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

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, δ15N demonstrated a constant trophic shift (Δδ15N ≈+ 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 δ15N values. In contrast, δ13C did not display consistent shifts between trophic levels (range of Δδ13C: + 1 to + 1.7‰). The results from the pond experiment showed that larger individuals had a more enriched δ15N 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 δ15N.