To be a fish in the ocean void is to glow, according to a new paper in PLOS ONE: an astounding 80% of open water marine fish can make their own light. What’s more, the trait has arisen 27 separate times in ray-finned fish lineages, a number much higher than previously realized or expected.

In a world without sun, fish have repeatedly found ways to shine. But why is that ability so widespread in the ocean, yet absent among vertebrates on land?

Bioluminescence, for one thing, is useful. The potential uses are probably endless, but blending in with the environment, defense from being eaten, attracting someone else to eat, or sending any number of messages to your colleagues are probably the most common reasons. Fish may either generate their own light by an enzyme reaction between a protein called a luciferin and an enzyme called luciferase that results in the emission of a photon, or they may host bacteria that do that job for them. But this cannot be the whole story.

In this study, scientists at St. Cloud State University, the American Museum of Natural History, and the University of Kansas compared 11 gene fragment sequences from 300 different groups of fish, mostly genera. They used computer programs that help them infer the most likely evolutionary relationships between them and then superimposed the bioluminescent status of the groups on the resulting family tree. This is what they found:

Family tree of ray-finned fish with acquisitions of bioluminsecence highlighted. Click the credit for a larger version of image. Credit: Davis et al. 2016

The 27 instances among the ray-finned fish (that is, all fish except for those with cartilaginous skeletons like sharks, or the lobe-finned fish that gave rise to terrestrial vertebrates) occur in fish found throughout the ocean. They span from the deep-sea lanternfish or anglerfish, whose eerie lures beckon fish to their doom, all the way up to the coral reef, where cardinalfish and pineconefish softly illuminate the nighttime seas. Since bioluminescence also seems to have evolved once or twice in the cartilaginous fishes like sharks, the trait may actually have evolved at least 29 times in the marine vertebrates.

It is so common in the open ocean that bioluminescence seems "almost a requirement" for these fish, according to one of the authors. Indeed, the most common vertebrate on Earth, a fish called the bristlemouth, exists by the (not making this up) quadrillions in the world's oceans, is also bioluminescent.

Of the 27 instances in ray-finned fish, 17 resulted from fish shanghai-ing light-producing bacteria (a symbiotic acquisition, labeled green above), and these 17 events produced about 48% of all bioluminescent fish species. It may be relatively easy to do this, which would explain the large number of independent instances. Bioluminescent bacteria that can live inside fish are common in the environment and they are not picky about their hosts, seemingly being content to be adopted by any fish that care to feed and house them.

Because bioluminescent bacteria are not under fishes’ direct control, they must devise structures (or other inducements) around them that are if they wish to regulate the glow. Fish that host symbiotic bacteria have many anatomical structures to feature, focus, or control the light their bacteria make, from shutters to windows to dangling lures. These structures may have encouraged rapid speciation among these lineages by giving evolution a shiny new plaything. Indeed, bioluminescent bacteria-hosting flashlightfish, ponyfish, and deep-sea anglerfish are species rich given the age of their groups.

The other eight acquisitions of light production by fish involve “intrinsic bioluminescence”, or the ability to glow without the help of bacteria (labeled blue above). But those eight origin events ultimately generated more than half of the species that can emit light – some 785 of 1,510 light-up fish.

The vast majority of the most diverse groups of deep-sea fish resulted from the evolution of intrinsic bioluminescence; almost 90 percent of groups with exceptional species richness glow with light they make themselves. One group, the netdevils (which sound like they should have their own NBA team), has even managed to acquire both types: a lure perched atop their head glows with bacteria, while a chin barbel shines with light they make themselves.

Groups that acquired their lighting by either method who use that light for communication or identification rather than camouflage seem to be particularly diverse: none of the most species-rich glowing fish lineages use it exclusively for hiding. In a place where there are no physical barriers like mountain ranges or deserts to promote speciation by physical separation, traits like illuminated pick-up lines could provide an easy way for species to diversify by natural selection. Small changes to the wording could result in big consequences for mate recognition and reproduction.

Thus, bioluminescence in marine fish may be so widespread not just because it is advantageous for fish to have a powerful new ability, or that picking up the bacteria that can make light is easy by comparison to land, but also that, once acquired, bioluminescence provides an easy platform upon which natural selection can act, promoting both the evolution of new species, and more and more twinkly fish.

Reference

Davis, Matthew P., John S. Sparks, and W. Leo Smith. "Repeated and Widespread Evolution of Bioluminescence in Marine Fishes." PloS one 11, no. 6 (2016): e0155154.