MEPS 624:89-102 (2019)  -  DOI: https://doi.org/10.3354/meps13002

ABSTRACT: Combining the use of predators as biological samplers together with measurements of the stable isotopic ratios (δ¹³C_Bulk and δ¹⁵N_Bulk) of their sclerotized beaks help investigate foraging ecology of poorly known oceanic cephalopods. However, high chitin content (an amino-sugar macromolecule) lowers beak δ¹⁵N_Bulk values, thus precluding direct isotopic comparison with other tissues and organisms. To overcome the chitin effect, compound-specific isotopic analysis of amino acids (CSIA-AA) was performed on squid beaks. The method was applied on beaks and muscle, and the resulting δ¹³C_AA and δ¹⁵N_AA values compared between tissues. The usefulness of CSIA was tested by defining the habitat and trophic position (TP_CSIA) of squids using their δ¹³C_AA and δ¹⁵N_AA values. Beak δ¹³C_AA values were reliably measured on 12 AA that included 5 essential and 7 non-essential AA, and δ¹⁵N_AA values were quantified on at least 7 AA that included 2 source and 4 trophic AA. Importantly, δsup>13C_AA and δ¹⁵N_AA varied little between muscle and lower and upper beaks, and TP_CSIA estimates were identical regardless of the tissue considered. Tissue δ¹³C_AA values of both essential and non-essential AA reflected the latitudinal baseline δ¹³C gradient that occurs in the Southern Indian Ocean, while beak δ¹⁵N_AA from source and trophic AA allowed the disentangling of the baseline effect from the trophic effect, and thus better calculations of squid TP estimates than from δ¹⁵N_Bulk values. Beak δ¹³C_AA and δ¹⁵N_AA defined isotopic niches of colossal and giant squids, the 2 largest living invertebrates. In subantarctic waters, they segregate by having species-specific foraging habitats (using δ¹³C_Gly or δ¹⁵N_Phe) and TP_CSIA (using δ¹⁵N_Glx and δ¹⁵N_Phe). TP_CSIA is higher in colossal (4.7) than giant (4.3) squids, and both values compare well with those of myctophid-eaters, suggesting very large squids prey primarily upon small zooplanktivorous fishes. As expected, CSIA-AA overcomes the chitin effect on beaks and it is a powerful tool to investigate trophic interactions of cephalopods. The method has a great potential with arthropods, because chitin is a main component of their exoskeleton but the deleterious effect of chitin is overlooked in isotopic studies focusing on crustaceans and insects.

KEY WORDS: Arthropod · Carbon · Chitin · CSIA · Habitat · Nitrogen · Stable isotope · Trophic position

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Cite this article as: Cherel Y, Bustamante P, Richard P (2019) Amino acid δ13C and δ15N from sclerotized beaks: a new tool to investigate the foraging ecology of cephalopods, including giant and colossal squids. Mar Ecol Prog Ser 624:89-102. https://doi.org/10.3354/meps13002 Export citation Share:    Facebook - - linkedIn