During his time in Daniel Morse's lab at the University of California Santa Barbara, USA, PhD student Daniel DeMartini has seen many Doryteuthis opalescens squid pass through the lab's doors. These squid provide DeMartini with a steady supply of the iridocyte cells that are responsible for the squid's shimmering opal-like markings. Iridocytes are found in many cephalopods, but what makes those of D. opalescens so special is their ability to adapt and produce a rainbow of different colours from the same cell. Most iridocytes are found in patches across the squid's body but DeMartini recalls: 'We started to notice that some squid had bright iridescent rainbow stripes underneath their fins. Sometimes most of the squid in a batch would have them, sometimes none. After a while we started to realise the rainbow stripes were only seen in the females.' So after a few years of observing this, DeMartini decided to investigate this female-only trait further, publishing his results in The Journal of Experimental Biology http://jeb.biologists.org/content/216/19/3733.abstract.
Upon the squid's arrival in the lab, DeMartini noted that on average, only half the females displayed colourful stripes, yet all were capable of producing them, as an hour after death all females had these vibrant markings adorning their bodies. When DeMartini examined the underlying tissue under a microscope he found it was full of iridocytes jam packed with layer upon layer of reflectins (the proteins responsible for reflecting the light as colour). The sheer number of iridocytes each packed with a high number of reflectin layers results in stripes that are six times brighter than other patches of iridocytes.
Sandwiched between the two colourful stripes, DeMartini also noticed a large bright white area whose appearance coincided with the emergence of the iridescent streaks. When he delved a l
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