Colour vision in animals

Josh Grosse – Contact

The light different animals can see covers a range from about 300 nm to over 750 nm in wavelength. Shorter than this is middle ultraviolet, which is absorbed by electrons in ordinary components of cells. Longer is infrared, which is associated with vibrations of molecules but not the energy transitions of electrons inside them.

All eyes detect light using proteins called opsins that combine with retinal or derived chromophores. Normally this exists in bent 11-cis form, converted by appropriate wavelengths to the straight all-trans form. This changes the shape of the opsin, which is picked up by transducin proteins that change the membrane potential and create a feedback loop.

The ability to see different colours depends on multiple opsins and is peculiar to two major groups, the arthropods and vertebrates. Some molluscs have very well-developed eyes but apparently only single opsins and so monochromatic vision. A few segmented worms have more than one but it is difficult to tell if they actually see differences from them.

Vertebrates

Rod and cone opsins

Vertebrate eyes have visual pigments in four main groups called LWS, Rh, SWS2, and SWS1 in order from long to short wavelengths. In most Rh is also split into Rh1 and Rh2, which goes more with the division of retina cells into rods and cones. (Rh is short for rhodopsin-like from Greek rodon, rose, since they tend to absorb green.)

Rod cells are more sensitive and accurate and so work for low-light vision. In most groups these only have Rh1. Cone cells on the other hand refresh faster and so work for bright-light vision, and usually have the other opsins. When there is more than one type of cone comparison between them allows discrimination of different colours.

Circles = cone pigments
Lines = rod pigments
* = partial sequences

Arthropods

Arachnid & crustacean opsins

Arthropods have two main types of eyes, lateral and median, as well as additional larval eyes in horseshoe crabs. Ancestrally the lateral eyes are compound and the others are simple light sensors or ocelli. However, sometimes the lateral eyes are broken up or lost and the median eyes are the principal ones, for instance in arachnid groups like spiders.

The visual pigments here come in five main groups named LWS1, LWS2, MWS1, MWS2, and SWS, with the first again specific to horseshoe crabs, and often include multiple variants. However in classes other than insects there are relatively few cases where the opsins have actually been related to different wavelengths.

There is also not much indication that these classes compare different pigments. Mantis shrimp eyes have a special midband with about a dozen combinations of opsins to detect colour, but only distinguish wavelengths to 15 nm or so. These then act for quick recognition of different ranges rather than being contrasted to determine finer shades.

M = maximum absorptions for visual pigments
S = spectral sensitivities, i.e. with screening effects
Circles = lateral eyes
Dots = median eyes (where reported)

Insect opsins

Insects have lost the crustacean MWS opsins but instead have double copies of the others, called LWS2a, LWS2b, SWS, and UVS in order from longer to shorter wavelengths. When present LWS2b is often associated with ocelli while the others generally give colour discrimination in the compound eyes, sometimes with further duplications or losses.

References

The following gives more complete versions of the references from the table. The opsin genes are all listed by accession number from GenBank. I have kept to examples where there are at least partial sequences, marked by asterisks, but for those interested Longcore has a repository including spectral sensitivities for some other terrestrial species.

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