Abstract
We describe the bi-directed eyes of a mesopelagic teleost fish, Rhynchohyalus natalensis, that possesses an extensive lateral diverticulum to each tubular eye. Each diverticulum contains a mirror that focuses light from the ventro-lateral visual field. This species can thereby visualize both downwelling sunlight and bioluminescence over a wide field of view. Modelling shows that the mirror is very likely to be capable of producing a bright, well focused image. After Dolichopteryx longipes, this is only the second description of an eye in a vertebrate having both reflective and refractive optics. Although superficially similar, the optics of the diverticular eyes of these two species of fish differ in some important respects. Firstly, the reflective crystals in the D. longipes mirror are derived from a tapetum within the retinal pigment epithelium, whereas in R. natalensis they develop from the choroidal argentea. Secondly, in D. longipes the angle of the reflective crystals varies depending on their position within the mirror, forming a Fresnel-type reflector, but in R. natalensis the crystals are orientated almost parallel to the mirror's surface and image formation is dependent on the gross morphology of the diverticular mirror. Two remarkably different developmental solutions have thus evolved in these two closely related species of opisthoproctid teleosts to extend the restricted visual field of a tubular eye and provide a well-focused image with reflective optics.
Highlights
As daylight in the ocean is very directional, several mesopelagic fish have developed upward-facing tubular eyes, the dorsal parts of these each being filled with a large spherical lens that produces a focused image on a well-developed main retina that lines the base of the tube
Some invertebrates use mirrors to form images [34,35,36], to our knowledge reflective optics have only been described in one vertebrate species [22]
It is perhaps surprising that mirrors are not more widely used as image-forming devices in vertebrates as reflective tapeta and argentea are readily available to form the basis of an image-forming reflector
Summary
As daylight in the ocean is very directional, several mesopelagic fish have developed upward-facing tubular eyes, the dorsal parts of these each being filled with a large spherical lens that produces a focused image on a well-developed main retina that lines the base of the tube. Dolichopteryx longipes, on the other hand, has dorsally directed tubular eyes as well as extensive ventro-laterally directed diverticula which, uniquely among vertebrates, produce focused images using Fresnel-type mirrors [22]. We describe the diverticulum of another mesopelagic species of opisthoproctid, Rhynchohyalus natalensis, that uses a mirror to produce a focused image in its diverticular eye. This is only the second vertebrate described to use a mirror in this way and it differs in some important respects from the mirror observed in D. longipes. The eye of R. natalensis has previously been described [3,19] but these authors studied a post-larval specimen and outlined an ocular structure significantly different from that of the larger animal described here
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