestion occurs after ingested food enters the elastic foregut, or proventriculus, where enzyme addition, trituration, and storage take place. Food passage through the foregut is rapid and it empties at a declining logarithmic rate. Partially digested food then passes to the midgut, or digestive gland, where the dual functions of enzyme secretion and absorption of digested food occur. Digestive enzymes secreted from the midgut include proteinases, carboxypeptidases, lipases, amylases, chitinases, and others. It may be possible for some penaeid species to alter their digestive enzyme profiles depending on their dietary history. Gut flora may be an important source of digestive enzymes for penaeids, although their role is not well established. While most soluble products of digestion are absorbed in the midgut and midgut diverticula, undigestible items pass through a simple tubular hindgut where longitudinal, muscular pads grasp the fecal string by its peritrophic membrane and rhythmical contractions expel it (Bailey-Brock & Moss, 1992).

Penaeid shrimp are known to ingest a variety of items and have been described as omnivorous scavengers, opportunistic omnivores, detritus feeders, carnivores, and predators. While some interspecific differences in the trophic status of penaeids may exist, ontogenetic changes in food preference and food availability can account for much of the reported variation in their diets. Epibenthic postlarvae and juveniles consume both animal and plant matter, including microalgae, detrital aggregates, macrophytes, foraminiferans, nematodes, copepods, tanaids, larval molluscs, and brachyuran larvae. As shrimp grow, many of the small invertebrates in their diet are replaced by larger invertebrate prey, such as mysid and caridean shrimp, amphipods, polychaetes, and molluscs, as well as fishes (Bailey-Brock & Moss, 1992).

Subadult and adult shrimp also consume significant amounts of detrital aggregates. Depending on its source and age, detritus can be efficiently assimilated. The relative contribution of detrital particles per se versus attendant microbes (i.e. fungi, bacteria, and protozoans) to shrimp nutrition is not clear. Detritus originates mostly from primary producers, and is largely composed of structural polysaccharides and other polymers. The ability to hydrolyze these compounds is rare among marine invertebrates, and is not well documented for penaeid shrimp (Bailey-Brock & Moss, 1992).

Bacteria rapidly colonize and decompose detritus in marine and brackishwater ecosystems, and were thought to be important components in penaeid diets. Recent evidence indicates, however, that bacterial biomass associated with detritus or sediments accounts for a relatively small fraction of total organic carbon and nitrogen, and animals using this resource generally do not consume these bacteria rapidly enough to meet their energetic requirements. Furthermore, bacteria are conspicuously lacking in long chain polyunsaturated fatty acids, sterols, and the sulfur-containing amino acid methionine, all of which are important dietary components for penaeid shrimp (Bailey-Brock & Moss, 1992).

Less is known about the potential nutritional importance of other microbial groups. Protozoans are considered important bacterial grazers in planktonic food chains, capable of consuming a large fraction of the bacterial biomass and channeling energy to higher trophic levels. A similar stiuation may occur in sediments and on detrital aggregates, where protozoans (especially ciliates) are thought to be important bacterial grazers. Although protozoans are presumed to synthesize their own long chain polyunsaturated fatty acids, thus making them desirable dietary items for penaeid shrimp, virtually nothing is known about macrofaunal consumption of these potential food items. Fungi are considered to play a minor role in the decomposition of detritus in marine and brackish water ecosystems, and are probably nutritionally unimportant to shrimp (Bailey-Brock & Moss, 1992).

Studies using stable carbon isotope techniques have helped to clarify food web relationships between penaeid shrimp and potential food items in coastal habitats. These techniques are useful in determining the origins and transformations of organic matter that is channeled through benthic food webs by providing a time-integrated dietary history of the species under study. Penaeid shrimp inhabiting coastal mangroves and saltmarshes appear to derive little carbon from macrophytes or macrophyte-derived detritus and associated heterotrophic microbes. Instead, a substantial portion of carbon ending up in shrimp biomass originates from planktonic microalgae, epiphytic algae, or benthic algae. These studies provide evidence that microalgae, or organisms feeding on microalgae, can be a major source of nutrition for penaeid shrimp in coastal habitats (Bailey-Brock & Moss, 1992).