First male, then female

Ribbon eels are bizarre animals, colourful and with a surprising nose. Among its many strange traits are the tubular nostrils at the front of the head which flatten out into leaf-shaped protrusions, and the two round openings near the eyes.
Another peculiarity of the ribbon eel (Rhinomuraena quaesita) is the transformations it undergoes during its life. Not only a transformation of colour, but also of sex. All ribbon eels are born male, and almost all black. Slowly, the animals become more colourful. Adults are azure blue with a bright yellow stripe over the dorsal fin. At the end of their life, all ribbon eels are female. The size of the school determines the speed of the transformation from male to female.
This gender transformation is called protandry, and is present in hermaphrodites to prevent self-fertilisation. The ribbon eel is the only moray protandrous species; other animals with this trait include various fish species (including clownfish) and flatworms.

Adult ribbon eels are more than a metre long and often hide during the day in coral crevices and niches, only emerging at dusk. They move through the water like snakes. These animals can be observed in the aquarium at Artis zoo in Amsterdam.