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First published online December 14, 2006
Journal of Experimental Biology 210, i (2007)
Copyright © 2007 The Company of Biologists Limited
doi: 10.1242/jeb.02671
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WHELK WONDERMATERIAL
laura{at}biologists.com
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Whelks make sure that their young get a pretty good start in life by enveloping their developing embryos in a tough and protective, yet flexible, egg capsule. `It is a very interesting natural rubber' says Scott Rapoport, of the Children's Hospital of Philadelphia. Rapoport explains that researchers are interested in biomaterials like whelk egg capsule because they are trying to crack the problem of making biomaterials as good as the natural version. Scrutinising a material's structure, and how it's made, throws up vital clues for curious scientists, so Rapoport and his colleague Robert Shadwick examined the structure and properties of egg capsule material during its production to try and find out more (p. 12).
The egg capsule production line begins in glands inside the whelk's shell, before the snail delicately manoeuvres the immature egg capsule out of its body and along a groove in its foot into another gland known as the ventral pedal gland. Before the egg capsule enters the ventral pedal gland, it is very fragile, unlike the tough and rubbery finished product. Because of this, Rapoport suspected that the ventral pedal gland plays an important role in the material's production, and wondered if the structure and mechanical properties of the egg capsule material were changed while it was in a gland, or if the gland helped to glue the material together.
To look at the structure of the capsule material in more detail, the team first isolated immature egg capsules before they entered the ventral pedal gland by chemically inducing the snails to lay them, and collected mature capsules after they had left the gland. Examining immature and mature capsule material using SEM, they found that both materials had a very similar layered structure. This suggested that the material's structure is built inside the snail's body. Because the immature material is so fragile, it also suggested that the ventral pedal gland contributes a `glue' to hold the capsule together and stop it falling apart.
To find out how the ventral pedal gland was strengthening the material, Rapoport subjected the mature material to a barrage of tests, treating it with heat or acid and then pulling on it and measuring the stress, or how it distorted. He already knew that the mature material had a two-phase stress response when he pulled on it. At low strain, the material behaves like a stiff elastic, where a small strain causes a big stress in the material. Increasing the strain causes a different response: the stress on the material doesn't increase that much with very big increases in strain, and the material behaves more like a floppy rubber band.
Treating the material with heat and acid changed how the material responded to strain. When strips of capsule material were bathed in acid, or heated up, and then given a tug, the material behaved like a floppy rubber band at both low and high strains - there wasn't a two-phase response. However this change was reversed when the team rinsed the acid off, or removed the heat. This told the team that the glue added in the ventral pedal gland stabilises the material: firstly causing the stiff elastic response, and secondly allowing the structure to reform after it is `unravelled' by the heat and acid treatment. The most important question researchers now have to answer is to find out exactly what the glue is that is holding whelk egg capsules together.
References
Rapoport, H. S. and Shadwick, R. E. (2007).
Reversibly labile, sclerotization-induced elastic properties in a keratin
analog from marine snails: whelk egg capsule biopolymer (WECB). J.
Exp. Biol. 210,12
-26.
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