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.

Chiton Bulldozer!

MolluscPOW has been very gastropod-heavy in recent weeks, not good for a blog about diversity! So, here is a groovy mollusc of an entirely different sort- a polyplacophoran.

Watch this Caribbean Chiton bulldoze a little tidepool winkle out of its way! Coming through! Annoyingly, there were no clues which species this one is. Any Polyplac fans out there in-the-know drop me a line!

There has been some exciting news in the world of Chiton research in recent weeks. More details to come in the next MolluscPOW post...

Mollusc Giant: Gumboot Chiton- King of the slippery tide pool things.

Polyplacophorans make me feel uneasy, but in a good way. Whenever I find one of these armour-plated lumps stuck to a boulder in a rockpool I ask myself 'why don't I know anything about these?' Then I move on to watching the mussels feeding or annoying a whelk (which kind of answers the question- too many other distractions). I did finally get around to reading a little about them recently and I was pretty floored by what I found- it seem, against all the odds, polyplacophorans are amazing!
These beasts, better known as chitons, slide around most of our rocky coasts on their big molluscan foot, breathing with their long gills tucked into grooves in their underside scraping algae up with their remarkable radula (molluscan tongues). Why are their tongues remarkable? Well, the teeth lining them are tipped with magnetite- an iron-based material which is very tough and, importantly, magnetic. Magnetite has been found in various organs in bees and birds, creatures which need to navigate with precision and its speculated that the magnetite deposits they have help them detect the earths magnetic field and use it to help steer their journeys. Why would a little slow-moving mollusc need this biological GPS? Well, chitons have a habit of returning loyally to a the same spot to sit out their low tide exposure so actually finding their way home after a hard tides grazing is pretty vital to them actually. They have a compass for a tongue.
That is very cool, but its just one of many fascinatingly weird features of this group which I'd like to  explore in future Mollusc POW posts.
To get to the point of this post, another of the Mollusc Giant series, lets look at the biggest of all Polyplacophorans; the Gumboot Chiton Cryptochiton stelleri.
Its odd. It looks like the sole of a Wellieboot (AKA a 'gumboot' by you Aussies and Norte Americanos). Its armour plates (known as 'valves' in the trade) are covered in a funky leathery skin. They live in rocky intertidal zones in an arc around the northern Pacific from California to southern Japan. Its huge (for a chiton). It grows to over a foot long and can weight 2kg! They apparently neatly illustrate the difference between 'edible' and 'tasty' so they don't feature highly in human diets. In fact they have few predators- their main pursuer is Ocinebra lurida, a carnivorous marine gastropod which allegedly just nibbles its ample mantle. Being such a big thing it needs more grown-up food than algae so it occasionally makes holes in giant kelp and other seaweeds.
My only complaint about Gumboot Chitons is that the lack of quality photos of these monsters on the web doing chiton-y things. Oh well. Instead here is a good one of a handsome specimen being wrangled by a fearless rockpool-er.