Opsin proteins are essential molecules in mediating the ability of animals to detect and use light for diverse biological functions. Therefore, understanding the evolutionary history of opsins is key to understanding the evolution of light detection and photoreception in animals. As genomic data have appeared and rapidly expanded in quantity, it has become possible to analyse opsins that functionally and histologically are less well characterized, and thus to examine opsin evolution strictly from a genetic perspective. We have incorporated these new data into a large-scale, genome-based analysis of opsin evolution. We use an extensive phylogeny of currently known opsin sequence diversity as a foundation for examining the evolutionary distributions of key functional features within the opsin clade. This new analysis illustrates the lability of opsin protein-expression patterns, site-specific functionality (i.e. counterion position) and G-protein binding interactions. Further, it demonstrates the limitations of current model organisms, and highlights the need for further characterization of many of the opsin sequence groups with unknown function.

Ultraviolet (UV) light occupies the spectral range of wavelengths slightly shorter than those visible to humans. Because of its shorter wavelength, it is more energetic (and potentially more photodamaging) than 'visible light', and it is scattered more efficiently in air and water. Until 1990, only a few animals were recognized as being sensitive to UV light, but we now know that a great diversity, possibly even the majority, of animal species can visually detect and respond to it. Here, we discuss the history of research on biological UV photosensitivity and review current major research trends in this field. Some animals use their UV photoreceptors to control simple, innate behaviors, but most incorporate their UV receptors into their general sense of vision. They not only detect UV light but recognize it as a separate color in light fields, on natural objects or living organisms, or in signals displayed by conspecifics. UV visual pigments are based on opsins, the same family of proteins that are used to detect light in conventional photoreceptors. Despite some interesting exceptions, most animal species have a single photoreceptor class devoted to the UV. The roles of UV in vision are manifold, from guiding navigation and orientation behavior, to detecting food and potential predators, to supporting high-level tasks such as mate assessment and intraspecific communication. Our current understanding of UV vision is restricted almost entirely to two phyla: arthropods and chordates (specifically, vertebrates), so there is much comparative work to be done.

Many sea urchins can detect light on their body surface and some species are reported to possess image-resolving vision. Here, we measure the spatial resolution of vision in the long-spined sea urchin , using two different visual responses: a taxis towards dark objects and an alarm response of spine-pointing towards looming stimuli. For the taxis response we used visual stimuli, which were isoluminant to the background, to discriminate spatial vision from phototaxis. Individual animals were placed in the centre of a cylindrical arena under bright down-welling light, with stimuli of varying angular width placed on the arena wall at alternating directions from the centre. We tracked the direction of movement of individual animals in relation to the stimuli to determine whether the animals oriented towards the stimulus. We found that responds by taxis towards isoluminant stimuli with a spatial resolution in the range of 29-69 deg. This corresponds to a theoretical acceptance angle of 38-89 deg, assuming a contrast threshold of 10%. The visual acuity of the alarm response of was tested by exposing animals to different sized dark looming and appearing stimuli on a monitor. We found that displays a spine-pointing response to appearing black circles of 13-25 deg angular width, corresponding to an acceptance angle of 60-116 deg, assuming the same contrast threshold as above.

Stomatopod crustaceans, or mantis shrimp, are renowned for their complex visual systems. Their array of 16 types of photoreceptors provides complex color reception, as well as linear and circular polarization sensitivity [1][2][3][4][5][6]. The least-understood components of their retina are the UV receptors, of which there are up to six distinct, narrowly tuned spectral types [4]. Here we show that in the stomatopod species Neogonodactylus oerstedii, this set of receptors is based on only two visual pigments. Surprisingly, five of the six UV receptor types contain the same visual pigment. The various UV receptors are spectrally tuned by a novel set of four short-and long-pass UV-specific optical filters in the overlying crystalline cones. These filters are composed of various mycosporine-like amino acid (MAA) pigments. Commonly referred to as ''nature's sunscreens,'' MAAs are usually employed for UV photoprotection [7,8], but mantis shrimp uniquely incorporate them into powerful spectral tuning filters, extending and diversifying their preeminently elaborate photoreceptive arsenal.