Saturday, 10 November 2012

Variety in Vision

Molly Hawes

As you read this article, your eyes flit over the page, jumping every three hundred milliseconds or so. Furthermore, if you turn your head slightly, reflexes force you to maintain your previous eye direction for at least twenty milliseconds. This avoids the inevitable blurring associated with shorter glances due to the brain’s inability to register differences in light intensity over shorter time spans. Most vertebrates and crustaceans share our two dimensional binocular visual set up.

Now try to imagine if you can (and, of course, you can’t) being able to see only a single strip of light, corresponding to just three single receptors wide, yet hundreds of receptors long. The essentially one-dimensional strip spans just ninety degrees, less than half the field of view of a human.  This is how the heteropod sea snail Oxygyrus differentiates partner from prey in the underwater world it inhabits. Should it decide the hapless object is prey, it will (in the delightful words of Professor Michael Land) “have a go with its snuffling tube”. 

Perhaps yet a more bizarre form of vision is that of the Odontodactylus or Mantis Shrimp. This species has, for four hundred million years, been quietly evolving a singularly unique visual mechanism. In each of its two circular eyes it has six colour detecting lines of receptors, positioned like the strings over the hole in a guitar. These can detect eight visible colours and four from the ultraviolet spectrum, as well as both planar and circular polarised light. The colour strip is overlain by a perpendicularly orientated row of three pseudopupils through which light can enter to allow movement detection and, potentially although not confirmed, triangulation. A question not yet answered by biologists is whether the twelve colours the Mantis Shrimp can see are processed using a ratio based system (like we use with red, green and blue) or a different type of ‘software’ altogether. Certainly any system capable of processing this number of colours must be highly complex and computationally demanding. 

Perhaps the most unlikely visual system is that of the Copilia quadrata. The blind males of this species have the vestigial cuticular lens, but in the females we see two eyes each containing five or seven rods. These fixed receptors, not including any form of retina, relay information through a single nerve in the totally transparent body to the animals’ brain.  eye continually scans and area just three degrees high and wide and so essentially sees a single, “zero-dimensional” dot. Discovered by Sigmund Exner in 1891, it is the only animal to create a two-dimensional image from an array of point images from a single lens. To complicate matters the animal also has another pair of simple eyes (more like our own) positioned at the front of the body.
Photomicrograph of Copilia quadrata showing the lenses

These animals and others (like the Saltians scenicus and the Labidocera) have developed highly specialised solutions to having primitive eyes. These niche solutions have evolved independently and remind us that a variety of hardware (some quite unlike our own) can provide a functional solution to the challenge of vision, and can provide organisms with the power of sight. 

This article is based on Professor Mike Land’s Richard Gregory Memorial Lecture, ‘Scanning Eyes’, from the 22nd October 2012.