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.