This article presents a summary and critical review of what is known about the 'grouped retina', a peculiar type of retinal organization in fish in which groups of photoreceptor cell inner and outer segments are arranged in spatially separated bundles. In most but not all cases, these bundles are embedded in light-reflective cups that are formed by the retinal pigment epithelial cells. These cups constitute a specialized type of retinal tapetum (i.e., they are biological 'mirrors' that cause eye shine) and appear to be optimized for different purposes in different fishes. Generally, the large retinal pigment epithelial cells are filled with light-reflecting photonic crystals that consist of guanine, uric acid, or pteridine depending on species, and which ensure that the incoming light becomes directed onto the photoreceptor outer segments. This structural specialization has so far been found in representatives of 17 fish families; of note, not all members of a given family must possess a grouped retina, and the 17 families are not all closely related to each other. In many cases (e.g., in Osteoglossomorpha and Aulopiformes) the inner surface of the cup is formed by three to four layers of strikingly regularly shaped and spaced guanine platelets acting as an optical multilayer. It has been estimated that this provides an up to 10fold increase of the incident light intensity. In certain deep-sea fish (many Aulopiformes and the Polymixidae), small groups of rods are embedded in such 'parabolic mirrors'; most likely, this is an adaptation to the extremely low light intensities available in their habitat. Some of these fishes additionally possess similar tapetal cups that surround individual cones and, very likely, also serve as amplifiers of the weak incident light. In the Osteoglossomorpha, however, that inhabit the turbid water of rivers or streams, the structure of the cups is more complex and undergoes adaptation-dependent changes. At dim daylight, probably representing the usual environmental conditions of the fish, the outer segments of up to 30 cone cells are placed at the bottom of the cup where light intensity is maximized. Strikingly, however, a large number of rod receptor cells are positioned behind each mirroring cup. This peculiar arrangement (i) allows vision at deep red wavelenghts, (ii) matches the sensitivity of rod and cone photoreceptors, and (iii) facilitates the detection of low-contrast and color-mixed stimuli, within the dim, turbid habitat. Thus, for these fish the grouped retina appears to aid in reliable and quick detection of large, fast moving, biologically relevant stimuli such as predators. Overall, the grouped retina appears as a peculiar type of general retinal specialization in a variety of fish species that is adaptive in particular habitats such as turbid freshwater but also the deep-sea. The authors were prompted to write this review by working on the retina of Gnathonemus petersii; the data resulting from this work (Landsberger et al., 2008; Kreying et al., 2012) are included in the present review.
Keywords: Behavioral adaptation; Grouped retina; Guanine; INL; Light reflection; Mesopic vision; OLM; ONL; OS; Photonic crystals; RGCs; RPE; Retinomotoric activity; Tapetum; inner nuclear layer; outer limiting membrane; outer nuclear layer; outer segment(s); retinal ganglion cells; retinal pigment epithelium.
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