Biological and environmental signals recorded in shells of Argonauta argo (Cephalopoda, Octobrachia) from the Sea of Japan

Abstract

The argonauts (genus Argonauta) are enigmatic cephalopods. They have a cosmopolitan distribution in subtropical and tropical seas, where they inhabit the epipelagic zone. Their biology, ecology, and life cycle are poorly understood. It is for the first time that stable isotope (δ13C, δ18O) and element ratios (Mg/Ca, Sr/Ca, Ba/Ca) from three argonaut shells have been analyzed in order to test whether their life cycle and habitat are reflected in these data. The three studied shells have been collected in October 2012 from a mass stranding in Yoichi Bay, Hokkaido, Japan. The specimens were sampled along the keel of the shells to acquire ontogenetic records and along growth-sections of the shells to obtain synchronous data of differing shell growth rates. Carbon and oxygen isotope values as well as Mg/Ca ratios are in part controlled by the shell growth rate. Sr/Ca values show similar ontogenetic trends in the three shells. Comparison with measured sea surface temperature data indicates a temperature control on δ18O, Mg/Ca, and Sr/Ca, and a fast growth of the argonaut shell. Ba/Ca ratios of the shells might record environmental parameters. These new data highlight the influence of vital effects, but they also demonstrate that argonaut shell isotopic and elemental records can be used to understand the life cycle of these animals better.