Sea mice (Aphroditidae, Polychaeta) are furry-looking marine worms that for
age on, or in, the sea-bed. They are distinguished by an amazing iridescenc
e along the lower sides of the body, associated with both hairs and spines
known as chaetae. We show by electron microscopic and optical analysis that
the iridescence of spines is caused by a highly regular, sub-micron scale
structure resembling that being developed for photonic crystals, and that i
n fact the sea mouse exploits a partial photonic band gap to achieve its re
markable coloration effects.