The Oceans are Aglow With Light-Up Fishes

June 8, 2016 PLOS Blogs Ecology New Research

*An Idiacanthus with bioluminescent barbel. Photo: Matthew Davis.*

Bioluminescence – the production of light from a living organism – is found in a surprising variety of ocean fish. A new study in the journal PLOS ONE shows the phenomenon has evolved independently many times, in many marine habitats.

Matthew Davis from St. Cloud University and colleagues analyzed genes from over 300 taxa of ray-finned fishes. They found that bioluminescence has evolved independently 27 times in 14 major fish clades (groups of fishes that come from a common ancestor).

The researchers suggest that all the bioluminescent fish they examined evolved the trait sometime around the Early Cretaceous, some 150 million years ago. Further, the team found that once an evolutionary line of fish developed the ability to produce light, it tended to soon thereafter branch into many new species.

When a trait evolves independently multiple times, it suggests that the feature is useful. The vast majority of bioluminescent fish live in mid-water or deep sea habitats, where light is scarce to non-existent. Under these circumstances, the only light is coming from other animals.

*A Tarletonbeania with bioluminescent photophores. Photo: Matthew Davis.*

“Bioluminescence has a lot of different functions,” says Davis. “Most of the groups that have evolved bioluminescence are using it for some sort of predator avoidance.”

This means camouflage for some fishes, such as those that use counter-illumination to help hide their silhouettes in the water column. Then there are tube-shoulder fishes, which shoot out a bioluminescent goo to help them escape predators, much like squid release a cloud of ink.

Others, such as many deep sea predatory fishes, have bioluminescent barbels or lures on their heads that they use to attract prey. Still other fishes use bioluminescent emissions to communicate.

Fish can produce bioluminescence in one of two ways.

*A Porichthys emitting light from ventral photophores. Photo: Matthew Davis.*

“Certain groups of fishes are capable of producing and emitting light all by themselves,” says Davis. “In those cases, their bioluminescent structures are often innervated by nerves, so they can turn the lights on and off.”

Other fishes house symbiotic bacteria with the ability to produce light in various structures on their bodies. About two-thirds of the times bioluminescence has evolved, Davis says, it is in symbiosis with bacteria.

After discovering this evolutionary diversity, Davis and his colleagues are working on the genomics of bioluminescence, hoping to identify specific genes associated with the production of light in fishes.