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...

Sympathy for Snails

Book review; “The Sound of a Wild Snail Eating” by E.T. Bailey

This is the second book review to appear on Mollusc POW and it wont be the last. I hope you readers are finding them of interest.

I have been yearning for years for a book to be written which finally does justice to snails. Something which would bridge the yawning gulf between the often obtuse science of malacology and the everyday experience people can have with land snails. A book which basically ‘sells’ snails to the masses and establishes them as the unforgettable lead characters in the ecological epic being played out in nature. A book which comes but rarely and penetrates the cultural life of people of all types, not simply nature-nerds or popular-science fans. 

“The Sound of Wild Snail Eating” was not that book, and I found it hard to set my feelings of unreasonable disappointment aside as I read it. I was of course completely unfair- Bailey never set out to deliver a molluscan version of “A Brief History of Time”. She instead  has crafted a neat package of malacological/biographical essays which have two themes at their core: her personal experience of suffering a degenerative neuro-skeletal illness, and the doings of a snail which hitch-hiked into her bedside in a pot plant. 

Some reviewers have railed against the malady-focused content, harshly accusing Bailey of self-indulgence and irrelevance. I agree up to a point: “my illness” stories are usually only of any interest to the afflicted themselves. At times I found myself skipping paragraphs of medical content to get to the nourishing molluscan meat whilst cursing Bailey’s editor for not ruling with an iron fist. But not everyone is quite as misanthropic as myself so perhaps some readers out there will find this part of the narrative of interest? I wonder if the problem with this book is simply one of marketing- it may have achieved less criticism were it sold as a human-interest story with good mollusc content, rather than as a piece of molluscan nature-writing with lots of subjective medical content shoehorned in. 

The actual snail-centered content is quality stuff. Although based in the USA, Bailey neatly avoids the parochiality which often infects American nature writing. The discussion of her personal journey from snail-ignoramus to snail-disciple is compelling. She clearly took the time to dig hard in search of background information on snails- the big-picture stuff about their evolution and biology, as well as the thoughts on matters molluscan from science giants such as Darwin which can be so hard to find casually. You may find some of her content slightly twee, but not disastrously so. Occasionally she does drift into relating factoids which have an inauthentic parroted quality to them, but who among us could do anything else when explaining the deuterostome/protostome split in the tree of life? The details of that obscure moment back in the Cambrian Era at which the molluscan and our own evolutionary heritage appear to diverge gives me headaches and I have struggled to explain it in my own words on several occasions. The real strength of the book for me is that she has a sympathy for the animals themselves as individuals which I strongly share yet is starkly missing from other works on the subject. On the whole Bailey does a good job of expressing the admiration she came to feel for her gastropod companion I can only hope that this moves the casual reader to look again at some of the smaller yet remarkable creatures we share our world with.

In conclusion I would say that “The Sound of Wild Snail Eating” is certainly an important tome in the regrettably small canon of popular (ie.non-specialist) writings on molluscs. It may have a future as a ‘gateway’ text, a book you loan to a potential new recruit to snail-fan-dom who would not be hooked-in by other, drier, fact-avalanche-styled efforts. It will also while away a few hours while you wait for the mollusc book to be written…

Serious Tasmanian Snail-business.

Field Guide Review: Tasmanian Land and Freshwater Molluscs by B.J. Smith and R.C. Kershaw

This is the first of what I intend to be a steady trickle of mollusc book reviews hosted here at MolluscPOW. As a field-focused zoologist I have a particular fondness for identification guides so I think it fitting that I begin with one of the best I have found in Malacology, an oldie but a goldie: Smith and Kershaw’s excellent field guide “Tasmanian Land and Freshwater Molluscs”. 

I acquired this book a couple of years ago from a charming bookshop in Hobart for the quaintly anachronistic price of $5. I spent many a happy day with it snail hunting in the field. My appreciation of this little gem over time has grown steadily with use.

Its brown paper cover, monochrome illustrations and industrial typesetting have in my opinion passed the stage where they look simply dated. They have now taken on an air of retro-chic. It serves to emphasise that with this book you are not getting another style-over-content glossy stocking-filler such as passes for a field guide in many high street bookshops. This book means serious snail-business!

Published in 1981 as part of the University of Tasmania’s excellent “Fauna of Tasmania” series it describes itself as an illustrated checklist of the terrestrial and freshwater snails of the island. I don’t think this really does it justice- it has clear and effective keys and distribution maps. The illustrations by Rhyllis Plant are of a very high standard and are of great assistance when poring over animals in the field. The sections at the beginning of the book describing collecting and preserving techniques, biogeography of the species described. It also sports one of the best introductions to the jargon of snail anatomy and shell description I have yet found. It is simply one of the best simple field guides I have had the pleasure of using.

The information in the guide is distilled from the 1979 “Field Guide to the Non-Marine Molluscs of South-Eastern Australia” by the same authors- I have not come across a copy of this book but if it were of the same standard as the Tasmanian guide it will be very useful.

No doubt dear reader you are thinking that 1981 was a long time ago. Names and taxonomy of groups has changed over the years and no doubt there will be fair slice of the content of this little guide which would need revising to be current but I wouldn’t say this detracts from the utility of the book. For the intended purpose of empowering an amateur to set out in the wilds of Tas and name the snails they luck upon this book is as good today as it ever was. It would be a huge boon for malacology were regional guides of this standard available for other parts of the world- guides sufficiently geographically focused to be portable and dipped into quickly in the field.

Although other title’s in the series have gone out of print over the years, “Tasmanian Land and Freshwater Molluscs” remains in print for the time being. The University of Tasmania has a useful list of stockists on their website. I would strongly recommend this volume anyone with an interest in Tasmanian wildlife, Australian snail fauna or simply the art of producing a good simple field guide. Be warned though, inflation has taken its toll over the years: a new copy today now costs a whole $5.50!

Overall MolluscPOW verdict: 8 Caryodes out of 10.

Desert Snails: Bowerbirds use shells as decoration

The dead sunbleached shells of desert land snails make a tempting addition to the mating bower of a Western Bowerbird. This splendid structure was photographed near Mimili in the Everard Ranges, South Australia by Dennis Allely. The walls of the bower are woven from sticks, the green objects are mostly old eroded glass and Quandong Berries  The snail species are possibly Sinumelon pedasum and/or S. mugravesi.
This part of South Australia is truly awash with hardy desert snail species. I visit the area fairly regularly and I always come away astounded by the quantity and diversity of the desert land snail fauna down there.