A light in the dark: ecology, evolution and molecular basis of copepod bioluminescence

Within the calanoid copepods, the bioluminescent species comprise 5-59% of the abundance and 10-15% of the biomass in the world's oceans. Most of the luminous species belong to the superfamily Augaptiloidea. The composition of bioluminescent species within the calanoid copepods shows latitudinal patterns; 5-25% of total calanoid copepods are found in high-latitude oceans, while 34-59% are in low-latitude oceans, reflecting a prey-predator relationship. Bioluminescent species of calanoid copepods are able to produce the light-emitting substrate coelenterazine. It is then transferred to higher predators through the food chain, and might be used for bioluminescence in other luminous organisms. A notable feature of copepod bioluminescence is the secreted-type, and its major function may be as an antipredatory response or a defensive behavior. Identification of more than 20 luciferase genes from calanoid copepods has revealed the highly conserved sequences of those genes. This leads us to the speculation that the genes for luciferase within the group of calanoid copepods have evolved independently of comparable genes outside of this group. We discuss here the ecological and biological functions of copepod bioluminescence, the significant diversity in luminous intensity, which might be evolutionarily relevant to their motility and habitat depth, and the promising future directions of bioluminescence studies.