Difference in nutritional status and food sources for hard- and soft-shell crabs (Portunus trituberculatus) using amino acids and isotopic tracers.

The swimming crab, Portunus trituberculatus, supports the world's largest crab fishery. Hard-shell crabs have fully developed exoskeletons and are rich in meat, while soft-shell crabs, which have recently molted, contain less meat. To determine their nutritional status and feeding behavior, we measured amino acids (AAs), organic carbon (OC), δ(13)C, δ(15)N, and (210)Po in the muscle of hard- and soft-shell crabs collected from the eastern Yellow Sea in May 2024. The concentrations of total AAs (TAAs) and OC were approximately 1.4 and 1.3 times higher, respectively, in hard-shell crabs than in soft-shell crabs. A significant positive correlation between TAAs and OC (r(2) = 0.74, p < 0.05) suggests that hard-shell crabs have a better nutritional status, due to the consumption of higher-quality food. Hard-shell crabs also exhibited significantly higher δ(13)C values and (210)Po activities than soft-shell crabs, whereas δ(15)N values showed no significant difference between the two groups. These results indicate that hard-shell crabs primarily consume higher trophic-level prey compared with soft-shell crabs, despite both occupying the same trophic position. Thus, this difference in food sources may be influenced by post-molt hiding behavior in soft-shell crabs, which limits access to high-trophic prey, or alternatively, by competition for food in coastal waters.