Kelley and Davis 2016

Light detection is essential for vision through retinal photoreceptors but also occurs in other body parts, such as skin, pineal complex, and deep brain.
Photopigments (opsins linked to light-sensitive 11-cis retinal chromophore) mediate color vision in vertebrates' eyes and are found in the skin of reptiles, amphibians, crustaceans, and fishes.
- Related photoreceptive molecules are also present in cephalopods.
Non-visual photosensitivity in skin allows animals to adjust coloration using chromatophores, which aids in camouflage, thermoregulation, and social signaling.
This review examines opsin-based photopigments in the skin, their roles in behavior, and biochemical pathways linking light shifts to pigment expression and chromatophore responses.
The importance of non-visual light detection in color functions is emphasized as it enables various animal applications.

All taxa, from unicellular organisms to vertebrates, have developed light sensitivity (Wolken & Mogus, 1979).
Light-response behaviors are evident in marine invertebrates, leading to actions such as body orientation and withdrawal reflexes (discussed by Steven, 1963).
- Example: Annelid worms exhibit tail withdrawal responses to light intensity changes.
Photokinetic responses observed in ancestral and juvenile forms before the development of functional eyes, such as in zebrafish larvae post-enucleation (Fernandes et al., 2012).
Eyeless fishes, like the Mexican cave tetra and others, demonstrate light-motivated responses, mediated by non-visual pigment expression in the brain (Tarttelin et al., 2012).
Notably absent photoresponses were seen in some cavefish due to mutations in non-visual pigment genes, indicating evolutionary adaptations (Cavallari et al., 2011).

Extraocular Photoreception:
- Refers to light detection occurring outside the eyes, with critical roles identified in processes of pupil constriction and circadian rhythms (Freedman et al., 1999; Lucas et al., 2001).

Chromatophores used for color change include:
1. Melanophores (brown/black, utilize melanin)
2. Erythrophores (red, use carotenoids/pteridines)
3. Xanthophores (yellow, from pteridine pigments)
4. Iridophores (silvery/iridescent, have guanine platelets)
5. Leucophores (white, made from colorless pteridines)
Each type is characterized by pigment composition affecting color and absorbance properties (refer to Table 1 in original document for absorbance data).

Chromatophores respond to light, influencing behavioral and ecological adaptations, such as camouflage behaviors (Cott, 1940; Stevens & Merilaita, 2009).
- Color responses include aggregating or dispersing pigments based on light conditions.
Identifying photoreceptors in skin provides localized mechanisms for light detection, independent from vision-mediated processes.
Rapid color change capability from skin light detection is advantageous as it avoids complex signaling pathways involving the eye and brain.

Research has shown opsins expressing photoreception in skin can accept different wavelengths and are functionally significant in various species (Davies et al., 2015).
Non-ocular responses in chromatophores suggest interactions with environmental phasing of biological and behavioral actions are considerable (Kelley et al., 2016).

Direct light sensitivity observed within chromatophores showcases behavioral significance across taxa, emphasizing this ability as facilitating survival strategies.
Opacity of environments greatly affects spectral sensitivity, with wavelengths considered crucial for habitat adaptation and activity timing (Oshima & Yokozeki, 1999).

Studies on zebrafish, tilapia, and cephalopods have reinforced findings linking light detection in skin with specific reactions in chromatophores (Fujii & Oshima, 1986).
Mechanisms of color change can be rapid or slower (as seen with fish and reptiles), depending on the chromatophore type involved (Bagnara & Hadley, 1973).

Non-visual light detection plays crucial roles in behaviors and pigment regulation, yet understanding of these underlying molecular and functional mechanisms remains limited.
Vestigial opsin types in other animals suggest wider adaptive strategies involving light detection capacity beyond classical visual photoreception; exploring this further is essential for comprehending ecological and evolutionary dynamics.
The evolutionary pathways of non-visual phototransduction are speculative and hold significant implications for future biophysical studies.

Acknowledge support from the Australian Research Council and the University of Western Australia for contributions to this work.

For complete reference details, refer to the corresponding list in the original document provided.

Key takeaways