The deep-sea glasshead barreleye, rhynchohyalus natalensis.
Credit: D. A. Flynn (CSIRO)
The University
of Tübingen's Institute of Anatomy has discovered a fish with a
previously unknown type of eye. The aptly-named glasshead barreleye
lives at depths of 800 to 1000 meters. It has a cylindrical
eye pointing upwards to see prey, predators or potential mates
silhouetted against the gloomy light above. But the eye also has a
mirror-like second retina which can detect bioluminescent flashes
created by deep-sea denizens to the sides and below, reports Professor
Hans-Joachim Wagner in the latest Proceedings of the Royal Society.
Professor
Wagner examined an 18cm long glasshead barreleye, rhynchohyalus
natalensis, caught in the Tasman Sea between Australia and New Zealand,
as part of an international research
project. The results were unexpected -- reflector eyes are usually only
found in invertebrates, such as mollusks and crustaceans, although one
other vertebrate, the deep-sea brownsnout spookfish or dolichopteryx
longipes, also uses a combination of reflective and refractive lenses in
its eyes. The light coming from below is focused onto a second retina
by a curved mirror composed of many layers of small reflective plates
made of guanine crystals, giving the fish a much bigger field of vision.
The glasshead barreleye is therefore one of only two vertebrates known to have reflector eyes; but significantly, although rhynchohyalus natalensis and dolichopteryx longipes belong to the same family, their reflective lenses have a different structure and appear to have developed from different kinds of tissue. That indicates that two related but different genera took different paths to arrive at a similar solution -- the reflective optics and a second retina to supplement the limited vision of the conventional refractive cylindrical eye.
The prisms in the brownsnout spookfish eye grew out of a layer of pigment on the retina and the angle of the reflective crystals varies depending on their position within the mirror, but in the glasshead barreleye, the crystals are flatter and images are formed depending on the roundness of the reflective surface. "The mirror here is formed from the silvery skin of the eye, and the crystals are arrayed almost parallel to the surface of the mirror," says Wagner. Models of the reflector showed that it is capable of throwing a bright, sharp image onto the retina below. "Obviously, a broad field of vision is an advantage even at great depths if similar structures develop independently to ensure it."
The glasshead barreleye is therefore one of only two vertebrates known to have reflector eyes; but significantly, although rhynchohyalus natalensis and dolichopteryx longipes belong to the same family, their reflective lenses have a different structure and appear to have developed from different kinds of tissue. That indicates that two related but different genera took different paths to arrive at a similar solution -- the reflective optics and a second retina to supplement the limited vision of the conventional refractive cylindrical eye.
The prisms in the brownsnout spookfish eye grew out of a layer of pigment on the retina and the angle of the reflective crystals varies depending on their position within the mirror, but in the glasshead barreleye, the crystals are flatter and images are formed depending on the roundness of the reflective surface. "The mirror here is formed from the silvery skin of the eye, and the crystals are arrayed almost parallel to the surface of the mirror," says Wagner. Models of the reflector showed that it is capable of throwing a bright, sharp image onto the retina below. "Obviously, a broad field of vision is an advantage even at great depths if similar structures develop independently to ensure it."
Story Source:
The above story is based on materials provided by Universitaet Tübingen. Note: Materials may be edited for content and length.
The above story is based on materials provided by Universitaet Tübingen. Note: Materials may be edited for content and length.
Journal Reference:
- J. C. Partridge, R. H. Douglas, N. J. Marshall, W.- S. Chung, T. M. Jordan, H.- J. Wagner. Reflecting optics in the diverticular eye of a deep-sea barreleye fish (Rhynchohyalus natalensis). Proceedings of the Royal Society B: Biological Sciences, 2014; 281 (1782): 20133223 DOI: 10.1098/rspb.2013.3223
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