Observations on the retina and ‘optical fold’ of a mesopelagic sabretooth fish, Evermanella balbo
The ‘optical fold’ of Evermanella balbo covers the ventro-lateral cornea and is presumed to capture illumination that would otherwise remain undetected by the tubular eye of this mesopelagic teleost. It contains alternating bands of cellular and acellular material, running approximately perpendicular to the lateral surface of the eye. Only parts of this lamellar body lie within the eyelid-like structure. The cellular lamellae are 2–2.5 μm thick centrally and composed of fibroblast-like cells. The extracellular bands (4.5–5 μm thick) contain regular arrays of collagen fibrils, with layers of thin fibrils sandwiching a region of thicker fibrils. The thin fibrils are organised in alternating sheets where fibrils, although all parallel, change their orientation by 90° between each sheet. All thick fibrils are oriented parallel to the lateral surface of the ‘optical fold’. In the main retina, small bundles of rod inner/outer segments are separated by the processes of the retinal pigment epithelium (rpe) laterally. Centrally, the length of tightly packed rods increases, but rpe processes no longer divide them into bundles. Medially, rod length increases further, but packing is less dense. The accessory retina is significantly thinner, and less well-developed than the main retina. Ventrally, the rods show no regular arrangement and are not grouped. Dorsally, however, rods are arranged into bundles, separated by melanosome-filled rpe processes. The thickness of the retina increases as it approaches the crystalline lens. It is on this dorsal accessory retina that light traversing the ‘optical fold’ most likely falls, facilitating the detection of moving objects in the ventro-lateral field of view.
KeywordsRetina Deep-sea ‘Optical fold’ Tubular eye Grouped photoreceptors
We are grateful to the master and crew of the FS Sonne and the chief scientist during cruise 234-1, Reinhard Werner. The technical expertise of Ulrich Mattheus is, as always, much appreciated. We are indebted to Adrian Flynn, who was responsible for the identification and photography of the specimen. Fanny de Busserolles and Fabio Cortesi kindly provided unpublished data on Evermanella photopigment opsins.
Compliance with ethical standards
Conflict of interest
The authors declare that they have no conflict of interest.
HJW was funded by BMBF 03G0233B and 03G0258B.
All applicable international, national, and/or institutional guidelines for the care and use of animals were followed. The single animal used for this study was already dead due to temperature and pressure gradients when the net came on board, and was donated by the University of Queensland capture team for further study.
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