Eukrohnia fowleri is a deep-sea marine arrow worm.

[2] The bioluminescent organ of Eukrohnia fowleri is found along the center of its tail fin on both its dorsal and ventral side.

It has a secreted bioluminescence that is thought to be coelenterazine based.

So far, there is no other bioluminescent organism that uses hexagonal packing in order to hold bioluminescent materials/[2] E. fowleri evolved through the adaptation to hypoxic water and due to the recent oxygenation of water they have been experiencing bottleneck events.

These events have been seen as one of the reasons that E. fowleri have such low biodiversity.