The purpose of this study was to investigate shell growth performance in two thin-shelled pelagic gastropods from cold seawater habitats. The shells of Arctic Limacina helicina and Antarctic Limacina helicina antarctica forma antarctica are very thin, approximately 2-9 μm for shells of 0.5-6 mm in diameter. Many axial ribbed growth lines were observed on the surface of the shell of both Limacina species. Distinct axial ribs were observed on the outermost whorl, while weak or no rib-like structures were observed on the inner whorls in the larger shell of L. helicina antarctica forma antarctica. For L. helicina, no ribs were observed on small individuals with three whorls, while larger individuals had distinct ribs on the outer whorls. Shell microstructure was examined in both species. There is an inner crossed-lamellar and extremely thin outer prismatic layer in small individuals of both species, and a distinct thick inner prismatic layer was observed beneath the crossed-lamellar layer in large Antarctic individuals. Various orientations of the crossed-lamellar structure were observed in one individual. Shell structure appeared to be different between the Antarctic and Arctic species and among shells of different size.
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