Tough teeth - the radula of the common limpet

by David Walker, UK

 

A look at a classic type of microscope subject, prompted by scientists' studies in 2015 where the teeth were found to be the strongest biological material tested to date.

 

Image right, Victorian prepared microscope slide,
labelled 'Palate Patella Vulgata BB Apr 1897'.

 

It's not often that the finer points of invertebrate anatomy reach the news headlines, but the teeth which form part of the radula of the common limpet Patella vulgata achieved just that in February 2015. This humble little critter which lives on rocks found along the seashore quietly munching plant matter using its radula to scrape the hard surfaces had been harbouring a secret. Its teeth were made of an organic-mineral composite that has been found to be the toughest biological material tested to date, demoting the widely cited spider silk into second place.

As a microscopy enthusiast I was delighted because snail radula are one of my most favourite subjects to study—they respond well to a wide variety of microscopy techniques, both incident and transmitted, including autofluorescence for some dry mounted species. After reading the news story I browsed my prepared slides of radula to find that I didn't have an example of the limpet (no shortage of whelks though). Fortunately, keeping an eye on eBay I found an example for a very modest price—the limpet's newly gained celebrity status didn't prompt a buying frenzy.

The slide is shown above. If 'BB' are the mounter's initials, they are not familiar to me and have found no reference to them, but the seller offered some valuable provenance in the auction (with thanks) suggesting it was a B. Borrows of Darlington (UK):

The slide hasn't been made to the highest standards, the radula has been lain out linearly as is typical but a number of the teeth seem damaged (in life or during the preparation?) and the white sealant has encroached into the mount. But it provided a good enough example to explore.

The teeth of many radula are opaque so a mix of transmitted and incident lighting is a useful method to provide some detail on the teeth as well as of the underlying structure. I do have the proper incident lighting head for my Zeiss Photomicroscope but rarely use it for covered subjects—I find the on-axis lighting rarely suited for the subjects I like studying. For very little cost and more effective for many subjects, is off-axis lighting using for example a white LED on flexible neck. I changed out the 5 mm LED on a commercial example for a 3 mm so can shine light at close quarters using objectives with short working distances. I was first alerted to the commercial USB LEDs on goosenecks from the Micscape March 2012 article by Andy Chick who was in turn inspired by Andrew Entwistle's LED lighting project presented in Micscape June 2006.

The two images below show off-axis lighting using the LED combined with transmitted light, firstly DIC, then phase.
 

Off-axis incident lighting + transmitted DIC, Zeiss 6.3X NA0.16 with lambda tint plate. Stack of three images.
(Not true DIC as don't have the correct matching prism for the 6.3X, the number II prism with condenser lens off gives reasonable results albeit without an even background.)
 

 

 

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