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Richard R Dubielzig, Kari Musgrave, Charles S Schobert; Comparative Anatomy of Avian Ciliary Muscles Among Underwater Sight Hunters. Invest. Ophthalmol. Vis. Sci. 2014;55(13):4444.
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Compare the anatomy of avian ciliary muscles in birds with attention to birds that use their eyes for underwater hunting and how that ocular anatomy reflects the environmental and behavioral niche.
Cases were selected from the archive of the Comparative Ocular Pathology Laboratory of Wisconsin (COPLOW). Five micron H & E sections were examined. Crampton’s muscle and Brücke's muscle were identified. Length and area of the muscles were measured. We could not reliably identify Müller’s muscle.
Crampton’s muscle was absent in 5 species of penguins.(Sphenisciformes), cormorants (Pelecaniformes), loons (Gaviiformes), and puffins (Charadriiformes). However, a robust Brücke's muscle was present in all these underwater sight hunters. In the majority of birds examined both muscles were present and no clear pattern of relative size could be detected.
These results suggest that birds that hunt underwater do not have corneal accommodation. These birds also have relatively flat corneas making the ocular optics nearly the same above and below water. We speculate the flat corneal is suspended from limbal sclera that is uniquely rigid and these two features make corneal accommodation impossible, hence Crampton’s muscle is not needed. These underwater sight hunters are not closely related species. Some of these birds have close relatives with a terrestrial life-style that have a robust Crampton’s. This supports the conclusion that absence of Crampton’s in these birds is an example of convergent evolution.
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