Isotopic reconstruction of marine food webs using cephalopod beaks:: new insight from captively raised Sepia officinalis

Cephalopod beaks are a valuable source of material for stable isotope analysis to reconstruct the feeding ecology of cephalopods and their predators. We conducted a controlled captive-rearing experiment on the cuttlefish Sepia officinalis L., 1758 to 10 months of age. During the first and second months of life, animals were fed mysids (Mysidopsis sp.) and shrimp (Palaemonetes pugio Holthuis, 1949), but after 2 months, they were fed a homogeneous diet of shrimp (Far-fantepenaeus aztecus (Ives, 1891) and Litopenaeus setiferus (L., 1767)) until sampling. Soft tissues such as buccal mass, arm, and mantle were enriched in N-15 over the shrimp diet by about 3.3 parts per thousand, with little change in C-13, a result in keeping with previous findings for other marine predators. However, beaks showed little isotopic enrichment over the diets at the time of beak formation, with the beak rostral tips representing the neonatal diet and the remaining beak material representing the adult (shrimp) diet. Additionally, for four wild cephalopod species (Todaropsis eblanae (Ball, 1841), Illex coindetii (Verany, 1839), Loligo vulgaris (Lamarck, 1798), and S. officinalis) obtained commercially in France, we measured delta C-13 and delta N-15 values in lower beaks and soft tissues. Similar to the results of our captive study, soft tissues were consistently enriched in N-15 over beaks by an average of 4.8 parts per thousand (range 3.9 parts per thousand-6.1 parts per thousand) but were slightly depleted in C-13 by 0.8 parts per thousand (0.7 parts per thousand-1.2 parts per thousand). The isotopic measurement of cephalopod beaks can thus be used to directly trace the isotopic composition of cephalopod diets and will be a powerful tool in the reconstruction of higher-trophic-level predators of cephalopods, since beaks are often the only material remaining for analysis from field samples.