Study of the diet effect on δ13C of shell carbonate of the land snail Helix aspersa in experimental conditions

This study aims to demonstrate the influence of the metabolic CO2 derived from the diet and of the atmospheric CO2 on the shell carbonate delta(13)C of the pulmonate snail Helix aspersa maxima raised under controlled conditions. Adult snails were analyzed and compared with three hatching and 1-day old young snails stemming from the same breeding. One day after, the 2-day old individuals were raised during I month. Three groups of gastropods were fed with fresh lettuce (C-3 plant, delta(13)C= -27.49 parts per thousand), three groups with corn (C-4 plant, delta(13)C= -11.7 parts per thousand), and three groups ate alternately both (C-3+C-4). The difference between the average VC values of the adult snails on the one hand and the hatched and 1-day old snails on the other hand indicates a depletion of 2.47 parts per thousand. Therefore, the isotopic parents offspring signal is not preserved. The depleted ingested albumen by the snail embryo in the egg during the building of the shell could explain this depletion. The C-3 diet experiment gave the expected isotopic composition difference between the diet (lettuce) and the shells (average Delta(13)C(shell-lettuce) = 13.75 parts per thousand +/- 0.52). This result shows a clear diet effect on the isotopic composition of the snail shells. For the C4 experiment, the difference in carbon isotope composition between the corn and the shell (Delta(13)C(shell-corn)) yielded an average value of 4.89 parts per thousand +/- 0.87. The main result is that Delta(13)C is not constant and appears to depend on the type of ingested food. Several hypotheses can arise from this study to explain the different fractionations: (a) differences in the quality of the two diets, (b) differences in turnover rate for C-3 and C-4 feeders. The groups regularly fed with mixed diet yielded delta(13)C values showing a preferential use of C-3 food for most values. The C-3-C-4 mixed dietary alternation probably led snails to use mainly the lettuce instead of the corn powder. (C) 2003 Elsevier Science B.V. All rights reserved.