Fireflies are one of the animals in Under the Ocean that has an actual use. The firefly can be attached to a Spear to make it a Light Source. It can also be put into a slot in the Toolbelt to make you emit light. This only works until the firefly dies, upon which it stops emitting light. Visually, the firefly tend to sparkle and emit glittery particles. The firefly can be found almost only in caves. There are rare reports of it appearing above, on beaches and such.