The size of a very small fish scale, equally as flat and even more transparent, there is an open ocean, mind-blowing, male, modified copepod called Sapphirina that has caught the eye of many a holidaymaking snorkeller in the tropics.

Put on a face mask, leap off your luxury launch, hang at the surface and look down into deep blue waters and ten to one you'll catch a glimpse of a little flash of iridescence now and again. The perpetrator is a copepod, not very copepod-like, but none the less related to the "cattle-of-the-sea" - that huge group of bewhiskered crustaceans that are the staple diet of many a fish and many a whale.

More precisely it is a male copepod of the genus Sapphirina, whose female is strikingly dissimilar. Her mate is totally transparent and very flat and lives inside transparent salps which he parasitizes. So how come the iridescent colour? If a beam of light (sunlight works well!) is directed at a certain critical angle at the surface of this diminutive character, the white light is prismatically split into some of the most distinctive colours in the spectrum. Polygonal stacks of transparent chitinized plates adorn the segments of this woodlouse-shaped copepod and they split the light by diffraction and refraction. No two individuals are the same and it is now believed by some researchers that the phenomenon is used in communication.

For me though "opal" describes the shifting colours better than "sapphire" - but then we'd have to call it "Opalina" and that name is already booked!

This article was written by Peter Parks

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