Abstract:Sea cucumber Apostichopus japonicus, belonging to the family Stichopodidae, is mainly distributed in seaweed bed in Bohai Sea and Yellow Sea of China. It is characterized as high nutritional value and delicious taste, containing polysaccharides, saponins, unsaturated fatty acids, amino acids and other bioactive substances. It possesses multiple biological functions, such as cancer prevention, immune regulation, prevention digestive tract injury, reduction inflammation and other pharmacological effects. The multiple nutrients and biological functions of sea cucumber are influenced by environment and output process. Food sources are considered the main factors leading to variation in their chemical composition. This study revealed 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 our research area 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 sowing area were -19.13‰ and 9.97‰, significantly higher than those of wild sea cucumbers (p < 0.01), which had value of -19.84‰ and 7.66‰ in the seaweed bed. These results indicated that macroalgae in the seaweed bed were dominated the food sources of the wild sea cucumbers, while sowing sea cucumbers retained some stable isotopic characteristics acquired during the breeding process. The average relative trophic level of wild sea cucumbers was 2.32, the lowest among the marine animals collected, which was 0.68 lower than that of sea cucumbers in the sowing area (3.00). Sea cucumbers exhibited a low trophic level similar to other echinoderms, such as sea urchin (2.39 ± 0.15), and shared this characteristic with most shellfish including Abalone rugosa (2.33 ± 0.08), Mussel purpurea (2.48 ± 0.12), Oyster (2.56 ± 0.12). However, the trophic level of mollusks like shellfish was not significantly higher than that of sowing sea cucumbers. The trophic level of wild and sowing sea cucumber was significantly lower than that of sea star Asterias amurensis (3.33 ± 0.50) (p < 0.01). The trophic level results indicated that wild sea cucumbers were the primary consumers in the seaweed bed with their potential food sources being local primary producers. The δ13C values of macroalgae ranged from -29.32‰ to -11.19‰, and δ15N values ranged from 3.32‰ to 14.64‰, encompassing the stable isotope values of sea cucumbers, indicating that macroalgae contributed the most diet of wild sea cucumbers with a contribution degree of 0.78. Based on this result, macroalgae were further subdivide 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 (p < 0.01). The average δ15N 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 (p < 0.01), and close to that of wild sea cucumber (average δ13C 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 (p < 0.01). The average contribution degree of brown algae to the diet of wild sea cucumber was 0.320, ranged from 0.02 to 0.76. The average contribution of red algae was 0.224, ranging from 0 to 0.56. The average contribution of green algae was 0.222, ranging from 0 to 0.52. The feeding preferences of wild sea cucumber in seaweed beds were as follows: large algae (brown algae > red 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 Dalian, Penglai, Laizhou and other coastal areas were induced 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 provided essential evidence for traceability.
