Inter-specific and ontogenic differences in δ13C and δ15N values and Hg and Cd concentrations in cephalopods

Abstract

Chemical tracers (e.g. stable isotopes or trace metals) are increasingly used to study trophic ecology and feeding habits of marine organisms and various factors can affect their values. The aim of the present study was to provide information on ontogenic effects on stable carbon and nitrogen values (δ 13 C and δ 15 N) and on cadmium (Cd) and mercury (Hg) concentrations in 5 cephalo- pod species from the Bay of Biscay (North-East Atlantic Ocean). To this end, individuals belonging to 3 species showing the widest range of sizes were analysed for muscle δ 13 C and δ 15 N values and mus- cle Hg, and digestive gland Cd concentrations. Results showed that stable isotope ratios allowed dis- crimination of specific feeding strategies during ontogenesis. Segregation between 5 cephalopod species in terms of trophic ecology was also evidenced (different isotopic niches). In contrast, Hg con- centrations varied over the same order of magnitude in these 5 cephalopod species, despite higher levels in the benthic octopus Eledone cirrhosa. Consistently, Hg concentrations followed the same ontogenic pattern and increased with increasing body size/age of cephalopods. Finally, Cd concen- trations varied over 3 orders of magnitude among the 5 species. Despite possible effects of physiol- ogy in terms of metal bioaccumulation, Cd concentrations were likely to reflect specific feeding pref- erences or feeding zones and ontogenic variability within a single species. Thus, ontogenic effects have to be taken into account when stable isotope ratios of carbon and nitrogen or trace metals are used as ecological tracers; the best recommendation being to focus on a given class age.