The Chitons have rocks for eyes.

Mollusc sensory perception is fascinating. In the past I have touched on the Chiton's bizarre magnetic tongues which serve as an in-built compass. Turns out their vision is pretty odd too. 

Mollusc vision is a cool topic. We humans are pretty proud of our visual acuity but to be honest our abilities are kind of 'middle of the road' when compared to the vast spread of molluscan eye designs. Future posts will explore a range of eye-innovation throughout the phylum but today I'm going to focus (no pun intended) specifically on Chitons.

In this weeks edition of the science journal 'Current Biology' there is a great article on some research into the eyes of Chitons. Its been known (or guessed?) for hundreds of years that the dozens of black speckles across the Chiton's valves (armour plates) are simple eyes but it was presumed that due to their tiny size and simple build that they were purely for detecting light with no resolution ability at all. The animal's lack of a clear brain structure probably led many to believe that anything further than detecting presence and absence of light would be beyond their ability. Well, a simple experiment has proved this assumption to be wrong in the case of one Chiton species at least.

The experiment was simply to observe the Fuzzy Caribbean Chiton Acanthopleura granulata's response to an approaching dark object and compare this to a general raising or lowering of the ambient light level. (Coincidentally, Acanthopleura granulata is the mystery Chiton shown in last week's MolluscPOW post.) Chitons 'clamp down' in a defensive posture when they feel threatened and they consistently did this on the approach of an object but did not do so when the ambient light changed. This neatly proves that the animals could 'see' approaching objects rather than just react to the presence and absence of light.

These little Chiton eyes are unusual: they are made from a solid crystal made of Aragonite (a form of calcium), the same hard material their valves are built of. Solid lenses in eyes have traditionally been seen as inferior to protein eyes such as our own due to their inability to focus by changing shape. This is not entirely fair: solid crystal eyes have advantages. Variations in the cross section of the lens allow them to have two separate refractive indices, that is, be able to achieve focus both in air and in water and also have a good field of view allowing both near and far objects to be resolved. They are also extremely tough. Both of these features are clearly helpful for Chitons which spend their whole lives in and out of water and being assaulted by crashing waves and grinding sands. 

Until reading about Chiton eyes I had been under the impression that the ancient arthropod group the trilobites were the only animals to have tried out solid crystal lenses in their eyes (Richard Fortey makes this claim in his otherwise excellent book 'Trilobite'). Interesting that the Chitons should have separately evolved 'eyes of rock', indicating that for some animal lifestyles at least this design has a lot to offer.

The experiment doesn't tell us much about the amount of detail the chitons can resolve, nor does it explain how the images from the dozens of little eyes are processed by the beast's internet-like nervous system. However it does make a step forward in understanding more about these awesome and poorly understood creatures. The fact that we get so much new information from such a simple experiment really spells out how much we have yet to learn about many of the less famous Molluscan Classes.