Giant Tube Worm, Riftia pachyptila

The Giant Tube Worm (Riftia pachyptila) is a species of marine invertebrate related to tube worms and commonly found in the intertidal and pelagic zones. This species lives from a mile to several miles deep underwater, on the floor of the Pacific Ocean near black smokers, where it is able to tolerate extremely high hydrogen sulfide levels.

The common name “giant tube worm” is also applied to the largest living species of shipworm, Kuphus polythalamia. Despite given the name “shipworm,” it is actually a bivalve mollusc and not an annelid.

R. pachyptila can reach a length of 7 to 8 feet. This species has a highly vascular-like red plume at the tip of its free end which is an organ for exchanging compounds with the environment. It has very few predators, as there are few creatures that live on the sea bottom at such depths. But when threatened, the worm’s plume may be retracted into the worm’s protective tube. The plume provides essential nutrients to bacteria living inside a specialized organ within the worm’s body and forms a symbiotic relationship.

This worm has no digestive tract; yet, the bacteria, which can make up nearly half of the total body weight of the worm, turn oxygen, hydrogen sulfide, carbon dioxide, etc. into organic molecules on which the worm feeds.

The bright red colors of the plume structures result from the complex array of hemoglobins found in the plume, which contain up to 144 globin chains. These hemoglobins are remarkable for carrying oxygen in the presence of sulfide, without being poisoned or inhibited by the molecule, as hemoglobins in most other species are.

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, the worm concentrates it in their blood to a level 100 times more concentrated than the surrounding water. The exact mechanism that allows the worm to withstand and concentrate such levels of nitrate is not known.

With no sunlight available at the depths this species is found, it relies on bacteria in their habitat to oxidize hydrogen sulfide, using dissolved oxygen in the water for respiration. As a large animal, this species is unique in the fact that it relies on bacteria to indirectly obtain all the materials it needs for growth. Tube worm growth resembles that of hydroponically grown fungi, more than it does that of typical animals which need to “eat”.

Reproduction occurs when females release lipid-rich eggs into the surrounding water so they start to float upwards. The males then release sperm bundles that swim to meet the eggs. Once the eggs hatch, larvae swim down to attach themselves to a rock on the sea floor.

This species has the fastest growth rate of any known marine invertebrate. It is known to colonize a new site, grow to sexual maturity and increase body size to nearly 5 feet in less than two years.

Image Caption: Tube worms at a Pacific hydrothermal vent are related to hydrocarbon seep worms. Riftia pachyptila. Credit: C. Van Dover/Wikipedia