Body Structure
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, as few creatures live on the sea bottom at such depths. 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 its 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.
The bright red color of the plume structures results from several extraordinarily complex hemoglobins found in them, which contain up to 144 globin chains (presumably each including associated heme structures). These tube worm hemoglobins are remarkable for carrying oxygen in the presence of sulfide, without being completely "poisoned" or inhibited by this molecule, as hemoglobins in most other species are.
Nitrate and nitrite are toxic but nitrogen is required for biosynthetic processes. The chemosynthetic bacteria within the trophosome are able to convert this nitrate to ammonium ions, which then are available for production of amino acids in the bacteria, which are in turn released to the tube worm. In order to transport nitrate to the bacteria, R. pachyptila are able to concentrate nitrate in their blood to a concentration that is 100 times more concentrated than the surrounding water. The exact mechanism of R. pachyptila’s ability to withstand and concentrate nitrate is still unknown.
Read more about this topic: Giant Tube Worm
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