Bioluminescence is a beautiful evolutionary phenomenon which has aided organisms in defending against predators, attracting mates, attracting prey, communicating, and even coping with metabolic stress. A ton of groups contain bioluminescent members (fungi, echinoderms, cnidarians, the list goes on and on) including some real evolutionary stand-outs.
In most cases (but not all!), bioluminescence results from enzyme-catalyzed oxidation of luciferins—light-emitting compounds—by luciferases. There can be many different luciferase compounds used even in closely related species.
New luciferin found in glowworms
A newly identified luciferin was discovered in caves in New Zealand (because of course it would be in a cave in New Zealand) in glowworms. This luciferin uses Xanthurenic acid and tyrosine as the two precursors to the glow. This particular glowworm is Arachnocampa luminosa, a species of fungus gnat that, in its larval stage, produces sticky threads by building a long muscousy tube and moving along the tube sort of vomiting up little sticky threads to trap insect prey. How disgustingly beautiful nature can be!
Glowworms are not really worms, but rather, larvae of several families of beetle and fungus gnat–however the bioluminescence is not homologous among the groups (so it’s arisen independently many times over). While it’s not always just the larvae that glows, the larvae emits the brightest blue-green glow. The glow helps the glowworms attract insects, attract mates, and protects them from predation (it also inspired James Cameron to make the blockbuster hit,
Pocahontas with Glowworms Avatar).
Deep sea Cephalopods like to flash each other
An especially cool evolutionary example is of a deep-sea octopus, whose “suckers,” which still retain a sucker appearance and sucker-like traits, have had many of their muscle cells replaced with light producing cells. Researchers suspect this may have occurred as the result of once being a shallow-water bottom dwelling octopus, and moving to a deep open-ocean environment where suckers were less necessary.
Now it appears, the octopi use these glowing suckers for communicating to one another via visual signaling. They may also be using them for attracting a favorite prey item of theirs—copepods (small crustaceans). This is an unusual prey item for an octopus, but the copepods are attracted to the bioluminescence.
For a more flashy light show, I’d recommend the firefly squid. Their deep blue lights (produced by photophores) are used for communicating with mates and perhaps rival squid. The light can also be used to break up the body pattern to confuse predators and attract small fish to prey on (because deep sea fish just cannot seem to learn which glowing lights mean danger). The really cool thing about firefly squid though, is not so much the light they produce, but the evolution of their eye that seemed to come with it. They are thought to be one of the only cephalopods to have color vision (by the way, cephalopod eyes: a fascinating topic). They have three visual pigments while other cephalopods only have one, and this may be so they can better distinguish ambient light from bioluminescent light, and perhaps because their light color is pretty unique from other bioluminescence emitted in the deep sea.
Symbiotic bacteria- lux operon helps flashlight fish and bobtail squid
In perhaps the most of obvious function of bioluminescence: The flashlight fish (Anomalous katoptron in this case, though there are several species), produces light using symbiotic bacteria. The fish’s light organs are located under it’s eyes so it can turn the light on and off by blinking. These organs are packed with bioluminescent bacteria to produce a greenish-blue light. Researchers found that the fish blink less (meaning their organs are open) in the presence of their planktonic prey indicating they use their bioluminescence for finding prey.
Quorum sensing and a beautiful tale of symbiosis
One of my favorite bioluminescent evolutionary excerpts is that of Vibrio fischeri and Euprymna scollops (the Hawaiian bobtail squid). V. fischeri is a symbiotic bacterium that produces bioluminescence through the lux operon (which involves another luciferase oxidizing a compound to produce blue-green light). The Vibrio interact with the squid (using type IV pili) which starts the maturation of light organs in the squid. These bacteria help the squid conceal its shadow while its foraging for food under the moonlight. This protects the squid from predators while providing the bacteria with a stable home.
What makes this bacterium especially notable, is that it was one of the first bacteria to be discovered to use quorum sensing. Quorum sensing is a gene expression regulation tool (often called “bacteria communication” and totally going to be on your exam tomorrow) where the Vibrio’s gene expression responds to changes in bacteria cell density. A signal molecule- N-acylhomoserine lactone (AHL), is synthesized by LuxI (a protein produced by the lux operon I mentioned earlier) and leaves the bacteria cells. LuxR forms a complex with AHL and binds the lux box causing the activation of luminescence genes. The bacteria colonize the squid’s light organ at a very high density producing lots of this AHL molecule.
Millipedes: Glow first used for coping with climate, co-opted for warning signal
If you’re ever in California, be on the lookout for the Motyxia millipedes. They’re pretty easy to spot as they emit a teal glow from their entire body. They also produce poison cyanide which many other millipedes do as well. Instead of concentrating their glow to one light organ and instead of emitting light from a luciferase reaction, they glow all over their exoskeleton using a photoprotein whose homology is unknown.
But what’s REALLY cool about the Mytoxia is that for a while it was thought that bioluminescence evolved in the millipedes as a way to warn predators. However, when researchers discovered that another species (previously Xystocheir bistipita, now reclassified as Mytoxia bistipita) glows, but much more faintly, they looked more into it.
They found that Mytoxia may have actually evolved to cope with hot, dry climates (this species is found in the Sierra Nevada Mountains). The glow of M. bistipita is much less intense and they also have fewer predators than other species. Millepedes have difficulty metabolizing oxygen in hot, dry climates which creates toxic byproducts (like peroxide). Their bioluminescent photoprotein actually helps to neutralize these toxic byproducts. The researchers concluded that the millipedes colonized higher elevations more recently than the bioluminescence evolved, and that with that colonization came more predation. Only then did they co-opt the trait for warning predators of their poison cyanide production. The brighter the millipede, the more cyanide it contained!
- Paul E. Marek, Wendy Moore. Discovery of a glowing millipede in California and the gradual evolution of bioluminescence in Diplopoda. Proceedings of the National Academy of Sciences, 2015.
- Jens Hellinger, Peter Jägers, Marcel Donner, Franziska Sutt, Melanie D. Mark, Budiono Senen, Ralph Tollrian, Stefan Herlitze. The Flashlight Fish Anomalops katoptron Uses Bioluminescent Light to Detect Prey in the Dark. PLOS ONE, 2017.
- Quorum Sensing in the Squid-Vibrio Symbiosis. Subhash C. Verma and Tim Miyashiro. Int J Mol Sci. 2013 Aug.