Although amino acid flux models have been developed in fish and other crustaceans, this is the first study to present an amino acid flux model for crabs. This study investigates the influence of different feeds and the deprivation of food on the free amino acid (FAA) pool, the rate of protein turnover and their relation to the growth and flux of amino acids in the shore crab Carcinus maenas. A model was developed for amino acid flux describing food consumption rates, assimilation rates, protein synthesis rates, growth and protein degradation, and losses. Crabs were fed frozen mussel (Mytilus edulis) tissue (Diet 1) or (fresh) white muscle from salmon (Salmo salar) (Diet 2) at 7% of their body weight per day while other crabs were starved for ten days. In the starved crabs, the amino acids partitioned into protein synthesis made up 36% of the free amino acid pool. There was a 50% reduction in the rate of protein synthesis in the starved crabs compared with the fed crabs. It was estimated that daily dietary amino acid intake might compose up to seven times the crabs' FAA pool. In addition, daily protein synthesis and degradation might respectively remove and return the equivalent of up to 4 times and twice the size of the FAA pool in the fed crabs, respectively. Specimens of C. maenas deposited in body proteins (as net growth) 14% in diet 1 and 40% in diet 2 of their amino acid consumption. The amino acid flux in the fed crabs suggested low protein conversion efficiency compared to other decapod crustaceans, including shrimps and lobsters.