Many species of insects, crustaceans, worms, and fish use glowing signals to attract mates. According to a new Current Biology study, animal lineages with bioluminescent courtship displays have more species than their non-luminous relatives.
Evolutionary biologists have long wondered why closely related lineages accumulate species at different rates. According to theory, populations undergoing strong sexual selection will diversify more quickly because of their increased potential for genetic isolation. When male traits and female preferences co-evolve, isolated populations will split off into new species.
To see if sexual selection contributes to bursts in diversification, Emily Ellis and Todd Oakley from the University of California, Santa Barbara, focused on one sexually selected trait: bioluminescent courtship. Light production evolved separately at least 40 times in a wide diversity of both marine and terrestrial animals, ranging from little crustaceans like ostracods and shrimp to sharks to beetles.
After conducting an extensive literature search, they identified 17 lineages with bioluminescent courtship and 23 lineages with bioluminescent counter-illumination, which is a type of camouflage. Some squid, for example, emit light on their lower surfaces to match the light coming down from above – becoming invisible to predators that hunt by looking up as they swim. The team also studied the evolutionary history, divergence, and relationships of 10 distantly related lineages with bioluminescent courtship in detail: an octopus, an ostracod, a click beetle (pictured above), a railroad beetle, a firefly, a lanternfish, a dragonfish, a ponyfish, and a lantern shark.
Not only did the lineages with bioluminescent courtship have more species than their non-luminous sister lineages, they also had faster rates of species accumulation. But that’s not the case with animals that use light for counter-illumination camouflage. The researchers didn’t find that same pattern of species richness for animal linages that glow for anti-predation defense, likely because that function is not sexually selected.