MEPS 629:67-86 (2019)  -  DOI: https://doi.org/10.3354/meps13121

ABSTRACT: Cephalopods compose a vital component of marine food webs worldwide, yet their trophic roles remain largely unresolved. This study used stable carbon and nitrogen isotopes to describe the trophic structure, ontogeny, and isotopic niche overlap of cephalopod groups from pelagic and near-bottom habitats around Bear Seamount in the Northwest Atlantic Ocean. Beaks from 225 specimens (13 families; 27 species), primarily from juvenile and sub-adult life stages, were collected during a deep-sea biodiversity cruise conducted in 2012. Differences in mean δ¹⁵N and δ¹³C values were detected among some families and across species within the families Ommastrephidae, Histioteuthidae, Mastigoteuthidae, and the superfamily Argonautoidea. Trophic positions ranged from 2.7 to 4.5 across assemblage members, with top positions held by Illex illecebrosus, Histioteuthis reversa, Octopoteuthis sicula, Taonius pavo, and Haliphron atlanticus. Cephalopod families exhibiting the broadest and most diverse isotopic niche widths overall included Ommastrephidae, Cranchiidae, and Octopoteuthidae. Families with the narrowest isotopic niches included Onychoteuthidae and the monospecific Joubiniteuthidae, and Vampyroteuthidae. Trophic position increased significantly with body size (mantle length) across all individuals sampled, and ontogenetic shifts in δ¹⁵N values were detected in 7 species. The continuous gradient and broad range of isotope values across families, species, and body sizes suggest an unstructured assemblage comprised of generalist and specialist foragers distributed throughout a vertical depth range of pelagic (depleted δ¹³C values) to near-bottom bathy/benthopelagic (enriched δ¹³C values) habitats. The results provide some of the first quantitative trophic metrics for many poorly studied species and advance our understanding of the diversity of cephalopod ecological roles in marine ecosystems.

KEY WORDS: Cephalopods · Deep-sea · Mesopelagic · Bathy/benthopelagic · Seamount · Stable isotope analysis · Community structure · Trophic position · Ontogenetic shifts