Zits in tubeworms: part of growing up.

Young tubeworms in the deep ocean break out with skin infections as a rite of passage to adulthood, according to a new notion of their growth.

As the youngsters settle down in their permanent homes, they lose their mouths and digestive systems. To survive, each young tubeworm must acquire a new energy source, a live-in colony of bacteria that capture energy from sulfur-spewing vents and other deep-sea chemical bonanzas.

That bacterial colony probably starts as an outbreak on a tubeworm's skin, Monika Bright of the University of Vienna and her colleagues contend in the May 18 Nature. This idea overturns an older one that tubeworms pick up their new bacterial friends by eating them.

The tubeworms begin their lives as tiny, swimming larvae. They waft through deep ocean waters until they find a suitable surface, such as a hydrothermal-vent chimney or a cold seep, where inner-Earth compounds leak out.

Studying animals that live in ocean abysses has been difficult, but Bright designed traps for the young tubeworms. She and her colleagues left the traps out for a year 2,500 meters deep near the East Pacific Rise. When the researchers retrieved them, they found tubeworms of a variety of ages. The team pieced together a series of individuals representing the early stages of tubeworm development.

The bacterial infections didn't show up in the worms' guts as predicted but were instead in the outer layers of a young animal's body. The bacteria probably migrate through the outer parts of the body to reach a layer that transforms into their new home, an organ called a trophosome.--S.M.