Calcitic belemnite guards are often used for temperature reconstructions of ancient seawater by using oxygen isotope thermometry. These geochemical studies discuss diagenesis and vital effects but neglect ecological or biogeographic effects on the isotope signature. To estimate the impact of seasonal temperature variations, short-term salinity changes and biogeography on the isotope signals we compare the δ18O and δ13C signals of ten cuttlebones with local water temperatures. The cuttlebones (aragonitic internal shells) come from five different species of recent cuttlefish (Sepiidae, Sepia sp.) from seven different regions (North Sea, Baltic Sea, Mediterranean, Red Sea, Angola, North Australia and Tasmania). All analysed specimens reflect the temperature-characteristics of their habitat perfectly. The δ18O signal and calculated temperatures follow annual temperature changes of up to 15 °C. The δ13C values show no clear pattern and are thought to be controlled by vital effects. Freshwater influence is recognizable in the negative δ18O and δ13C values of the Baltic Sea specimen, although sudden short-term salinity changes are not reflected by the signatures.