Sea urchins 
- Revisited -
by Jean-Marie Cavanihac, France 
Some months ago I wrote an article about sea urchins. So why am I writing again on this topic? Perhaps it's because I always have an urchin spine in my left forefinger! But the real reason is different: at that time I didn't use a color still camera and the images were 'hand colored'. Some features of urchins are so beautiful in their true colors, that I have prepared a second article to show some of these features to Micscape readers; particularly the pedicellariae and their tiny internal 'bones', such as these below:
But to recap a little: some parts of an urchin are made of calcium carbonate, (calcite, with magnesium carbonate an important additional component), and these materials are able to rotate plane polarized light.

Some additional equipment is needed to add polarized lighting to your microscope. Some methods, including using clip-on Polaroid sunglasses, have been described in past articles in Micscape, but I have used a different approach. 

Liquid crystal displays (LCD's) like those used in handheld calculators, car temperature indicators, low priced watches etc. include a polarizing filter to increase the display contrast. Some filters for computer screens are also polarizers. On an LCD this filter is a film glued onto the glass display, in other cases it's just a removable plastic grey plate. It's this last type I have used:

With two pieces of this film, one in the light train (the polarizer) and the other on the eyepiece or in front of camera objective (the analyser), it's possible to obtain a 'dark field' when rotating one piece with respect to the other. The planes of polarisation are now 'crossed' and normally no light can pass through. If you introduce into the optical train a birefringent substance (i.e. having two different refractive indices, RI), multicolours caused by an interference phenomenon are seen.
These colours will be seen with an anisotropic material (i.e. the RI changes in different planes), and in our case calcite is precisely such a material. In some plastic plates, (CD cases etc), strains in the material modify the refractive index to create beautiful pictures. Note, that depending on the thickness of the plastic polarizing filter, you can obtain different results when revolving the polarizer or analyser or both. But coming back to sea urchins:
With urchin larva, the chalky skeletal rods appear brightly colored under polarized light, but the image is a little too dark for my camera to capture: For the image below, the polarizer was slightly rotated.
In adult sea urchins, the pedicellariae are a sort of small forceps located near the urchin's mouth and are used to clean the mouth, spines or to dislodge any kind of parasitic larvae. All of them possess three 'fingers'. 

Pedicellaria have different shapes according to their function: tridactyle forceps, delicate spoon, or the redoubtable venomous claw. I used the same method described in my first article (chlorine solution) to dissolve the epidermal tissues to obtain the skeletal structure:

The results are shown below: first, the tridactyle forceps are probably the strongest.
The second type: triphyllous forceps .... under both normal and polarized lighting. 
Another picture of the three valves of an entire open pedicellaria, under polarized light.
The most impressive: globiferous pedicellaria which is a venomous claw with three spines. A 40x objective shows the channel inside the main spine; it's probably to innoculate venom.


Click on the image right to view an animated gif image (285 kbytes, in real time). On the upper 'finger' the venomous spine is clearly visible. 


Skeletal structure of one of the three 'fingers'. 

Pedicellariae can continue to move as if they were autonomous, even after they have been separated from the urchin for many hours. (Or even for up to five or six days after removal, if they are kept in the refrigerator!) The stalk is covered with a ciliated epidermis which is probably sensitive to parasitic larvae. 

So how can you remove these interesting features of an urchin for closer study? The pedicellariae are less than a quarter the size of the spines: a nail clipper is useless because they are, in practice, invisible amongst the spines. But here is a simple, efficient and quick method: I direct a strong seawater jet around the mouth by using a 50 ml syringe, with the sea urchin placed in a cup. The pedicellariae which break off look like tiny three leaf clovers in the cup bottom. 

Note: It's the first time I have 'mirrored' an article about the same topic with the French microscopy magazine ''. Many readers of it think Micscape is the best e-zine for microscopy! But some of them have difficulties reading English and have requested I write  a French translation of my articles. Additional pictures will also be on this other site. 

Comments to the author Jean-Marie Cavanihac are welcomed.

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All photographs © Jean-Marie Cavanihac 2002

Published in the May 2002 edition of Micscape Magazine.

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