Spectral Tuning in the Eyes of Deep-Sea Lanternfishes (Myctophidae): A Novel Sexually Dimorphic Intra-Ocular Filter

Abstract

Deep-sea fishes possess several adaptations to facilitate vision where light detection is pushed to its limit. Lanternfishes (Myctophidae), one of the world's most abundant groups of mesopelagic fishes, possess a novel and unique visual specialisation, a sexually dimorphic photostable yellow pigmentation, constituting the first record of a visual sexual dimorphism in any non-primate vertebrate. The topographic distribution of the yellow pigmentation across the retina is species specific, varying in location, shape and size. Spectrophotometric analyses reveal that this new retinal specialisation differs between species in terms of composition and acts as a filter, absorbing maximally between 356 and 443 nm. Microspectrophotometry and molecular analyses indicate that the species containing this pigmentation also possess at least 2 spectrally distinct rod visual pigments as a result of a duplication of the Rh1 opsin gene. After modelling the effect of the yellow pigmentation on photoreceptor spectral sensitivity, we suggest that this unique specialisation acts as a filter to enhance contrast, thereby improving the detection of bioluminescent emissions and possibly fluorescence in the extreme environment of the deep sea. The fact that this yellow pigmentation is species specific, sexually dimorphic and isolated within specific parts of the retina indicates an evolutionary pressure to visualise prey/predators/mates in a particular part of each species' visual field.

Colours and colour vision in reef fishes: Past, present and future research directions

“…The absorption of different reef-fish lenses and corneas and the short wavelength cut-off of combined ocular media often sharpen and shift spectral sensitivities of photoreceptors from the visual pigment absorbance alone (Figure 11(c)). In a few fish species, there are also retinal inclusions around the photoreceptors in the light path that provide further spectral sensitivity tuning (Collin & Potter, 2000;de Busserolles et al, 2015). These are very poorly understood in reef fishes and relatives (Fineran & Nicol, 1974).…”

Spatial resolving power and spectral sensitivity of the saltwater crocodile, Crocodylus porosus, and the freshwater crocodile, Crocodylus johnstoni

“…In an attempt to explain the remaining deviations between the normalised absorbance curve of a photoreceptor and the pure A 1 /A 2 visual pigment template, a mixed chromophore model was used (Temple et al, 2010;de Busserolles et al, 2015). The model assumed that each photoreceptor contained only one opsin, but possibly a varying percentage of A 1 -and A 2 -based chromophores.…”

The evolution of specialized dentition in the deep‐sea lanternfishes (Myctophiformes)

“…Previous studies investigating lanternfish diversification in open‐ocean environments have focused on their bioluminescent communication (Davis, Holcroft, Wiley, Sparks, & Smith, 2014; Davis, Sparks, & Smith, 2016), vision (de Busserolles et al, 2015), and jaw size (Martin & Davis, 2016); while others have examined diversification in relation to historical oceanic partitioning events (Denton, 2018). These previous works offer an insight into lanternfish speciation and provoke new questions, inspiring a need for deeper understanding of diversification in this abundant and diverse group.…”