https://academic.oup.com/bioscience/article-pdf/31/6/458/586188/31-6-458a.pdf

Ecology of Vision

Changes in the visual environment affect colour signal brightness and shoaling behaviour in a freshwater fish

A major goal of sensory ecology is to identify factors that underlie sensory-trait variation. One open question centers on why fishes show the greatest diversity among vertebrates in their capacity to detect color (i.e. spectral sensitivity). Over the past several decades, λmax values (photoreceptor class peak sensitivity) and chromacy (photoreceptor class number) have been cataloged for hundreds of fish species, yet the ecological basis of this diversity and the functional significance of high chromacy levels (e.g. tetra- and pentachromacy) remain unclear. In this study, we examined phylogenetic, physiological and ecological patterns of spectral sensitivity of ray-finned fishes (Actinoptergyii) via a meta-analysis of data compiled from 213 species. Across the fishes sampled, our results indicate that trichromacy is most common, ultraviolet λmax values are not found in monochromatic or dichromatic species, and increasing chromacy, including from tetra- to pentachromacy, significantly increases spectral sensitivity range. In an ecological analysis, multivariate phylogenetic latent liability modeling was performed to analyze correlations between chromacy and five hypothesized predictors (depth, habitat, diet, body coloration, body size). In a model not accounting for phylogenetic relatedness, each predictor with the exception of habitat significantly correlated with chromacy: a positive relationship in body color and negative relationships with body size, diet and depth. However, after phylogenetic correction, the only remaining correlated predictor was depth. The findings of this study indicate that phyletic heritage and depth are important factors in fish spectral sensitivity and impart caution about excluding phylogenetic comparative methods in studies of sensory trait variation.

Spectral sensitivity in ray-finned fishes: diversity, ecology and shared descent

Vertebrate photoreceptors are commonly distinguished based on the shape of their outer segments: those of cones taper, whereas the ones from rods do not. The functional advantages of cone taper, a common occurrence in vertebrate retinas, remain elusive. In this study, we investigate this topic using theoretical analyses aimed at revealing structure–function relationships in photoreceptors. Geometrical optics combined with spectrophotometric and morphological data are used to support the analyses and to test predictions. Three functions are considered for correlations between taper and functionality. The first function proposes that outer segment taper serves to compensate for self-screening of the visual pigment contained within. The second function links outer segment taper to compensation for a signal-to-noise ratio decline along the longitudinal dimension. Both functions are supported by the data: real cones taper more than required for these compensatory roles. The third function relates outer segment taper to the optical properties of the inner compartment whereby the primary determinant is the inner segment’s ability to concentrate light via its ellipsoid. In support of this idea, the rod/cone ratios of primarily diurnal animals are predicted based on a principle of equal light flux gathering between photoreceptors. In addition, ellipsoid concentration factor, a measure of ellipsoid ability to concentrate light onto the outer segment, correlates positively with outer segment taper expressed as a ratio of characteristic lengths, where critical taper is the yardstick. Depending on a light-funneling property and the presence of focusing organelles such as oil droplets, cone outer segments can be reduced in size to various degrees. We conclude that outer segment taper is but one component of a miniaturization process that reduces metabolic costs while improving signal detection. Compromise solutions in the various retinas and retinal regions occur between ellipsoid size and acuity, on the one hand, and faster response time and reduced light sensitivity, on the other.

/pdf/functional-significance-of-the-taper-of-vertebrate-cone-5d6u9npwex.pdf

Functional significance of the taper of vertebrate cone photoreceptors.

Structure and spectral sensitivity of photoreceptors of two anchovy species: Engraulis japonicus and Engraulis encrasicolus.

We investigated the spectral and morphological features of the photoreceptors of five atherinomorph teleosts, representing two different orders, and with different life styles and habitats, the Beloniformes and Atheriniformes The retinae of Belone belone (Belonidae), Dermogenys pusillus (Hemiramphidae), Atherina boyeri (Atherinidae), Marosatherina ladigesi (Telmatherinidae), and Melanotaenia maccullochi (Melanotaeniidae) were examined by light and electron microscopy and microspectrophotometry In addition to rods, five morphologically different cone types were identified: short, intermediary and long single cones, and double cones which are arranged in distinct specific mosaics Sporadically, triple cones were also found Double cones were longer-wave-sensitive, but no general correlation between single cone morphology and spectral sensitivity could be demonstrated The rods had λmax close to 506–509 nm The λmax of cone visual pigments ranged from about 368 nm to 578 nm Ultraviolet-sensitive single cones were present in the three freshwater species, M ladigesi, M maccullochi and D pusillus and three spectrally distinct short-wave-sensitive single cone classes were identified in M maccullochi In M ladigesi, spectral sensitivity varied among individuals due to varying rhodopsin/porphyropsin mixtures In D pusillus and M maccullochi polymorphism of the longer-wave cone pigments might occur These findings are discussed with respect to phylogeny, photic habitat, behavior and feeding habits

The retina of five atherinomorph teleosts: photoreceptors, patterns and spectral sensitivities.

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The retina of five atherinomorph teleosts: photoreceptors, patterns and spectral sensitivities.