But the microscopic larvae of these giants are born bacteria-free, with a complete digestive system. Juveniles swim, hunt, and eat before permanently settling down and taking up with their microbial partners. Now the idea that the larvae acquire their symbionts by eating them has been overturned. By collecting the giant worms' tiny spawn from traps laid on the ocean floor, oceanographers have shown that the sulfur-eating bacteria infect the larvae through their skin.
Andrea Nussbaumer and Monika Bright of the University of Vienna, and Charles Fisher, professor of biology at Penn State, report their findings this week in the British journal Nature.
Previous groups had shown that, after a larva quits swimming and attaches itself to the bottom of the ocean near a volcanic vent, its mouth disappears and its stomach shrinks away, even as it grows a specialized organ called the trophosome that houses the symbiotic bacteria it collects. "It is an absolutely obligate symbiosis for the worm," Fisher explains. "If the larvae do not get the right symbiont, they die."
The prevailing hypothesis was that the appropriate bacteria were gathered into the stomach during feeding, somehow escaped digestion, and by remaining in the stomach caused it to undergo metamorphosis into the trophosome.
But those conclusions were based on a very
small set of observations, due to the extreme
difficulty of obtaining the tubeworm's larval
and juvenile stages. The only way to collect
these delicate organisms is directly from the
ocean floor, at 2500 meters
Contact: Barbara K. Kennedy