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Eye in the sky
  1. Ivan R Schwab
  1. Correspondence to: Ivan R Schwab University of California, 4860 Y St, Suite 2400, Sacramento 95817, USA; irschwab{at}

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Most aquatic animals do not rely upon the cornea for refraction; in fact some do not even have a cornea. Evolutionarily, the cornea probably appeared early in vertebrate phylogeny although not for refraction. Rather, the cornea probably first appeared for protection, especially for the crystalline lens. Early fish developed a thin protective layer, and some species subsequently developed a second layer of cornea called the spectacle. In many fish neither layer of cornea is especially good optically, but they do not have to be. Parenthetically, echoes of those two separate layers still exist in our own cornea.

Neither of these two corneal layers has to be particularly smooth, because when the cornea is immersed in water, there is almost no refraction obtained from the cornea, permitting light to pass through unchanged. This creates a problem for any fish that must navigate the aerial world in any meaningful visual manner. What do such creatures do to overcome the generally poor visual characteristics of the lacklustre piscine cornea?

Conceptually, flying fish seem curious to us but these winged creatures simply occupy an unusual niche. Found in …

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