This study examined the feeding and nutrition of Penaeus semisulcatus using a combination of stable isotope (813C and 81N) analysis and gut contents analysis. Field collections included post larvae, juvenile and adult shrimp inhabiting intertidal and subtidal habitats on the coast of Qatar. Post-larvae were collected in shallow water seagrass beds while juveniles and adults were caught in deeper macroalgal beds. Post-larvae and juvenile shrimp in seagrass beds were found to feed mainly on benthic organism such as foraminifera, polychaetes, diatoms and small crustaceans (amphipods, isopods and ostracods), whereas larger shrimp in the macroalgal beds fed mainly on bivalve molluscs and to a lesser extent polychaetes. Post larval stages of Penaeus semisulcatus were found in seagrass beds, in contrast to Metapenaeus affinis which settle at a comparatively younger age and were found in seagrass beds and over the intertidal zone where microbial mats were present. Stable isotope analysis indicated that primary production in mangroves (813C -28.5 to -27.4 %0 and 815N 0.8 to 0.9 %0) did not contribute to nutrition of either shrimp species, but suggested an influence from seagrass beds (813C %0 -7.9 ± 1.3and 815N 1.5 ± 0.42 %0) and microbialmats (813C -7.7 %0 and 815N -0.9 to -0.2 %0). In laboratory studies, the very earliest stage P. semisulcatus post-larvae, not completely adapted to a benthic life style, exhibited reduced survival when fed on microbial mat. However, within a few days of metamorphosis microbial mats supported high growth and survival. The possible role of intertidal mudflats in supporting shrimp populations merits further investigation. Analysis of stable isotope ratios in sieved size-fractions of the microbial mat and the shrimp tissue supports the hypothesis that the shrimp are gaining most of their nutrition from the associated infauna, primarily nematodes. The results indicate that both the C and N isotope contents in shrimp muscle tissue take about 2-3 weeks to equilibrate after a change in dietary isotopic signature. A simple dilution model showed that most of this change was explained by the high growth rate in early life stage shrimp, rather than turnover. A wide range of trophic enrichment in tissue stable isotope signatures relative to diet in the laboratory, supporting interpretation of field studies which confirm the linkage between sensitive shallow water habitats and early life stages of penaeid species and indicate the need for suitable assessment of the potential indirect impacts of coastal developments involving dredging and land reclamation.