There might be plenty of fish in the sea, but evolution seems to prefer the glowing ones.
Impressed by the diversity and pervasiveness of light-emitting sea creatures, a team of zoologists recently began to puzzle out the evolutionary origins of bioluminescence. After tracing the branches that these fish took along the evolutionary tree, the scientists determined that fish developed some type of bioluminescence in at least 27 independent evolutionary events. This repeated evolution signifies that bioluminescence are an extremely useful adaptation for fish, according to the recent article in the journal PLoS ONE.
These results are especially curious since land animals rarely evolve bioluminescent features. That the trait evolved so many different times in fish is partly due to the multitude of reasons why fish glow.
Many sea creatures use bioluminescence as locating beacons in dark or murky waters, where finding another member of your species isn’t always easy. During mating season, these fish announce their presence by luminescing steadily or flashing a special pattern.
The same glow that attracts mates can also attract prey, especially in deep sea fish. The fantastical black dragonfish—imagine a black eel with fangs and a bioluminescent barbel— uses its glow as a deep-sea flashlight to facilitate hunting.
On the defensive side, certain species use bioluminescence to confuse predators. A family of fish known as tubeshoulders shoots bioluminescent ink to evade attackers, similar to a squid or an octopus. Many fish develop lightly-colored glowing bellies to hide themselves from predators below by mimicking the diffuse lighting of the water above.
In the case of the myctophid lanternfish, scientists believe that their bioluminescence initially evolved for camouflage, but developed into a mechanism for intraspecies communication. Unique underwater lights and patterns give fish the ability to quickly recognize their compatriots from the many other fish in the sea, and this may have profound consequences for their evolution. With Ars Technica, Annalee Newitz writes:
Many glowing fish groups are called “species rich” because they tend to diverge into new species very quickly. Smith and his colleagues believe this is probably because they’re using species-specific light patterns to recognize mates and communicate. If a group of fish finds a nice ecological niche that works, and it has a very obvious way of communicating with its immediate group, members of that group of fish will mate only with each other and rapidly diverge from their neighbors.
In other words, bioluminescence in fish is an example of both convergent evolution—the same trait arises many different times—and divergent evolution—species which possess bioluminescence tend to differentiate into new species more rapidly—making bioluminescence a particularly useful trait to study if you’re an evolutionary biologist.
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