How important are mangroves as a carbon source for decapod crustacean larvae in a tropical estuary?

In order to evaluate the importance of mangrove leaf detritus as a food source for decapod crustacean larvae, stable isotopes (δ13C and δ15N) were measured in the tissue of these larvae and other selected organisms. Decapod crustacean larvae and planktonic adult decapods,Lucifer faxoniandAcetes americanus, were sampled in the Itamaracá estuarine system, Brazil. For comparison with stable isotope values in the estuary, plankton and POM samples were also taken on the adjacent continental shelf. In the Itamaracá estuarine system, δ13C of decapod larvae and the holoplanktonic decapodsL. faxoniandA. americanusranged from about –17 to –23‰ (mean: –19.8 ± 1.5‰). This indicates that the contribution of mangrove carbon (δ13C: –26.8‰) to larval nutrition was generally negligible. Among decapod larvae, porcellanid zoeae showed relatively low δ13C values, while penaeid shrimp postlarvae showed the highest δ13C. Porcellanid zoeae were thus the only decapod larvae with a considerable percentage (13 to 40%) of mangrove carbon in their tissue. However, these low values may also be due to a negative isotope shift that occurs during embryogenesis. Feeding experiments with zoeae ofPetrolisthes armatus(Anomura: Porcellanidae) andSesarma rectum(Brachyura: Grapsidae) showed assimilatory shifts of up to 1.0 ± 0.2‰ for13C and up to 1.4 ± 0.6‰ for15N. Analysis of embryonic isotope fractionation for 5 decapod crustacean species,P. armatus,S. rectum,Aratus pisonii,Uca thayeri, andU. maracoani, showed that larval tissue was13C-depleted by up to 3.8‰ and15N-depleted by up to 2.3‰ in relation to parental tissue.