They have a highly vascularized, red "plume" at the tip of their free end which is an organ for exchanging compounds with the environment (e.g., H2S, CO2, O2, etc). The tube worm does not have many predators, if any, for few creatures live on the sea bottom at such depths, but if threatened, the plume may be retracted into the worm's protective tube. The plume provides essential nutrients to bacteria living inside a specialized organ within their body (i.e., trophosome) as part of a symbiotic relationship. They are remarkable in that they have no digestive tract, but the bacteria (which may make up half of a worm's body weight) turn oxygen, hydrogen sulfide, carbon dioxide, etc. into organic molecules on which their host worms feed. This process, known as chemosynthesis, was first recognized by Colleen Cavanaugh while she was a graduate student at Harvard.[2] This evolutionary adaptation presumably came about from living so far away from sunlight. With sun not available as a form of energy, the tubeworms rely on bacteria in their habitat to obtain nutrients.
The bright red color of the plume structures results from several extraordinarily complex hemoglobins found in them, which contain 24 or 144 globin chains (presumably each including associated heme structures). These tube worm hemoglobins are remarkable for being able to carry oxygen in the presence of sulfide, and indeed do also carry sulfide, without being completely "poisoned" or inhibited by this molecule, as hemoglobins in most other species are.
To reproduce, the Riftia Pachyptila females release lipid-rich eggs into the surrounding water so they start to float upwards. The males then unleash sperm bundles that swim to meet the eggs. After the eggs are fertililized, the larvae swim down to attach themselves to the rock.
^ Ruppert, E.; Fox, R.; Barnes, R. (2007). Invertebrate Zoology: A functional Evolutionary Approach, 7th edition, Belmont: Thomson Learning. ISBN 0030259827.
^ Cavanaugh, Colleen M.; et al. (1981). "Prokaryotic Cells in the Hydrothermal Vent Tube Worm Riftia pachyptila Jones: Possible Chemoautotrophic Symbionts". Science213 (4505): 340–342. doi:10.1126/science.213.4505.340. PMID 17819907.
Further reading
Minic, Zoran; Hervé, Guy (2004). "Biochemical and enzymological aspects of the symbiosis between the deep-sea tubeworm Riftia pachyptila and its bacterial endosymbiont". European Journal of Biochemistry271 (15): 3093–3102. doi:10.1111/j.1432-1033.2004.04248.x.
