Bioluminescent fireflies and fungi

flashing signals, murderous intent, and foxfire

Bioluminescence is the production of light by living organisms. This light is the result of a chemical interaction between certain kinds of molecules (called luciferins in most cases) that react with oxygen when they are activated by suitable enzymes (called luciferases) and sometimes additional factors. The root of these words is ‘lucifer’, from the Latin word meaning ‘bearer of light’, referring to the morning star. (I note, parenthetically, that the term has been applied to matches as well as the devil before his fall from grace.)

Bioluminescence has evolved many times, in many organisms, from bacteria to insects and fish and even plants.

Some time ago I began to think that bioluminescence would be an interesting topic for an essay. But I rapidly discovered that there is so much research on bioluminescence, and so many organisms can produce light, that I would have to start small, with a tiny subset of the available information, in order to portray a few of the fascinating behavioral and ecological stories that research has revealed. So I will start with a terrestrial story, but I hold open the option to write more, particularly about bioluminescence in marine organisms, at a later time.

Possibly the most well-known examples of bioluminescent organisms are the fireflies, also known as lightning bugs. They are actually beetles. The larvae generally produce light, and it is thought that the sudden flash of light probably deters predators. Although adults of the day-flying species do not produce light, the night-flying species produce light chemically, at the ends of their abdomens. Sadly, we don’t seem to have any fireflies here, but during my long years in the Midwest, their dancing lights were a summertime evening treat. A friend from the Southwest once came to visit for the sole purpose of seeing fireflies!

There are many kinds of fireflies, and males of the night-flying species typically fly around during the mating season, flashing their lights. Each species flashes in a particular pattern, in sequences of short and long pulses of light, which allows females to recognize the right kind of male. If she likes what she sees, she flashes back after a specific, short lapse of time; the length of that delay allows the male to tell that this is the right kind of female. That initial attraction is followed by a prolonged close-up courtship.

However, at least in some species, finding the right species is only the beginning. Female fireflies can mate more than once, collecting sperm from several males, but they can be very choosy. Not all males are good choices! Males package up their sperm along with some nutritional material (called a nuptial gift), and the package is passed on to the female when male and female mate. Females thus obtain extra nutrients for their eggs, and bigger gifts mean more nutrition for the eggs. Females prefer a male with a large nuptial gift, which is indicated by the male’s individual flash pattern. Even when a female has mated several times, the male that offers the largest nuptial gift is likely to be the father of most of the fertilized eggs. Of course, there is more to be learned. For instance, how do females discriminate among the sperm and nuptial gifts of their several mates after mating has occurred?

Although adult fireflies of some species don’t eat anything during their short life, others feed on nectar and pollen, and still others are predators, often feeding on other insects. The now-classic story is unusually interesting. Females of Photuris fireflies like to eat males of Photinus fireflies. They lure in those victims by flashing with the time delay that is characteristic of the males’ species. So in comes the unsuspecting, eager Photinus male, only to be grabbed and eaten by the ‘femme fatale’.

Why would a deceptive, voracious female want to eat males of another species? She obtains important protective chemicals, called lucibufagins, from them! Those chemical compounds are apparently quite nasty and even toxic to many would-be predators, including birds, lizards, and spiders. She does not produce lucibufagins for herself (they are expensive to make) but gets them from selected prey. Males of her own species do not contain much of this protective chemical, but Photinus males do. Careful experiments clearly showed that female Photuris that had not eaten male Photinus usually became spider victims, but those that had feasted on such males usually escaped. And high doses of lucibufagins worked better: the more males were eaten by a female Photuris, the more likely she was to escape the spiders’ attentions. Therefore, females are better protected late in the season, after they have had a chance to feast on a number of hapless males.

Furthermore, females protected by acquired lucibufagins put some of those protective compounds into their eggs. And those eggs were, in turn, protected from predation by ants and predatory ladybird beetles.

What a great story!

In contrast to the detailed story of firefly bioluminescence, there seems to be little known about the ecology of bioluminescence in fungi. Many wood-rotting fungi, including some that live in our area, can produce glowing light, which is sometimes called foxfire. Two suggested functions are deterrence of grazing animals that might damage the fungus and attraction of animals that might touch or eat the reproductive part and disperse the spores. I have not found reference to experiments that might support or reject these conjectures. Research needed!

 

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