Echoes in the Chambers: Exploring the Ecology and Evolution of Mesophotic Zone Scavengers in Chambered Cephalopods

Abstract

Cephalopods are ecologically and evolutionarily significant marine invertebrates, yet major gaps remain in our understanding of their trophic ecology and physiological adaptations, particularly among deep-living and morphologically conserved lineages such as nautiloids. This dissertation investigates the ecological and metabolic dimensions of phragmocone-bearing cephalopods through a multi-scalar isotopic approach. In Chapter 1, stable isotope analyses (δ¹³C and δ¹⁵N) of multiple Nautilus and Allonautilus populations across the Pacific reveal strong geographic structuring of isotopic niches, with region-specific variations in baseline carbon sources and trophic levels. Chapter 2 focuses on mesophotic scavengers, including nautiloids and other reef slope taxa, and applies nitrogen-based trophic level estimates to show convergence in foraging roles across distinct geographic locales in Fiji and Papua New Guinea. Chapter 3 compares metabolic signatures of extant nautiloids, sepiids, and extinct ammonoids using Cₘₑₜₐ, a carbon-isotope-based proxy for metabolic rate. Results show that modern nautiloids exhibit lower metabolic signals than both extinct ammonites and extant cuttlefish, consistent with their slow-growing, low-energy lifestyles. Together, these findings illuminate the trophic ecology, niche dynamics, and physiological diversity of chambered cephalopods across evolutionary time, enhancing our understanding of how environmental constraints shape deep-sea and mesophotic life.