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Out of the darkness


Bioluminescent bay

Late night beach visitors often encounter shorelines that glow in the dark. Often mistakenly referred to as phosphorescence, this Avatar-like glow is caused by masses of tiny dinoflagellates 'lighting up' in alarm when they are disturbed by waves or currents, thanks to a chemical reaction called bioluminescence.

Unlike phosphorescence, where light is absorbed and later re-emitted, bioluminescence is an enzyme-driven reaction that occurs in living cells. The enzyme luciferase converts a pigment called luciferin into a new compound that emits light.

Bioluminescence is surprisingly common — especially in the ocean where it is found in everything from fish to sea stars, microscopic bacteria, jellyfish and squid. And while it is predominantly a marine phenomenon, bioluminescence has evolved independently in a broad range of organisms, including some terrestrial organisms like fungi, glow worms, fireflies and cockroaches.

"It's not really known why bioluminescence is more widespread in the oceans, but it is certainly influenced by the conditions that oceans provide," explains Dr Nerida Wilson, senior research scientist and marine biodiversity specialist at the Western Australian Museum.

"Large parts of the ocean are dark or dim all the time, and organisms that could produce and manipulate light would have a significant evolutionary advantage.

When light penetrates the oceans, it is absorbed quickly — at a depth of 10 metres, 85 per cent of light has been absorbed. The shortest wavelengths of light (blue and green) penetrate deepest, which Wilson points out make those colours the most efficient light to project, and is probably why most bioluminescent organisms use this part of the spectrum.

Seeing the light

Light can be used for communication, to attract prey or potential mates or to distract or scare away predators. When pursued by predators one Caribbean shrimp Parapandalus sp spews out a bioluminescent cloud, leaving the predator to hunt around in the shining mess while it makes its escape.

The jellyfish Periphilla uses bioluminescence as a burglar alarm or decoy defence if it is blundered into by other animals in the darkness. It then releases a packet that bursts into rocket-like sparkles as it travels away from the jellyfish.

Wilson's own favourite glow in the dark animal is the clusterwink snail, Hinea brasiliana, which is found in the intertidal zone along the NSW coastline.

"It has a very fast flash, which is striking compared to the slower glow of some other organisms," she says.

"It's also neat because it sends out its glow while remaining inside its hard, protective shell," Wilson adds. "The shell itself has some very interesting properties that helps diffuse the glow, so instead of being a single bright spot, the light spreads out across the shell, making the signal even bigger."

She says that the flashing is probably a predator deterrent, because the snails flash when they are bumped into by other animals.

"They come together at low tide and gather in the cracks and crevasses under rocks. When lots flash simultaneously, the effect is amplified."

Another common use of bioluminescence is counter-illumination, where an animal in the sea lights itself to match the light coming down in the water column. This makes them virtually invisible to predators who may be looking upwards for a silhouette.

The Hawaiian bobtail squid Euprymna scolopes uses a symbiotic relationship with bioluminescent bacteria to light up its underside, so that predators looking upward can't see it silhouetted against a moonlit sky.

Many jellyfish species are bioluminescent, especially in the deep ocean, where they use it for defence. Comb jellies startle would-be predators with bright flashes of light — others species use their glowing tentacles as decoys. And some jellyfish exude glowing slime that sticks to potential predators and makes them vulnerable to predation instead.

Luminescence on land

On land, fireflies use flashes of light from their abdomens to communicate with each other, and to attract mates, while larvae glow to warn off predators. More sinisterly, North American female Photuris fireflies fake the flashes of another firefly genus (Photinus) to attract and then eat the hapless males. They do this to acquire the toxic chemical lucibufagins produced by Photinus species, which it uses to ward off other predatory species.

Some caves and secluded gullies are home to glowworm larvae, which use their lights to lure prey to a sticky snare comprising long trailing threads coated with beads of mucus. These so-called fishing lines can catch large insects such as beetles and cockroaches, as well as their preferred prey of midges and small flies.

Scientists recently discovered that some species of the bioluminescent South American cockroach genus Lucihormetica, flash the same shade of green as a highly toxic genus of click beetles. This is the first known case of bioluminescence being used for defensive mimicry.

One of the most remarkable groups of bioluminescent terrestrial organisms is fungi. Only three fungal lineages glow (Omphalotus, Armillaria and Mycenoid species), and the intensity of the light they give out varies — many Australian species are very luminous, while others from North America give off only a faint glow.

Researchers believe that the fungi get their glow in the same way as bioluminescent animals. However, they are yet to identify luciferin and luciferase in fungi, which can glow 24 hours a day, unlike animals that only produce light intermittently.

Scientists are also unclear on why fungi glow: in species where the spore-bearing part glows, some researchers think it may attract insects to help disperse the spores. But in other species, it is the fibrous, nutrient-gathering mycelium that glows. Any insects attracted to the glowing threads would be likely to do more harm than good if they ate them, and researchers have postulated that maybe they glow to attract beneficial insects.

While advances in technology in recent years have shed new light on the story of bioluminescence on land and in the sea, each new find throws up new questions.

"We suspect that many more organisms — including nudibranchs — have the capacity to glow, but catching them in the act, and discovering how and why they do it isn't easy," says Wilson.

"There are a lot of exciting discoveries still to be made."
