TABLE 2.

List of studies providing evidence of opsin expression in nonhuman animal skin

Year	Author	Species	Photosensitive protein	Detection method	Potential role	Major finding
1998	Provencio et al	Frog (Xenopus laevis)	OPN4	In situ hybridization, Western blot	Mediates melanosome migration for color change	OPN4 is detected in dermal melanophores
2005	Isoldi et al	Frog (Xenopus laevis)	OPN4	RT-qPCR	Mediates melanosome migration for color change	Light activates melanopsin phosphoinositide cascade, resulting in melanosome granule dispersion within melanophores
2010	Xiang et al	Fruit fly (Drosophila melanogaster)	Gr28b (rhodopsin-like protein)	Electrophysiology, light-activated signaling assay (GCaMP3 imaging)	Mediates light-avoidance behavior	Ablation of class IV dendritic arborization neurons decreases light-avoidance behavior
2010	Mäthger et al	Cuttlefish (Sepia officinalis)	r-opsin	RT-PCR	Mediates distributed light sensing	r-opsin mRNA is found in the skin
2013	Ullrich-Lüter et al	Sea urchin (Strongylocentrotus purpuratus)	c-opsin	Immunohistology, in situ hybridization	Mediates shadow reflex	c-opsin mRNA and protein are found in the epidermal cells
2015	Ramirez et al	Octopus (Octopus bimaculoides)	r-opsin	Immunofluorescence, gene expression analysis	Mediates the skin color change	Isolated skin tissue shows color alteration upon light illumination
2015	Kingston et al	Octopus, cuttlefish, longfin squid (Doryteuthis pealeii)	r-opsin, RGR	Immunofluorescence, Western blot, RT-qPCR	Mediates skin color change	Phototransduction components of the retina are found in the skin
2015	Davies et al	Zebrafish (Danio rerio)	OPN5	RT-PCR	Not shown	OPN5 is detected in the skin
2018	Schweikert et al	Hogfish (Lachnolaimus maximus)	SWS1	De novo transcriptome	Mediates color change	Skin expresses distinct phototransduction signaling cascade from that found in the retina
2019	Delroisse et al	Velvet belly lanternshark (Etmopterus spinax)	OPN3	De novo transcriptome	Mediates color change	Opsins are differentially expressed in the retina and skin
2019	Buhr et al	Mouse (mus musculus)	OPN5	Immunofluorescence, RT-PCR	Synchronizes the circadian rhythm	OPN5 regulates the amplitude of clock gene expression

Abbreviations: G_qα, G_q α-subunit; Gr28b, gustatory G protein–coupled receptor 28b; OPN3, encephalopsin; OPN4, melanopsin; OPN5, neuropsinRGR, retinochrome; r-opsin, rhabdomeric opsin; RT-PCR, reverse transcriptase PCR; RT-qPCR, reverse transcription-quantitative PCR; SWS1, short-wavelength sensitive opsin.