Giant tube worms have no mouth or gut, and are fed by bacteria inside them.
Around volcanic vents on the Pacific seafloor, red-plumed worms up to three metres long grow faster than any other known marine invertebrate. As adults they have no mouth, gut or anus. Instead they house bacteria that turn toxic hydrogen sulfide into food, with no sunlight involved at all.
In 1977 an expedition to the Galápagos Rift went looking for hot springs on the seafloor, which were assumed to be lifeless. Instead it found crabs, clams, mussels and thickets of giant tube worms, Riftia pachyptila; one site was so crowded with their red plumes that it was named the Rose Garden. Many more vent ecosystems have since been found along geologically active stretches of ocean floor.
Riftia lives in a tough tube of chitin anchored near the vents. Its plume, red with haemoglobin, takes up hydrogen sulfide, oxygen and carbon dioxide from the water; unusually, that haemoglobin can carry oxygen in the presence of sulfide without being poisoned by it. The blood delivers these chemicals to an organ called the trophosome, packed with bacteria that use energy from sulfide to build sugars from carbon dioxide, a process called chemosynthesis. The worm then digests the older bacteria. This was the first partnership of its kind described between a marine invertebrate and chemical-fuelled bacteria; Colleen Cavanaugh first recognised chemosynthesis in the trophosome.
The deal suits both sides. Free-living sulfur bacteria face a problem: sulfide comes from vent fluid, oxygen from seawater, and the two react on contact. The worm solves it by holding its plume where they mix and carrying both through its blood. Its larvae do have a gut, and pick up bacteria from the water; useful ones are kept, and the gut is then remodelled into the trophosome. Fed this way, Riftia can colonise a new vent, mature and grow to 1.5 metres in under two years.
Speed matters because vents are temporary; most last less than ten years. Once settled, a worm is tethered for life and dies if its vent goes cold, though larvae can drift 100 to 200 kilometres to find new ones. The Rose Garden had been overrun by clams and mussels by 1985 and was later buried by lava, but a new colony, 'Rosebud', was found nearby. Claims that Riftia is among Earth's longest-lived organisms are unlikely; it is some related tube worms of calmer cold seeps that are known to exceed a century.
Source: Wikipedia — Riftia · Text summarised from Wikipedia (CC BY-SA 4.0)
