Reflecting optics in the diverticular eye of a deep-sea barreleye fish ( Rhynchohyalus natalensis )

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.

Application of biomimetics principles in space optics

“…Rhynchohyalus natalensis is a mesopelagic, probably circumtropical, fish species of the Opisthoproctidae family [8,12]. The eyes of Rhynchohyalus natalensis consist of two parts separated by a septum -a dorsal tubular eye and a ventro-lateral diverticulum dividing the visual space in similar way as in Dolichopteryx longipes (Fig.…”

“…The mirror probably works as "chaotic" reflector. Hence the diverticulum can form an image by reflection (though the reflectance is approximately only 20-30%), with probably good preservation of spatial information [8]. …”

“…The eyes of Rhynchohyalus natalensis consist of two parts separated by a septum -a dorsal tubular eye and a ventro-lateral diverticulum dividing the visual space in similar way as in Dolichopteryx longipes (Fig. 3) [8].…”

“…The retinular cells in the rhabdom are specially arranged so they can determine even the polarization of light [5]. Although the mirror eyes of these two species are similar, they vary in two important attributes -the origin of the reflective tissue and the orientation of their reflective plates [8].…”

Eye development in the four-eyed fish Anableps anableps : cranial and retinal adaptations to simultaneous aerial and aquatic vision

“…Thus, the vertebrate limb has become a key model for the study of morphological innovations. Other anatomical structures, such as the vertebrate eye, have remained largely unchanged in its general configuration, except in instances where it has been reduced or lost [3][4][5][6][7] and in the remarkable tubular eyes of deep sea fish [8]. However, in at least four fish lineages, including the foureyed fish of the genus Anableps, the eyes have undergone partial duplication [9].…”