But the microscopic larvae of these giantsare born bacteria-free, with a completedigestive system. Juveniles swim, hunt, andeat before permanently settling down andtaking up with their microbial partners. Nowthe idea that the larvae acquire theirsymbionts by eating them has beenoverturned. By collecting the giant worms'tiny spawn from traps laid on the oceanfloor, oceanographers have shown that thesulfur-eating bacteria infect the larvaethrough their skin.
Andrea Nussbaumer and Monika Bright of theUniversity of Vienna, and Charles Fisher,professor of biology at Penn State, reporttheir findings this week in the Britishjournal Nature.
Previous groups had shown that, after a larvaquits swimming and attaches itself to thebottom of the ocean near a volcanic vent, itsmouth disappears and its stomach shrinksaway, even as it grows a specialized organcalled the trophosome that houses thesymbiotic bacteria it collects. "It is anabsolutely obligate symbiosis for the worm,"Fisher explains. "If the larvae do not getthe right symbiont, they die."
The prevailing hypothesis was that theappropriate bacteria were gathered into thestomach during feeding, somehow escapeddigestion, and by remaining in the stomachcaused it to undergo metamorphosis into thetrophosome.
But those conclusions were based on a verysmall set of observations, due to the extremedifficulty of obtaining the tubeworm's larvaland juvenile stages. The only way to collectthese delicate organisms is directly from theocean floor, at 2500 meters depth, in thedeep sea vehicle Alvin. Bright invented"tubeworm artificial settlement cubes," or"baby traps" as the team calls t