Food Source Analysis of Sea Cucumber (Apostichopus japonicus) in Seaweed Bed of Changshan Archipelago Indicated by Stable Isotopes

Sea cucumber Apostichopus japonicus, belonging to the family Stichopodidae, is primarily distributed in seaweed beds in the Bohai Sea and Yellow Sea of China. It is characterized as having high nutritional value and valued for its taste, containing polysaccharides, saponins, unsaturated fatty acids, amino acids, and other bioactive substances. It also possesses multiple biological functions, such as cancer prevention, immune regulation, prevention of digestive tract injuries, reduction of inflammation, and other pharmacological effects. The multiple nutrients and biological functions of sea cucumbers are influenced by the environment and output process. Food sources are considered the main factors leading to variation in their chemical composition. This study analyzed the trophic level and food sources of sea cucumber in the seaweed bed of Changshan Archipelago based on the carbon and nitrogen stable isotope (δ13C and δ15N) analysis. The average δ13C value of macroalgae was –20.24‰, ranging from –29.32‰ to –11.19‰, while the average δ15N value was 8.13‰, ranging from 3.32‰ to 14.64‰, which covered the distribution range of sea cucumber values. The average δ13C and δ15N values of sea cucumbers from the study site were –19.48‰ and 8.81‰, respectively, with δ13C ranging from –22.04‰ to –17.15‰ and δ15N ranging from 6.40‰ to 11.54‰. The average δ13C and δ15N values of sea cucumbers from the sowing area were –19.13‰ and 9.97‰, significantly higher than those of wild sea cucumbers (PP15N values ranged from 3.32‰ to 14.64‰, encompassing the stable isotope values of sea cucumbers, indicating that macroalgae contributed the majority to the diet of wild sea cucumbers with a contribution degree of 0.78. Based on this result, macroalgae were further subdivided into brown algae, green algae, and red algae as different food sources. The average δ13C of red algae was –22.80‰, ranging from –29.32‰ to –17.70‰, which was significantly lower than that of green algae and brown algae (P15N of red algae was 8.10‰, ranging from 6.12‰ to 10.60‰. The average δ13C of brown algae was-20.10‰, ranging from –27.45‰ to –14.61‰. The average δ15N of brown algae was 7.26‰, ranging from 3.32‰ to 11.72‰, which was significantly lower than that of green algae and red algae (P13C and δ15N values were –19.84‰ and 7.66‰, respectively). The average δ13C of green algae was –19.33‰, ranging from –24.76‰ to –11.19‰. The average δ15N of green algae was 9.12‰, ranging from 4.72‰ to 14.64‰, which was significantly higher than that of red algae and brown algae (Pred algae>green algae)>microalgae>seagrass, and other marine organisms.Stable isotope analysis (SIA) has been proven to be a reliable and effective method for determining the geographical origin of aquatic products, which can be attributed to its contribution to food source traceability. The stable isotope characteristics of sea cucumbers indicate differences in food sources from different locations or production methods. To further compare these differences, sea cucumbers from other coastal areas were included in the discussion to reveal regional variations. In this study, we identified the differences between wild and cultured sea cucumbers from various growth localities in terms of food sources and stable isotope characteristics, which helped us understand the food sources of sea cucumbers in the seaweed beds and provide essential evidence for traceability.