Mangrove leaf litter, the dominant food of the detritivorous sesarmid crab Parasesarma erythodactyla, is highly deficient in nitrogen and long-chain polyunsaturated fatty acids. We experimentally investigated potential contributions of the gut microbiota to the nutrition of P. erythodactyla by providing crabs with 13C- and 15N-labelled substrates that could lead to biosynthesis of key amino acids and fatty acids. Antibiotic treatment suppressed the gut microbiota of the crabs by reducing bacteria density to 9 and 52 % of those in the non-treated crabs in the amino acid and fatty acid experiments, respectively. Compound-specific isotopic analysis revealed significant 13C-enrichments in threonine (Thr), isoleucine (Ile), phenylalanine (Phe), and docosahexaenoic acid (DHA) in crabs fed 13C-enriched glucose and palmitic acid, respectively, suggesting that these essential nutrients were de novo synthesized in the crab. Antibiotic treatment affected the incorporation of 13C-label to Thr, suggesting an involvement of the gut microbes in the biosynthesis of this amino acid. On the contrary, levels of 13C-enrichment in Ile, Phe, and DHA in crabs with intact and suppressed gut microbiota were comparable, suggesting that these compounds may be synthesized endogenously by the crab. Differences between crabs with and without an intact gut microbiota, however, may have been undetectable because of the high variability in 13C-enrichment of these compounds and the incomplete removal of gut micro-organisms in the antibiotic-treated crabs. Genetic and biochemical capacities enabling the gut microbes to de novo synthesize Ile, Phe, and DHA, therefore, need to be verified by additional approaches.