Sweepers on the Wreck of the Ulysses, Egypt, Red Sea.
Photo copyright Tim Nicholson, whose underwater photographs and photo books are available from http://www.photoboxgallery.com/timnicholson.
Taken on the Ulysses, Red Sea, Egypt.

The fish in the photo are pygmy sweepers, also known as glass fish and golden sweepers (Parapriacanthus ransonneti). You find dense schools of these small fish throughout the Indo-Pacific, from the Red Sea to Australia. They like shady places during the day: wrecks, overhangs and caves. At night they venture out to feed on zooplankton.

Sweepers have large eyes and a single fin on their backs.. They are 10 cm long with yellow heads, peach-coloured bodies and a distinct lateral line running from head to forked-tail.

The pygmy sweepers have light-emitting compounds in their guts, presumably gained from food they have eaten.

Pygmy sweepers living in the Red Sea used to be known as Parapriacanthus guentheri, but it is now thought that there is just one species and Parapriacanthus guentheri is an obsolete name.

 
Further Reading
The Underwater Photographer: Digital and Traditional Techniques, by Martin Edge, Paperback, 424 pages (2006)
The Essential Underwater Photography Manual: A Guide to Creative Techniques and Key Equipment, by Denise Nielsen Tackett and Larry Tackett, 176 pages (2005).
Shipwrecks from the Egyptian Red Sea by Ned Middleton; Immel Publishing, 196 pages.

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