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The 'Sea Mouse' (Aphroditidiae sp.) is a worm1 that crawls along the bottom of the sea. Due to the iridescent spines (or chaetae) it has a furry appearance - explaining the name, I suppose. The recent interest in this humble looking creature is due to the strange properties of these spines. It seems that optical physicists, trying to make sophisticated 'photonic band gap' materials, have been beaten to it by this 'mouse'. The spines are red when light falls on it one way - and blue/green when the light shifts to another angle.

Hardly a new phenomenon - nature often gets there first, having had millenia to practice in. However, it does raise the question : How'd evolution produce THAT? (or it did to s_alanet, anyway :). The argument (distilled) runs like this :

"There cannot be intermediate forms for such a complex structure"
Hmmm..now where have I heard that argument before?. There always has to be a reason for a structural feature, doesn't there? It couldn't possibly have been chance that was involved?

Importantly, the creature can live between 1m and 2000 metres underwater2. So the necessity for 'perfect' spines 'first-time' is removed - as Gorgonzola points out in the node mentioned above. So a gradual approach to this 'perfection' is possible, therefore? This is where the problem lies - the assumption that the organism 'strives' towards a goal. This confuses the struggle of the optical biomimetic researcher3 with that of the sea creature. I doubt that Aphrodita applies for funding! (on the basis of its spines' utility in optical networking, perhaps :)

All that can be said is that a mouse has spines because its parents had them. Even the researchers don't know :

"...we are uncertain as to the biological purpose of their iridescence - species recognition, mating or attraction of prey are all possibilities..."
which is exactly the position an 'evolving' sea mouse is in. It cannot know about 'pseudobiorefringence' or the importance of a 'high index contrast'. There is no purpose to changes made to the structure of its spines. Just as important is the combination of several selection pressures on the spines - structural and 'darwinian'.

Returning to the "what use is half a spine?" argument; it is possible to produce scenarios for Aphrotidia - which like all evolutionary 'just-so' stories are only plausible routes, and not 'true' histories. However, it is not necessary. Opals have photonic band gap effects, as do butterfly scales - they don't just exist in high-tech labs. It's easy to forget that we are only really familiar with the properties of everyday objects made of stone, metal and glass. Life, however, is constantly inventing materials that have spectacular and surprising effects.

In summary, the mouse got its spines by packing cylinders of chitin together into a tube shape. Fortuitously, this gave the chaetae some unusual optical properties which may have allowed some of the creatures to venture into deeper waters (or mate more easily, or attract prey).

1Since Linneus named it Aphrotidia, so who are we to argue? :)
2Most technical info taken from R. C. McPhedran, N. A. Nicorovici, D. R. McKenzie, L. C. Botten, A. R. Parker and G. W. Rouse, The Sea Mouse and the Photonic Crystal Aust. J. Chem. (2001). Which I found online, if you search you might (still) find it too.
3There really is such a field as 'optical biomimetics' - it's the study of living systems with interesting optical properties, such as butterfly wings and beetle shells.

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