The Tail-light Shark (Euprotomicroides zantedeschia) is a rarely found species of shark in the family Dalatiidae - the only member of its genus.
To date, only four specimens have been found, from the deep oceanic waters in the southern Atlantic Ocean and Pacific Ocean.
The sharks have an abdominal 'pouch' containing a blue bio-luminescent fluid which can apparently be ejected into the surrounding seawater.
The function of the fluid, which is unique in the shark family, is unknown. If other animals are considered, it's likely to have use either as a defence mechanism, or as a 'lure' for prey.
See : Cybium, 40(3): 187-197.