Tuesday, October 14, 2008


The phylum Annelida, the segmented worms, gets more interesting all the time. In addition to earthworms, leeches, and the marine polychaetes, there have been some recent additions. Creatures such as Riftia, the large, red-tentacled worms that live at hydrothermal vents, once considered to belong in a separate phylum, are now included within the annelids. Morphological and molecular evidence unambiguously supports uniting these vestimentiferans, along with the pogonophorans into a single taxon, the Siboglinidae within the annelids. Most analyses interpret the siboglinids as nesting within the Class Polychaeta, the largest annelidan class.
Even more interesting than their evolutionary relationships is the unusual approach to nutrition employed by some members of this group of worms. Although Riftia is a very large worm, significantly taller than I am, it doesn't eat or have a gut; instead it relies on symbiotic bacteria to fix carbon and generate food from the oxidation of inorganic molecules. The details of the biochemical pathways used in this process are being revealed in spite of the fact that these endosymbiotic bacteria cannot be grown in culture in the lab.

1 comment:

Mike said...

I'm curious how the bacterial symbiants ended up inside of these deep-sea worms in the first place. Maybe the bacteria were there first, were ingested by a hungry annelid, and then... protected themselves from digestion by secreting their sugars so that the digestive system need not have searched any further? Or maybe the bacteria which did so were the only ones that survived a worm invasion. Or perhaps the only worms down there that survived life in the deep were ones with weaker digestive systems that could not kill off the bacteria. In order to answer the question I wrote this Haiku:
Gutless, huddled together
In the deepest dark
My questions are all answered. Thank you. -Mike