Ocean Watch

Monday, January 4, 1999

Many marine
animals can make
light for protection

Two weeks ago, during the night Christmas boat parade, I noticed some sparkling lights in the water next to our boat.

"Look," I called to my friends. "It's bioluminescence."

"Bio-what?" one asked.

"Luminescence. It's what you call light given off by animals. You know -- like fireflies."

One of my friends peered at the sparkles in the water.

"So what animals are making these lights?" she asked.

I watched the greenish flashes, trying to think of an answer.

"Actually," I said after some thought, "I have no idea."

Truthfully, I did have an idea about what might be making the light. But since so many marine animals are capable of it, naming a specific one was hard.

Bioluminescence is rare among land and freshwater animals, but in the ocean, the phenomenon is as common as coral.

In shallow coastal waters, such as those off Honolulu, some species of bacteria, dinoflagellates, jellyfish, crustaceans, brittle stars and fish can be seen making their own light.

But it's in deeper water, 600 to 3,000 feet down, that marine animals truly shine. A whopping 70 to 80 percent of jellyfish, shrimp, squid and fish there are luminescent.

The way these creatures cast their light varies. Simple organisms are mostly flashers, creating chemical light in response to a mechanical disturbance, such as getting bumped by a boat.

Others, like squid and fish, have more sophisticated systems with nerves controlling their lights. Some squid even have folds of skin that act like tiny curtains. Closed, the curtains obscure the light; lifted, the light shines bright.

A few fish get continuous light from bacteria living in the animals' light organs. These fish turn the constant light on and off by means of a shutter that opens and closes.

Why do so many animals make light?

Researchers believe the most common function is to avoid predators. A hunting animal may be startled, blinded or repelled by sudden light. And in some cases, the light disguises the form of an animal by making it a confusing shape or even rendering it invisible.

But it works the other way, too. Some predators use homemade light to get close to potential meals.

The cookie-cutter shark, for instance, is a small fish with little muscle or fin power to chase down big prey. Still, this sit-and-wait predator regularly manages to take cookie-shaped bites out of tunas, dolphins and billfish.

How?

The cookie-cutter shark has bioluminescent organs over its underside, thus making it nearly invisible from below. One small patch on the shark's belly, however, has no light organs and thus remains dark. This patch, the size of tuna and billfish prey, acts as a lure. When a big, fast-moving fish comes close to check it out, the waiting shark strikes, getting a mouthful of flesh and leaving a cookie-size hole.

Glowing light organs have another important use: enabling a species to communicate with members of its own kind. The lights may be used to signal one another about food or danger, or to find members of the opposite sex.

Whatever these animals need to make known to one another, in the dark world of the deep ocean, light is one of the few means of doing it.

Given the characteristics of the lights off my boat on the night of the boat parade, I would guess we were hitting some light-making dinoflagellates. Some of these tiny planktonic organisms -- part plant, part animal -- sometimes flash pale green when disturbed.

Seeing such lights is always a treat, but that night, these creatures added a cheery note to our holiday mood.