Ocean Watch

A reader sent me an e-mail titled "Thing we found in our tank" and described a "pink worm-like creature with fuzzy hairs." The writer had just added some crabs to the aquarium, along with some new live rocks.

The animal, I wrote back, is likely a free-swimming bristleworm. These marine worms commonly hide inside the cracks and holes of rocks. And introducing a bristleworm to an aquarium is usually introducing trouble.

Some bristleworms, such as the graceful feather dusters, build tube-homes and stay in one place, sifting the water for passing plankton. The bristles on these worms' feet hold them securely inside their tubes.

Others, like my reader's worm, are mobile, and many of those carry poison in their foot bristles. These walkers are called fireworms.

Handling a fireworm is a memorable experience. The bristles break off in human skin, ooze their toxin and cause a nasty rash. There's no effective treatment for these stings, but they aren't medically dangerous and eventually go away on their own.

Carrying poison in their feet is a successful defense for fireworms. Fish don't eat these soft-bodied worms, and humans drop them in a hurry.

Hawaii's fireworms are orange, red, pink, green and often iridescent. But even though fireworms can be handsome, you don't want to expose your invertebrate pets to these creatures. Fireworms are carnivores that eat corals, snails, crabs, starfish or other worms, some of which are benign and quite beautiful.

"How do I get rid of it?" the reader wrote back of her stowaway.

People have designed traps that effectively catch fireworms in aquariums.

And fireworms have built-in traps that effectively catch prey wherever they find it. The front end of the worm looks rounded and harmless, like an earthworm. But when a fireworm sees something to eat, it rapidly everts its throat, which bears two sharp jaws inside. These jaws seize the prey, and then the worm retracts its throat, pulling the food inside.

In Australia, fireworms eat crown-of-thorns starfish, entering through wounds made by harlequin shrimp. It takes about a week for fireworms to kill a starfish.

Such a slow death is awful to imagine, but this is simply the fastest the worms can eat.

Such starfish eating is a good example of the marine food chain in action: Crown-of-thorns starfish eat coral, and harlequin shrimp eat starfish. Sometimes, however, a starfish escapes the shrimp. But if fireworms find shrimp wounds on a starfish, then they eat the starfish. The shrimp and worms, therefore, help keep the starfish population in check, which keeps the coral reefs thriving.

Of her fireworm, my reader wrote, "I don't want anything in my tank that is violent." One animal eating another, however, is not violence. It's the basis of all life on this planet.

Fireworms play an important role in reef ecology, but that doesn't mean you want them performing their act in your aquarium. If you get one, buy the trap.