Its all around us and performs some of the most fundamental ecosystem services on our planet. Plants, whether on land or in the shallow seas, use the power of light to catalyze a cascade of reactions that ultimately result in an amazing, complex web of interdependent organisms. Even in the deep sea the products of light find their way to the bottom to be recycled among the myriad critters inhabiting this temporal oasis. Light is not only produced by chemical reactions in the sun, but animals, fungi and bacteria have harnessed a particular chemical reaction for communication.
Fireflies are in the family of beetles called Lampyridae, the “lantern bearers”. Much like our own homes, fireflies have light bulbs which use switches to turn it on and off. Luciferase, the light switch, is an enzyme that fits lock and key with the bulb, a substrate called Luciferin, in a special organ within the fireflies' belly. This is a slow, two-step reaction with a fairly high cost to the firefly. In fact, this is why we can see the light decay after the initial flash. .
There are about 2000 species of lampyrids globally, with about 120 of those in North America. A phylogenetic study by Stanger-Hall and colleagues (2007), using a variety of genes, found that light production evolved more than once in this family and the current taxonomic classification was not supported by the molecular data. There were other anomalies too. For instance, two genera (groups of closely related species) which were classified outside of the Lampyridae family were clearly nested cozily within the group.
If lantern-bearing arose several times among one group of closely related beetles it must serve a unique purpose that the ancestors of the family were pre-adapted towards. But based on morphological analysis, light production evolved early, predating the Lampyridae, and was retained from the larval form. Like all beetles, their larvae are grubby little worm-like creatures that live a life apart from their adult counterparts. Branham & Wenzel (2003) report lampyrid larvae with a constant, but faint, glow in the abdominal larval light organs. This is supported by observations that adults vary widely in light production as well as light organ placement and use.
[caption align="aligncenter" width="480" caption="Video of firefly larva from Thailand. Note the constant glowing in the posterior segment."][/caption]
Typically, lampyrids use their light displays as sexual signals to attract a mate. But more ancestral family members use pheromones. Citing a chapter by Lloyd (1997), Stanger-Hall described three mating signal systems in the 120 North American species of firefly:
- Chemical signals (pheromones): so-called ‘‘dark fireflies’’ which produce no light as adults, are active during the day and release chemical signals to attract mates.
- Glows (continuous light signals): these ‘‘glowworm fireflies’’ tend to have larvae-like females who spend the day in underground burrows and emerge at night. They glow (short distance) in combination with using pheromones (long distance) to attract males who fly towards the glow, but usually do not signal themselves.
- Flashes (short intermittent light signals): the more common ‘‘lightning bug fireflies’’ that are active at dusk or in the dark. Both males and females use species-specific light signals to communicate with each other in an interactive visual morse-code.
There are no clear patterns in the evolution of the signaling pathway though. Whether they used flashes, glows or pheromones, each strategy is independent of any evolutionary lineage. Below, the tree on the left is color-coded by sexual signal modes (individual species are irrelevant for this discussion). Green signifies flashes, orange is glow, grey means using pheromones and weak glows, while black is only pheromones. It is obvious from the random arrangement of color in the figure that glows and flashes have multiple origins in the North American firefly fauna.
The tree on the right is the same, but this time orange branches are for both flashes and glows while black branches are for pheromones only. The asterisks denoted light signal origins (orange) or losses (black) while the letters A & B represent two possible evolutionary scenarios. In scenario A, light signaling originated once in ancestral adults, but were subsequently lost nine times. Alternatively, in scenario B, ancestral lampyrids used pheromones as the sexual signal, then independently transitioned to light signals four times, which were followed by four losses of light signaling.
Taken together with outside evidence, there is support for the hypothesis that North American fireflies invaded the continent multiple times with multiple origins (i.e. Europe or Asia). While scenario A allows for only a single origin of light production followed by 9 losses, scenario B is more parsimonious – that is, requiring fewer steps. Gaining light production appears more rare than losing that ability. So, we are left with a conundrum. It is clearly a useful strategy if it evolved repeatedly and independently in fireflies, yet these insects are clearly asking for it by announcing their presence to a very hungry world.
To reconcile this evolutionary problem, we need to know a bit more about their ecology. Using a technique called open flow respirometry, which measures carbon dioxide production, Woods and colleagues (2007) measured the energetic costs on the firefly during flashes and when at rest. More carbon dioxide was produced (meaning a greater metabolic rate) during light production than at rest, but it was a lower metabolic rate than walking while not flashing. When comparing lampyrids that are capable of light production versus those were not, there were no differences in metabolic rate meaning that bioluminescence carried no physiological costs.
But, as we might surmise from common sense, producing light must make you a beacon to predators. To test what cost there might be to switching on the light bulb to predation, Woods set up an experiment with arrays of sticky-trap cups with flashing LEDs that simulated the mating signals of Photinus greeni and sticky-trap cups without any light (see left). P. greeni is chemically defended against would-be predators and considered unpalatable. But there is one predator that remains unbothered by this defense. In fact, it is another species of firefly, Photuris versicolor, that sequesters the compound for use in its own defense. Naturally, P. versicolor hunts other fireflies by tracking their mating signals and capturing ‘grounded’ males in the midst of getting it on. Out of 218 individuals of P. versicolor trapped, only 4 were caught on non-flashing traps. Interestingly, 96% of all trapped Photuris were females.
The take home message? It doesn’t cost much to flash your stuff but you may attract the wrong company!
“Every single night, male fireflies are out there flying a fine line between sex and death. For us, it definitely rivals the most exciting television thriller! So, next time you’re outside on a summer night take a moment to admire the firefly romance and risk that’s playing out all around you.”-Sara Lewis, Professor of Biology at Tufts University (quoted from press release on physorg.com)
Branham, M., & Wenzel, J. (2003). The origin of photic behavior and the evolution of sexual communication in fireflies (Coleoptera: Lampyridae) Cladistics, 19 (1), 1-22 DOI:10.1111/j.1096-0031.2003.tb00404.x
Lloyd, J.E. (1997). Firefly mating ecology, selection and evolution In: Choe, J.C., Crespi, B.J. (Eds.), Evolution of Mating Systems in Insects and Arachnids. Cambridge University Press, London, 184-192 DOI: 10.1017/CBO9780511721946.011
Stanger-Hall, K., Lloyd, J., & Hillis, D. (2007). Phylogeny of North American fireflies (Coleoptera: Lampyridae): Implications for the evolution of light signals Molecular Phylogenetics and Evolution, 45 (1), 33-49 DOI: 10.1016/j.ympev.2007.05.013
Woods WA Jr, Hendrickson H, Mason J, & Lewis SM (2007). Energy and predation costs of firefly courtship signals. The American naturalist, 170 (5), 702-8 PMID: 17926292