Figure 2 : Dolichpteryx longipes , a fish with a conventional tubular eye with refractive optics , and a diverticulum based on reflecting optics . A Dorsal view of a living specimen ; the dotted line indicates the plane of sectioning of the following micrographs . B Thin section of the tubular eye and the diverticulum stained with methylene blue . C Diagram of the adjacent section : sclera-blue ; choroid-black ; retinal pigment epithelium-brown ; rod inner and outer segments-green ; remaining retinal layers-yellow ; mirror-grey . D Visual field angles of the tubular and the diverticular eye . E & F Bright field and dark field micrograph of the diverticulum : Note the bright reflection of the mirror in dark field illumination with polarised light . G Schematic representation of ray tracing modelling demonstrating the function of the focussing mirror of the diverticulum ( Fresnel principle ) H & I High magnification bright field and dark field light micrographs of the mirror structure showing reflective crystals . J Electron micrograph of stacks of guanine crystals showing alternation of crystals and cytoplasm , each layer about 120nm wide , corresponding to the quarter wavelength of the incoming light and resulting in mirror-like reflection .
of alternating layers ( bioluminescence : 480 nm ) of different optical densities is well known from other animals to act as a potent reflective device . In addition , we recorded the orientations of these crystals along the length of the mirror and observed that they started almost parallel to the septum ventrally , and then increasingly changed the orientation more dorsally . Modelling these data demonstrated that the incoming light from below was thus effectively reflected and focussed at the outer limiting membrane level , i . e . the rod outer segment acceptance opening of the lateral retina ( Fig 2G ). By adding such a downward-facing diverticulum to its tubular eye , therefore , Dolichopteryx almost doubles the range of its visual field ( Fig 2D ) and is capable of surveying the waters above and below .
We have been privileged to be the first to report this mirror-based focussing device in the diverticulum of the Dolichopteryxeye , an optical arrangement already known from several arthropods and described by K . Kirschfeld , one of the founders of the Tübingen Graduate Training Centre and IMPRS for Neuroscience , but never before observed in vertebrates . Interestingly , focussing lenses with a similar optical principle were developed by A . J . Fresnel in the 19th century for early lighthouses .
As exciting as our discovery of the Dolichopteryx mirror eye was at the time , it turned out to be the key to another surprising story when we looked at the eyes of the other members of the barreleye family . The opisthoproctids comprise seven genera and 19 species ; however , the taxonomic relationships are far from settled . All of these have tubular eyes with diverticula of various sizes and degrees of complexities . Two families Opisthoproctus ( Fig . 1 ) and Winteria have tiny outpocketings made up of all three ocular layers , i . e . sclera , choroid and retina , with an unpigmented ventrolateral “ window ” admitting light to a diverticular retina that lacks ordered , mirror-like crystals .
The intriguing observation here is that in larval specimens of Dolichopteryx , described earlier , the diverticulum looks very similar to this simple , or “ primitive ” situation in Opisthoprocus . It is tempting to speculate that this might be another case where ontogeny recapitulates phylogeny . A further barreleye species ( Bathylychnops ) has a diverticulum considerably larger than in the two previous families ; however , instead of a crystal mirror it uses a corneal ( connective tissue ) lens to focus light onto the retina of the diverticulum . Finally , Rhynchohyalus presents a situation at first sight very similar to the “ mirror eye ” diverticulum of Dolichopteryx . However , the mirror with its guanine crystals , which in Dolichopteryx is derived from the retinal pigment epithelium , in Rhynchohyalus is made up by iridocytes of the choroid .
These observations lead to at least two further questions : ( i ) What is so special about barreleyes that makes them the only family of deep-sea fish ( known to date ) to have evolved such a variety of different eye designs ? And ( ii ) What are the taxonomic relationships within the opisthoproctid family ?
Clearly more morphological data will not help ; instead , molecular data and markers are needed to solve these questions . And since these require fresh tissue for DNA or RNA analysis , more deep-sea cruises are planned . Apart from the excitement of exploring the fascinating fauna of the deep , the work on a research ship provides an unforgettable social experience : a combination of hard work on board , focussed and without the distractions of the lab at home , coupled with intense and productive interaction with friends and colleagues .
Prof . Dr . Hans-Joachim Wagner works at the University of Tübingen in the Department of Anatomy and enjoys mixing both scientific discovery and thrill-seeking adventure .
[ 1 ] Wagner , H . -J ., Douglas , R . H ., Frank , T . M ., Roberts , N . W . & Partridge , J . C . ( 2009 ). A novel vertebrate eye using both refractive and reflective optics . Current Biology , 19 , 106-114
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