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What's a pyrosome? That was my question a few weeks ago when I read that pyrosomes are food for leatherback turtles. Now, thanks to several good textbooks and readers who sent me information about these creatures, I am a pyrosome admirer.

These are the kinds of animals that make marine biology fun.

Pyrosomes are relatives of sea squirts (tunicates) that swim in the open ocean. These creatures are whitish, tube-shaped, closed on one end and open on the other. A pyrosome looks like a traveling condom.

In the past, people used other comparisons. In a British publication of the 1950s, a writer compared a pyrosome to an "old-fashioned, long, incandescent gas mantle." Another author wrote that a pyrosome is like an "elongated thimble."

But looks are deceiving. What appears to be one long creature is a colony of joined-together individuals, like reef corals except squishy. Each individual in a pyrosome colony has a mouth facing outward.

Tiny, beating threads inside the mouths draw plankton-laden water into the body. After extracting food and oxygen, each body expels its water and wastes into the hollow cylinder. This pumping system has a bonus. Because the colony can open and close its rear opening, the ejected water enables the creature to get around by jet propulsion.

Pyrosomes range in size from one inch to 30 feet long, depending on the species. The enormous ones are rare, but seeing one would be a diver's dream. The opening of the largest pyrosome is so big a person can swim inside.

As if all that isn't weird and wonderful enough, each body in a pyrosome colony bears two organs containing light-manufacturing bacteria. Thus, pyrosomes can glow brilliantly in the dark.

One voyaging naturalist in 1892 wrote that he caught a pyrosome four feet long and 10 inches in diameter: "I wrote my name with my finger on the surface of the giant Pyrosoma as it lay on the deck in a tub at night, and the name came out in a few seconds in letters of fire."

This blazing light is where the creature's scientific name comes from: Pyrosoma, the creature's first scientific name, means "fire body." Pyrosome light usually occurs in waves.

When a colony runs into trouble, the mouths at the front stop taking in water and turn on their lights. This signals surrounding individuals to do the same. As a result, the colony stops moving and a ripple of radiance sweeps down it.

Such luminous surges may frighten predators away. Or they might start a war. It has been suggested that the 1964 Gulf of Tonkin torpedo attack on an American ship, which drove the U.S. deep into the Vietnam War, were pyrosome flashes, because the creatures are common in that area.

As I read up on pyrosomes for this column, I realized that I saw one up close and didn't know it. While diving in the Galapagos Islands, I spotted a ghostly tube about two feet long. Since the creature was new to me, I didn't touch it, nor did I fetch my knowledgeable guide, who surely would have known what it was.

My excuse is that several hammerhead sharks were watching us descend, and they had my attention. By the time the sharks left and I got my wits about me, the weird creature was gone. Never again will I pass up on a pyrosome.

The message: If you love your plovers, don't use pesticides.

Thanks for taking the time to write me about these remarkable birds. The letters are a highlight of my Hawaii autumns.