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| The peacock mantis shrimp has the greatest colour vision in nature |
Just before you are about to sleep, take time to look around your room. You will be able to distinguish the shapes of objects, but everything appears as shades of grey. At low light levels our eyes cannot detect colours. Turn the lights on and suddenly colour returns. The large numbers of photons of different energy levels strike the retina to produce sensory inputs associated with green or red; the human eye can distinguish between an incredible 10 million different hues. Yet this pales in comparison to species elsewhere in the animal kingdom.
Human eyes contain two types of photo-receptors, the rod and cone cells. Rod cells function even in low light conditions, giving us the ability to detect shapes. The cone cells require light to function and result in colour sensitivity when triggered. The mantis shrimp, however, has no less than 12 different types of photo-receptors, giving it the ability to sense wavelengths of light, completely invisible to us. Many invertebrates outstrip vertebrates when it comes to ocular power, but there are many which outshine humans too.
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| The 300 million year old fossil of Acanthodes bridgei from Kansas |
Yet when colour vision itself originated is a contentious issue, but a recent fossil discovery shows that it is at least 300 million years old. Analysis of a fossil fish discovered in Kansas and housed at the National Museum of Nature and Science in Tokyo has revealed that the exceptionally well preserved eyes contain rod and cone cells. Analysis revealed the presence of granules within cells which, based upon their physical and chemical similarities to the light sensitive granules within modern fish eyes, most likely contained the pigment eumelanin which aids the detection of colour.
The fish species itself, Acanthodes bridgei, is a possible common ancestor of both living groups of fish, the chondrichthys and the osteichthys. We are descended from the latter. If Acanthodes is indeed the common ancestor then its colour vision is ancestral to all living classes of vertebrates bar the agnathans; the jawless lampreys and hagfish. By analysing fossilized vertebrate eyes it may be possible to determine the ocular acuity of other extinct species, such as the dinosaurs. This will be immensely useful when reconstructing ancient ecosystems. Colour plays a vital role in nature, for example the way it guides insects to flowers during pollination. Being able to understand its effect on ancient ecosystems is of utmost importance.
