While opsins have not been localized to particular cells in the skin of any cephalopods prior to this study, the same r-opsin used to detect light in the eyes of the cuttlefish Sepia officinalis is also expressed in its skin ( Mäthger et al., 2010). Because opsins are known to function as light receptors, the cells that express opsin may be dispersed light sensors that could underlie some light-mediated behaviors.
While c-opsins are typically thought to detect light in vertebrate eyes and r-opsins in invertebrate eyes, various opsins are expressed in the skin of many animals ( Ramirez et al., 2011), and opsins have been localized to receptors dispersed across the body of animals from multiple phyla, including cnidarians, echinoderms, annelids and vertebrates ( Plachetzki et al., 2012 Raible et al., 2006 Backfisch et al., 2013 Bellono et al., 2013 Fulgione et al., 2014).
There are at least three major groups of opsins: the r-opsins, c-opsins and Go/RGR (retinal G-protein-coupled receptor) opsins ( Porter et al., 2012 Feuda et al., 2012). Recent work on the molecular basis for light sensing in the skin of myriad animals suggest that cephalopod skin could detect light using the same families of proteins that detect light in the eyes of animals, including a subfamily of G-protein-coupled receptor proteins (GPCRs) called opsins. optic, peduncle and chromatophore lobes), leading to an overall darkening of the skin tone and sometimes even distinct patterns, which also demonstrates the importance of the eyes and CNS in controlling the activity of chromatophores ( Messenger, 1967 Boycott, 1961 Young, 1976 Dubas et al., 1986). Chromatophores can be experimentally controlled with electrical stimulation of the eyes or various brain regions (e.g. Cephalopods seem to use their well-developed, camera-type eyes to gather information about salient features of the light environment, such as brightness, contrast and edges, which strongly influence changes in the appearance of their skin ( Messenger, 1979 Chiao and Hanlon, 2001 Zylinski et al., 2009). When chromatophore muscles contract, the pigment sac at the center is stretched out, showing the chromatophores' color. Cephalopod chromatophores consist of an elastic sac filled with pigment granules and surrounded by radial muscles, which are innervated by nerves that extend directly from the brain ( Cloney and Florey, 1968 Young, 1971, 1974). Chromatophores are an evolutionary novelty because their morphology in coleoid cephalopods is distinct from those found in any other animal taxa, including other mollusks. In general, body-patterning behaviors in cephalopods depend on three major components: the eyes, the central nervous system (CNS) and pigmented organs called chromatophores embedded in the skin ( Messenger, 2001). While light in the environment influences which body patterns are produced, exactly how cephalopods gather and use environmental light to control their body patterning is still debated ( Buresch et al., 2015). Octopuses, like other coleoid cephalopods, create signals and camouflage themselves by altering the color, pattern and texture of their skin ( Holmes, 1940 Hanlon and Messenger, 1988 Packard and Sanders, 1971). Finally, our data suggest that a common molecular mechanism for light detection in eyes may have been co-opted for light sensing in octopus skin and then used for LACE. LACE in isolated preparations suggests that octopus skin is intrinsically light sensitive and that this dispersed light sense might contribute to their unique and novel patterning abilities. Consistent with our hypothesis, the maximum sensitivity of the light sensors underlying LACE closely matches the known spectral sensitivity of opsin from octopus eyes. We fit our action spectrum data to a standard opsin curve template and estimated the λ max of LACE to be 480 nm. By creating an action spectrum for the latency to LACE, we found that LACE occurred most quickly in response to blue light. We hypothesized that octopus LACE relies on the same r-opsin phototransduction cascade found in octopus eyes. To uncover how octopus skin senses light, we used antibodies against r-opsin phototransduction proteins to identify sensory neurons that express r-opsin in the skin. We call this behavior light-activated chromatophore expansion (or LACE).
Although these changes primarily rely on eyesight, we found that light causes chromatophores to expand in excised pieces of Octopus bimaculoides skin. Yet, we do not fully understand how cephalopods control the pigmented chromatophore organs in their skin and change their body pattern. Cephalopods are renowned for changing the color and pattern of their skin for both camouflage and communication.
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