Isotopic evidence concerning the habitat of Nautilus macromphalus in New Caledonia

Modern nautilids (Nautilus and Allonautilus) have often been studied by paleontologists to better understand the anatomy and ecology of fossil relatives. Because direct observations of these animals are difficult, the analysis of light stable isotopes (C, O) preserved in their shells has been employed to reveal their habitat and life history. We aim to (1) reconstruct the habitat depth of Nautilus macromphalus and (2) decipher the fraction of metabolic carbon in its shell by analyzing oxygen and carbon isotopes (δ18O, δ13C) in the septa of two specimens in combination with analyses of water samples from the area. Additionally, we investigate whether morphological changes during ontogeny are reflected in the isotopic values of the shells. Results reveal that the patterns of change of δ18O and δ13C in the septa of N. macromphalus pre- and post-hatching are consistent with previous studies. Values of δ18Owater range from 0.7 to 1.4‰ (VSMOW), with a maximum value coincident with a salinity maximum at ~150 m. We use the temperature and δ18Owater profiles to calculate equilibrium values of δ18Oaragonite with depth. Comparing these values with the measured δ18O of the septa shows that the habitat depth of N. macromphalus is ~140 m pre-hatching and ~370 m post-hatching. Using δ13C of shell carbonate and published data on metabolic carbon, the fraction of metabolic carbon is reconstructed as ~21% and 14% pre- and post-hatching, respectively. The reconstructed depth pre-hatching is slightly shallower than in N. pompilius from the Philippines and Fiji, but the post-hatching depth is similar. However, it is important to emphasize that these estimates represent average over time and space because nautilus is a mobile animal. Lastly, the changes in morphological parameters and the changes in δ13C and δ18O during ontogeny do not coincide except at hatching and at the onset of maturity.