Every summer, as the Earth enters a region of space containing high concentrations of solar debris, nighttime skywatchers are rewarded with a wonderful light display: the Perseid meteor shower. The annual Perseids occur when the Earth passes through a stream of dust from the Comet Swift-Tuttle, although a close to full moon may make this year’s meteor shower a little less intensive than last year’s, you are still likely to see 10–15 Perseids per hour during the peak night of Aug. 12–13.

Moon jellyfish

Moon jellyfish are limited in their ability to move. They move with the current, even while swimming. (Katie Wincek CC BY-NC-ND 2.0)

To get the best view, go to the darkest possible location and observe as much sky as possible directly above you. The rate of the meteor showers increases from around 10 p.m. through dawn, so the later you can look the better.

But you don’t have to be an astronomer to see summer light shows produced by nature. Some animals produce light from within their bodies. This phenomenon, bioluminescence, gives these creatures a visual aura.

Phosphorus was once thought to be the source of light in living creatures, but researchers now know that bioluminescence is accomplished through oxidation (adding of oxygen) to a protein in the animal known as luciferin. When oxygen, either in a gaseous form or mixed in a liquid, combines with the chemical luciferin in the presence of luciferase, a bioluminescent enzyme, a new “excited” compound is produced and gives off light. Unlike fuel combustion, there is no heat associated with luminescence.

The most familiar of the light producers is the firefly or lightning bug. During June and July, after spending most of the year underground, fireflies emerge to attract a mate. Light, produced in the firefly’s abdomen section, flashes on and off in a specific pattern or code. There are more than 2,000 species of fireflies and each species has its own light code.

In most firefly species, the males fly about while flashing their code. Females, usually on the ground, will flash the same code back. The male joins the female on the ground, where they mate. But trusting in this communication has its dangers. Some female fireflies are excellent mimics, flashing back the signal of another species. The enamored male, believing he has found his mate flies down, only to be devoured by an imposter.


The firefly is the state insect of Pennsylvania. (Fiskadoro / CC BY-NC-ND 2.0)

Our aquatic environments supports the greatest amount of bioluminescent animals. The Chesapeake Bay, especially its southern waters, supports its share of living lights. Many microscopic bacteria and plankton can produce light.

Noctiluca, about 0.0625 of an inch, belongs to a group of plankton known as dinoflagellates. Its name literally means night light. Noctiluca lights up in response to physical disturbances in the water. The light from one Noctiluca is tiny and brief. But many individuals gathered together create an eerie greenish glow in the water. Another dinoflagellate, Ceratium, has a three-pointed, anchor-shaped body that produces a twinkling light effect.

Other larger light creatures include the moon jellyfish and some species of comb jellies. The moon jelly sports four pink, horseshoe-shaped gonads atop its 10-12 inch “head.” Its tentacles are marginal and very short. The light from a moon jelly appears bluish.

Comb jellies are similar to other jellyfish, like sea nettles, in their gelatinous appearance, minus the stinging tentacles. Instead, eight rows of fringed plates, called combs, propel them through the water.

One comb jelly common in the Bay is the sea walnut, named for its oval shape. When disturbed, the sea walnut flashes a green light along its combs. Although only 4 inches, the effect created by hundreds of congregating sea walnuts can be quite startling.

In deep ocean waters, up to 80 percent of all jellyfish, squid, shrimp and fish are luminescent. In the pitch blackness of ocean depths, it seems logical that so many animals can produce their own light.

In shallower water like the Chesapeake Bay, bioluminescence is a response to an outside stimulus. Touch, wind, rain or extremely choppy water may cause a creature to light.

Researchers also believe that bioluminescence is used by some animals to startle or confuse a predator, or disguise the prey’s true size and form. Flashes and patterns of light may be a form of communication for attracting a mate, as with the fire fly, or as a warning to others of its kind.

Natural light shows are everywhere, from backyards to the Chesapeake Bay. So if you miss the Perseid shower, take a walk near a dark field and watch the firefly display. Lie on a dock after a rain and peer into the murky water. If you are lucky, the usually dark waters may provide a glimpse of fantastic creatures creating their own light.

Kathleen Gaskell is the Bay Journal's copy editor and author of Chesapeake Challenge. Contact Kathleen at kgaskell@bayjournal.com.

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