Bioluminescence

Bioluminescence was a natural occurrence in many kinds of insects and marine animals, as well as types of alien creatures, that caused them to give off light.

The Star Whale under the Starship UK was bioluminescent. (TV: The Beast Below)

The Mantasphids were bioluminescent and provided light for the humans on Myarr after their "surrender". (TV: The Infinite Quest)