Variability of diet-tissue isotopic fractionation in estuarine macrobenthos

Juveniles of bivalves Mactra veneriformis and Ruditapes philippinarum, and ghost shrimps Nihonotrypaea japonica and N.harmandi were reared on a microalga of a constant isotopic value to quantify their diet-tissue isotopic fractionation. The weights of the animals increased by > 7-fold, resulting in isotopic equilibria with their diet. Fractionation for bivalve soft tissues was 0.6 to 0.9 parts per thousand for carbon and 3.4 to 3.6 parts per thousand for nitrogen, which fell within the range of the currently accepted fractionation values (0 to 1 and 3 to 4 parts per thousand.). Examinations of acid-treated or untreated whole body, muscle and exoskeleton of the ghost shrimps showed (1) large variations in delta(13)C for untreated exoskeletons, (2) reduced delta(13)C for acid-treated exoskeletons by 3.5 to 6.2 parts per thousand, (3) confined ranges in C-13 and N-15 fractionations for muscles (2.0 to 2.2 and 3.6 to 4.0 parts per thousand), (4) only slight effects of acid treatment on C-13 and N-15 fractionation for muscles (<= 0.3 parts per thousand differences), (5) a significant difference in C-13 fractionation for acid-treated whole bodies between N. japonica (-0.3 parts per thousand) and N. harmandi (-1.7 parts per thousand), and (6) 2.3 to 3.0 parts per thousand of N-15 fractionation for whole bodies, which were smaller than for muscles due to negative fractionation for exoskeletons (-3.0 to -1.9 parts per thousand.). These findings suggest that carbonates in exoskeletons should be removed by acid and that muscle is the most appropriate tissue for isotopic analysis. Although 15 N fractionation for ghost-shrimp muscle was within the above-mentioned accepted range, 13C fractionation was outside this range. The present study highlights that fractionation is species and tissue-specific, and that the accepted fractionation values may not be universally applicable.