Dynamic light filtering over dermal opsin as a sensory feedback system in fish color change

**Background:** Dynamic color change in animals, such as fish, is a rapid physiological process involving the movement of pigment granules within chromatophores. While dermal photoreception (light sensitivity of the skin) has been hypothesized to play a role in regulating this color change, its functional organization in living animals was unknown. The hogfish (Lachnolaimus maximus) is a color-changing reef fish capable of switching between white, reddish-brown, and mottled appearances within seconds. Previous work identified SWS1 opsin as the most abundant opsin in hogfish skin, but its precise location and function remained unclear. This study aimed to investigate the physical and optical relationship between SWS1 opsin and chromatophores to understand how dermal photoreception might regulate color change.

**Methods:** The researchers used a multi-technique approach on skin samples from 16 hogfish (16.5–35.5 cm total length). Immunohistochemistry with an anti-SWS1 opsin antibody was performed on skin cross sections to localize SWS1 expression. Transmission electron microscopy (TEM) and immunogold labeling were used to examine ultrastructure and confirm opsin localization at the subcellular level. To estimate the spectral sensitivity of the hogfish SWS1 opsin, the authors aligned its amino acid sequence to that of the night aulonocara (Aulonocara hueseri), a fish with a known SWS1 peak sensitivity (λmax) of 415 nm, and compared known spectral tuning sites. Microspectrophotometry (MSP) was used to measure the transmission spectra of individual chromatophores (melanophores, erythrophores, and xanthophores) by passing white light through dispersed pigment and measuring transmittance from 400 to 700 nm (n = 60 cells per type).

**Key Results:** Immunofluorescence showed SWS1 opsin expression localized directly beneath chromatophores, not within them. TEM revealed a previously unknown cell type beneath each chromatophore, densely filled with a reticulated membrane structure. Immunogold labeling confirmed SWS1 opsin within this membrane structure, identifying these cells as putative photoreceptors. The spectral sensitivity estimate placed the hogfish SWS1 λmax at approximately 415 nm, with only two minor amino acid substitutions (S97C and M116V) compared to the night aulonocara. MSP showed that all three chromatophore types strongly attenuated short-wavelength light in the SWS1 sensitivity range: melanophores reduced transmission by ~90%, erythrophores by ~85%, and xanthophores by ~50%. The mean transmission spectra showed sharp transitions for erythrophores (at ~550 nm) and xanthophores (at ~488 nm), while melanophores had uniformly low transmission.

**Clinical Implications:** This study provides the first evidence of specialized photoreceptor cells in the skin of a vertebrate, dedicated to monitoring color change. The findings suggest a sensory feedback mechanism: when chromatophore pigment is dispersed, it blocks short-wavelength light from reaching the SWS1 photoreceptors; when pigment aggregates, more light reaches the receptors, activating them. This system likely allows the animal to fine-tune its color change performance by integrating feedback from the skin with visual input from the eyes. While not directly clinical, this research advances fundamental understanding of sensory biology and could inform future studies on bio-inspired sensors or treatments for pigmentary disorders. The closed-loop feedback model proposed here may be a common principle in color-changing animals, with implications for evolutionary biology and comparative physiology.