Blue-headed wrasse
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[Photo] Blue-headed wrasse, Thalassoma bifasciatum, San Pedro, Belize Barrier Reef.Date 03/08/2007. Author Tibor Marcinek (http://www.marcinek.sk/). License: public domain

The blue-headed wrasse (Thalassoma bifasciatum) is a species of saltwater fish in the wrasse family (family Labridae) of order Perciformes native to the coral reefs of the Caribbean Sea. Blueheads are small (less than 90 mm standard length) and rarely live longer than 2 years. They form large schools over the reef and feed primarily on planktonic copepods as well as small benthic crustaceans.

Like many other wrasse species, the bluehead is a protogynous sequential hermaphrodite: individuals begin life as either male or female, but females can change sex later in life and function as males. Females and young males are yellow and white in color, often with black lateral stripes and occasionally dark vertical bars. This coloration is known as the Initial Phase. These individuals can rapidly alter the presence or intensity of their yellow color, stripes, and bars, and these color changes appear to correspond to behavioral changes. Large females and some males can permanently change coloration and/or sex and enter the Terminal Phase coloration, which has a blue head, black and white bars behind the head, and a green body. It is this color phase that gives the species its name. One Terminal Phase male wrasse will usually coexist along with a group of roughly a dozen females; if the male dies, the largest female will transform, over a week, into a male, changing colour and behaviour to act as the new male over the remaining females.

http://en.wikipedia.org/wiki/Blue-headed_wrasse

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