Abstract
Biofluorescence has recently been found to be widespread in marine fishes, including sharks. Catsharks, such as the Swell Shark (Cephaloscyllium ventriosum) from the eastern Pacific and the Chain Catshark (Scyliorhinus retifer) from the western Atlantic, are known to exhibit bright green fluorescence. We examined the spectral sensitivity and visual characteristics of these reclusive sharks, while also considering the fluorescent properties of their skin. Spectral absorbance of the photoreceptor cells in these sharks revealed the presence of a single visual pigment in each species. Cephaloscyllium ventriosum exhibited a maximum absorbance of 484 ± 3 nm and an absorbance range at half maximum (λ1/2max) of 440–540 nm, whereas for S. retifer maximum absorbance was 488 ± 3 nm with the same absorbance range. Using the photoreceptor properties derived here, a “shark eye” camera was designed and developed that yielded contrast information on areas where fluorescence is anatomically distributed on the shark, as seen from other sharks’ eyes of these two species. Phylogenetic investigations indicate that biofluorescence has evolved at least three times in cartilaginous fishes. The repeated evolution of biofluorescence in elasmobranchs, coupled with a visual adaptation to detect it; and evidence that biofluorescence creates greater luminosity contrast with the surrounding background, highlights the potential importance of biofluorescence in elasmobranch behavior and biology.
Highlights
Marine organisms biofluoresce by absorbing the dominant higher energy ambient blue light via fluorescent compounds, and reemit it at longer, lower energy wavelengths, visually resulting in green, orange, and red fluorescence to the human visual system
We focus on cartilaginous fishes and examine the photoreceptor cells in two species of catsharks (Scyliorhinidae: Cephaloscyllium ventriosum and Scyliorhinus retifer) that exhibit bright green fluorescence patterns resulting from the presence of fluorescent compounds in their skin
Models for estimating the lightness perception of these monochromatic sharks do not exist, we used models developed for the human visual system to gain insight into how biofluorescence changes the appearance of the sharks in the eyes of their conspecifics, compared to the way they appear under white light
Summary
Marine organisms biofluoresce by absorbing the dominant higher energy ambient blue light via fluorescent compounds, and reemit it at longer, lower energy wavelengths, visually resulting in green, orange, and red fluorescence to the human visual system. Recent work has demonstrated that biofluorescence is widespread in marine cartilaginous and ray-finned fishes[4,21], very few studies have examined the fluorescent properties of these lineages. Further assessment of these spectral properties is critical to advancing our understanding of the evolution of biofluorescence in marine vertebrates and its potential impact on their evolution, behavior, diversification rate, and the composition of marine ecosystems in general. Models for estimating the lightness perception of these monochromatic sharks do not exist, we used models developed for the human visual system to gain insight into how biofluorescence changes the appearance of the sharks in the eyes of their conspecifics, compared to the way they appear under white light
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