Variability of diet-tissue isotopic fractionation in estuarine macrobenthos

Abstract

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 for carbon and 3.4 to 3.6 for nitrogen, which fell within the range of the currently accepted fractionation values (0 to 1 and 3 to 4 ). Examinations of acid-treated or untreated whole body, muscle and exoskeleton of the ghost shrimps showed (1) large variations in 13 C for untreated exoskeletons, (2) reduced 13 C for acid-treated exoskeletons by 3.5 to 6.2 , (3) confined ranges in 13 C and 15 N fractionations for muscles (2.0 to 2.2 and 3.6 to 4.0 ), (4) only slight effects of acid treatment on 13 C and 15 N fractionation for muscles ( 0.3 differences), (5) a significant difference in 13 C fractionation for acid-treated whole bodies between N. japonica (-0.3 ) and N. harmandi (-1.7 ), and (6) 2.3 to 3.0 of 15 N fractionation for whole bodies, which were smaller than for muscles due to negative fractionation for exoskeletons (-3.0 to -1.9 ). 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, 13 C fractionation was outside this range. The present study highlights that fractionation is speciesand tissue-specific, and that the accepted fractionation values may not be universally applicable.