Veloso, J. L.; Barord, G.; Dooley, F.; Ward, P. D.

Nautiloids, the only surviving externally shelled cephalopods, persist in isolated Indo-Pacific reef slopes despite life-history traits that limit dispersal and recovery. Yet the ecological basis of their persistence remains poorly understood. Here, we compare carbon ({delta}13C) and nitrogen ({delta}15N) isotope values from seven nautiloid populations (including Nautilus and Allonautilus) spanning the Pacific. Isotopic niches varied strongly among locations, but only weakly among species, suggesting that geographic context rather than phylogenetic identity is the primary driver of trophic differentiation. Populations from the Bismarck Sea and American Samoa exhibited elevated {delta}15N, consistent with regional nutrient cycling and nitrogen fixation, whereas Great Barrier Reef (GBR) nautiloids displayed unusually broad {delta}13C ranges linked to possible reef-derived carbon subsidies. These results reveal how local oceanography and resource availability shape isotopic niches in long-isolated populations, providing a framework for understanding both the ecological resilience and evolutionary divergence of ancient cephalopods in modern oceans.