Cannibalism contributes significantly to the diet of cultured sand crabs, Portunus pelagicus (L.): A dual stable isotope study

Abstract

The significance of cannibalism in the diet of juvenile pond-cultured blue swimmer crabs ( Portunus pelagicus (L.)) was investigated using dual stable isotope analysis of carbon and nitrogen. In a laboratory feeding experiment, δ 15N demonstrated a constant trophic shift (Δδ 15N ≈+ 1.6‰), and therefore seemed to be a reliable indicator for assessing trophic position for P. pelagicus. This agrees with previously reported trends. Difference in growth rate did not seem to influence δ 15N values. In contrast, δ 13C did not display consistent shifts between trophic levels (range of Δδ 13C: + 1 to + 1.7‰). The results from the pond experiment showed that larger individuals had a more enriched δ 15N than smaller individuals, which, when compared to the results from the laboratory experiment, indicates that larger individuals were at a higher trophic level. This is most likely due to cannibalism prevailing in the pond rather than a direct result of faster growth rate. Cannibalistic behaviour might further increase growth, resulting in the observed positive correlation between size and δ 15N.