Abstract
The cornea is the first optical element in the path of light entering the eye, playing a role in image formation and protection. Corneas of vertebrate simple camera-type eyes possess microprojections on the outer surface in the form of microridges, microvilli, and microplicae. Corneas of invertebrates, which have simple or compound eyes, or both, may be featureless or may possess microprojections in the form of nipples. It was previously unknown whether cephalopods (invertebrates with camera-type eyes like vertebrates) possess corneal microprojections and, if so, of what form. Using scanning electron microscopy, we examined corneas of a range of cephalopods and discovered nipple-like microprojections in all species. In some species, nipples were like those described on arthropod compound eyes, with a regular hexagonal arrangement and sizes ranging from 75 to 103 nm in diameter. In others, nipples were nodule shaped and irregularly distributed. Although terrestrial invertebrate nipples create an antireflective surface that may play a role in camouflage, no such optical function can be assigned to cephalopod nipples due to refractive index similarities of corneas and water. Their function may be to increase surface-area-to-volume ratio of corneal epithelial cells to increase nutrient, gas, and metabolite exchange, and/or stabilize the corneal mucous layer, as proposed for corneal microprojections of vertebrates.
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Acknowledgments
We thank Joshua Simmich for assistance with SEM and useful discussions, Michael Archer (UWA) and Lynn Tolley (UQ) for conducting EM services, and Wen-Sung for input on identification and phylogeny. We thank two anonymous referees for their contributions towards improving this manuscript. S.E.T. was supported by postdoctoral fellowships from The University of Queensland and the Natural Sciences and Engineering Research Council of Canada. TMJ was funded by EPSRC (Grant no. EP/E501214/1). Part of this work was supported by the Australian Research Council (SPC; NJM), the Biotechnology and Biological Sciences Research Council (Grant no. BB/G022917/1 to NWR), and the Engineering and Physical Sciences Research Council (Grant no. EP/E501214/1 to NWR), and the Asian Office of Aerospace and Research and Development (NJM).
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Talbot, C., Jordan, T.M., Roberts, N.W. et al. Corneal microprojections in coleoid cephalopods. J Comp Physiol A 198, 849–856 (2012). https://doi.org/10.1007/s00359-012-0755-9
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DOI: https://doi.org/10.1007/s00359-012-0755-9