REFINING THE HABITAT OF NAUTILUS MACROMPHALUS BASED ON THE ISOTOPIC COMPOSITION AND TEMPERATURE OF THE WATER COLUMN IN NEW CALEDONIA

The Nautiloidea is a group of externally shelled cephalopods that originated in the Late Cambrian. The only modern descendants include the genera Nautilus and Allonautilus—sometimes dubbed “living fossils.” Modern nautiloids have often been studied by paleontologists to better understand the anatomy and ecology of fossil relatives. In terms of their habitat and life history, direct observations of these animals are difficult because they live in deep-sea environments. One approach to investigate their habitat and life history is the analysis of light stable isotopes (C, O) preserved in their shell. The value of δ¹⁸O provides information on the temperature of the water in which the shell was formed, and thus is a useful tool to reconstruct the habitat of fossil and modern cephalopods. Application of this method requires information on the chemistry and temperature of the water column. However, such information is absent in most studies of extinct and modern forms due to the difficulty of collecting water samples at the same site. In this study, we analyzed oxygen and carbon isotopes (d¹⁸O, d¹³C) in the septa of two specimens of Nautilus macromphalus from New Caledonia. In addition, we analyzed samples of water for oxygen and carbon isotopes, as well as temperature and salinity, as a function of depth at two nearby stations. We also performed a morphometric analysis to examine if morphological and isotopic changes during ontogeny are correlated. Results reveal that the patterns of change of d¹⁸O and d¹³C in the septa of N. macromphalus pre- and post-hatching are consistent with previous studies. Values of d¹⁸O_water range from 0.7 to 1.4‰ (VSMOW), with a maximum coincident with a salinity maximum at ~150m. We use the temperature and d¹⁸O_water profiles to calculate equilibrium values of d¹⁸O_aragonitewith depth. Comparing these values with the measured d¹⁸O of the septa shows that the habitat depth of N. macromphalus is ~125 m before hatching and ~350 m after hatching. Our results do not reveal any correlation between morphological changes and isotopic values in either of the two specimens.