Validating the Incorporation of ¹³C and ¹⁵N in a Shorebird That Consumes an Isotopically Distinct Chemosymbiotic Bivalve

The wealth of field studies using stable isotopes to make inferences about animal dietsrequire controlled validation experiments to make proper interpretations. Despite severalpleas in the literature for such experiments, validation studies are still lagging behind, notablyin consumers dwelling in chemosynthesis-based ecosystems. In this paper we presentsuch a validation experiment for the incorporation of 13C and 15N in the blood plasma of amedium-sized shorebird, the red knot (Calidris canutus canutus), consuming a chemosymbioticlucinid bivalve (Loripes lucinalis). Because this bivalve forms a symbiosis with chemoautotrophicsulphide-oxidizing bacteria living inside its gill, the bivalve is isotopically distinctfrom ‘normal’ bivalves whose food has a photosynthetic basis. Here we experimentallytested the hypothesis that isotope discrimination and incorporation dynamics are differentwhen consuming such chemosynthesis-based prey. The experiment showed that neitherthe isotopic discrimination factor, nor isotopic turnover time, differed between birds consumingthe chemosymbiotic lucinid and a control group consuming a photosynthesis-basedbivalve. This was true for 13C as well as for 15N. However, in both groups the 15N discriminationfactor was much higher than expected, which probably had to do with the birds losingbody mass over the course of the experiment