It remains to be established whether opsin gene expression in the skin of S. officinalis
is correlated with a ‘visual distributed sensing’ capability because we have not yet demonstrated protein expression that could link the opsin gene with a physiological or behavioural function. Certainly, these dermal opsins may not have any functional significance. Nevertheless, some oceanic squid have well-developed extra-ocular photoreceptors that measure downwelling light and adjust ventral counterillumination (Young et al. 1979
; Young & Mencher 1980
), so it would not be completely surprising to find a similar system in shallow-water, benthic cephalopods such as cuttlefish that rely heavily on camouflage for protection in diverse habitats.
The skin opsins may provide an explanation for how cuttlefish can achieve their impressive camouflage and signalling body patterns in the absence of colour perception. Currently, we have found one opsin type in the retina and fin (which is transparent and could sense light above and below), suggesting that any skin photoreceptive abilities would be ‘monochromatic’, just like the cephalopod eye (Brown & Brown 1958
; Marshall & Messenger 1996
; Mäthger et al. 2006
). Even though the opsins found in the ventral skin are slightly different, they are most probably functionally the same as those of the retina and fin, so that spectral discrimination at the level of the skin opsin is unlikely. However, we do not want to rule out the possibility that future studies may reveal additional dermal opsins tuned to different wavelengths.
Nevertheless, even a single skin opsin could help regulate dynamic colour and body patterning and we mention three possibilities to stimulate future research. (i) Opsins may not convey any wavelength information but may detect reflectance properties of the environment, which in turn may influence the relative expansion of chromatophores for brightness matching of the habitat. (ii) Opsins may be closely associated with chromatophores that act as spectral tuning filters and convey wavelength information to the dermal opsins. Chromatophores expand and retract, and occur in different colour classes, and they could function much the same way as oil droplets function in colour vision of many animals such as turtles (Liebman & Granda 1975
). In fact, the butterfly Heliconius erato
has photoreceptors with one opsin pigment that provides colour discrimination using different perirhabdomal filter pigments (Zaccardi et al. 2006
). (iii) Iridophores, structural reflectors some of which are also actively controlled, could perform similar roles as spectral tuning filters. Another more distant, but exciting, possibility is that iridophores, which not only reflect narrow waveband light but also polarize reflected light at certain incident angles (Mäthger et al. 2009
), could pass polarization information from the ambient light field to the skin opsins, so that the skin may even function to analyse linear polarization in the animal's environment.
One further intriguing possibility is that the dermal opsins work in conjunction with the statocysts to drive the countershading reflex in cuttlefish. This reflex causes cuttlefish to expand whichever chromatophores face upwards, irrespective of the animal's body orientation (Ferguson et al. 1994
). Countershading is a widespread camouflage method in the animal kingdom and may be particularly effective in aquatic habitats (Ruxton et al. 2004
The exact physiological and/or behavioural functions of cuttlefish dermal opsin genes remain to be established. We are developing an S. officinalis opsin antibody for immunohistochemical studies so that protein expression can be investigated, which will allow us to begin addressing questions of functionality of this remarkable finding.