Potential metabolic records in isotope signals of chondrichthyan hard tissues

Recent advances have demonstrated that the field metabolic rates (FMRs) of teleost fishes can be quantified from stable isotope compositions of carbonate-biomineralized otoliths. Bones and teeth, in addition to bioapatite, contain carbonate and should retain a similar isotopic expression of metabolic rate. Chondrichthyan skeletons are cartilaginous, but in most species some degree of bio-apatite mineralisation of cartilage has been determined. Here we tested the potential to estimate the FMRs of chondrichthyan fishes from the isotopic composition of calcified cartilage. Our results indicate that δ¹³C values of vertebrae and jaws covary with assumed activity levels, body mass and temperature in a manner consistent with them being influenced by the metabolic rate of the organisms using δ¹⁸O values as a proxy. Highly active thunniform species had significantly more negative δ¹³C_vert (and higher C_resp) values than species with less active swimming types, conforming to expectations. Values of δ¹³C in carbonate from jaws were lower in immature elasmobranchs Raja asterias than in Mustelus sp., potentially reflecting mass effect differences in metabolic rates. Tooth enameloid showed anomalous carbon isotope compositions which imply direct exchange with dissolved inorganic carbon during crystal growth. Many knowledge gaps related to the relationships between structural tissue isotope compositions and carbon respiratory physiology of elasmobranch remain. However, our results provide evidence that carbon isotopes of structural carbonate in cartilaginous tissues of chondrichthyan fishes offer potential as a proxy for metabolic level.