Intra- and interspecific variability in offspring size in nautilids

Hatching size has been considered of great importance in the evolution of externally shelled cephalopods. However, our knowledge of how hatching size varies in response to biotic and abiotic factors is largely lacking. We present a comprehensive overview of hatching size in all known species of modern nautilids (225 specimens, representing eight species). Hatching size ranges from 22 to 33 mm, with the smallest hatchlings in Nautilus pompilius suluensis and the largest hatchlings in Nautilus belauensis. There is no significant difference in hatching size between males and females in the same species. In addition, hatching size does not affect the morphology of the embryonic shell; smaller hatchlings are identical in morphology to larger hatchlings in the same species. Although information pertaining to temperature and duration of embryonic development are limited, we conclude that there is no clear correlation between hatching size and the temperature at which embryonic development takes place. In contrast, there is a weak correlation between hatching size and the duration of embryonic development. In the Late Cretaceous nautilid Eutrephoceras for which data on hatching size are available, species in colder climates exhibit a larger hatching size than those in more temperate climates. A comparison of hatching size and adult size among modern nautilid species reveals a positive correlation, largely driven by N. pompilius suluensis, at one end of the spectrum, and N. belauensis, at the other. This relationship may be rooted in parental care strategy and/or predation pressure.