Ward, Peter DVeloso, Job Lukas2026-02-052026-02-052026-02-052025Veloso_washington_0250E_28778.pdfhttps://hdl.handle.net/1773/55147Thesis (Ph.D.)--University of Washington, 2025Cephalopods 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.application/pdfen-USnoneCephalopodEcologyIsotopesMesophoticPaleobiologyPhragmoconeBiologyEcologyBiologyThesis