Tissue-diet discrimination factors of isotopic ratios (△δ~(13)C and △δ~(15)N) in two brittle star species: Effect of reproductive state, diet and tissue composition

摘要

Tissue-diet discrimination factors (TDDFs) for carbon (△δ~(13)C) and nitrogen (△δ~(15)N) were estimated for two brittle star species (Ophiocomina nigra and Ophiothrixfragilis) in tissues of two body compartments (disk and arms) according to different diets (fish muscle, mussel and macroalgae). Variations in TDDFs are studied in the light of physiological changes, as two phases were revealed over the course of the experiment, i.e. a go-nadal rest and a gonadal maturation. Overall, the average △δ~(13)C and △δ~(15)N values ranged from — 4.1 ± 0.2 to 1.5±0.2‰ and from -1.8 ±0.2 to 4.0±0.3‰, respectively. Variability in TDDFs revealed little differences between species but appeared to be mainly driven by the diet and the physiological state of the organisms to a lesser extent. △δ~(13)C in gonadal resting organisms were systematically ca. 0.8‰ lower than in gonadal ma-turing species. TDDF values for organisms fell within the range of expected values, except for △δ~(13)C with macroalgae diet and △δ~(15)N with fish muscle diet. As revealed by %N and CN ratios, high dietary-protein seemed to play a key role in explaining △δ~(15)N values, while △δ~(13)C seemed to be affected by differential as-similation efficiency in dietary components. We also suggested a possible effect of isotopic signature of the diet on TDDFs showing that the more enriched the diet in δ~(15)N and δ~(13)C, the lower the TDDF.