The use of stable isotope ratios δ13C and δ15N to track the incorporation of Ulva and other important dietary ingredients into the gonads of the sea urchin Tripneustes gratilla

In this study, we demonstrate how stable carbon (C) and nitrogen (N) isotope analysis can be used to estimate the allocation of specific dietary ingredients, incorporated into four artificially formulated feeds supplemented with varying amounts of dried seaweed Ulva armoricana (0, 50, 150 and 200 g/kg), to the production of gonads by Tripneustes gratilla over a 20-week growth trial. Results indicate that U. armoricana is an important dietary ingredient for gonad production, accounting for an average of 37.33 +/- 1.03% of the isotopic signal recorded for gonads at the end of the growth trial. This is significantly more than fish meal and soya, which only contributed an average of 11.40 +/- 1.81 and 3.80 +/- 0.09%, respectively, of the isotopic signal of the gonads. Maize and wheat each account for almost 25% of all the dietary ingredients used to formulate the feeds, but their relative contribution to gonad production only averaged 8.55 +/- 0.58 and 3.80 +/- 0.05%, respectively, among the diets. This indicates that, at least for gonad development, these ingredients may not be required at such high levels. These findings support previous data on the importance of Ulva in abalone and urchin diets, and demonstrate the value of stable isotope analysis for assessing the contribution of specific dietary ingredients in new feed formulations, particularly where growth of specific tissues is being investigated.