Alvinella pompejana, the Pompeii worm, is a species of deep-sea polychaete worm (commonly referred to as "bristle worms"). It is an extremophile found only at hydrothermal vents in the Pacific Ocean, discovered in the early 1980s off the Galápagos Islands by French marine biologists.
Introduction
In 1980 Daniel Desbruyères and Lucien Laubier, just few years after the discovery of the first hydrothermal vent system, identified one of the most heat-tolerant animals on Earth — Alvinella pompejana, the Pompeii worm. It was described as a deep-sea polychaete that resides in tubes near hydrothermal vents, along the seafloor. In 1997, marine biologist Craig Cary and colleagues found the same worms in a new section of Pacific Ocean, near Costa Rica, also attached to hydrothermal vents. The new discovery and subsequent work led to important progress in the scientific knowledge of these special worms.
They can reach up to in length and are pale gray, with red tentacle-like gills on their heads. Perhaps most fascinating, their tail ends are often resting in temperatures as high as , while their feather-like heads stick out of the tubes into water that is much cooler, . Scientists are attempting to understand how Pompeii worms can withstand such extreme temperatures by studying the bacteria that form a "fleece-like" covering on their backs. The bacteria have also been discovered to be chemolithotrophic, contributing to the ecology of the vent community. Recent research suggests the bacteria might play an important role in the feeding of the worms.
Attaching themselves to black smokers, the worms have been found to thrive at sustained temperatures of from and even for a short time, making the Pompeii worm the most heat-tolerant complex animal known to science after the tardigrades (or water bears), which are able to survive temperatures over 150 °C.
Biology
Reaching a length of up to , Pompeii worms have "hairy" backs; these "hairs" are actually colonies of bacteria such as Nautilia profundicola, which are thought to afford the worm some degree of insulation. Glands on the worm's back secrete a mucus on which the bacteria feed, a form of symbiosis. The Pompeii worms form large, aggregate colonies enclosed in long tubes.
Alvinella pompejana has relatively simple organ systems centering around its rod-like heart. Its outermost organ is the gills along its feather-shaped head, four external gills present as leaf-like structures with a red colour due to their haemoglobin. The heart provides blood to these organs using contractions, pushing blood along the dorsal and ventral vessels. Beneath the heart lies the animal's stomach which connects to an oesophagus that is used to consume food. Finally, surrounding the organs is a coelom filled with coelomocytes, a type of phagocyte that acts as an immune system for the animal.
The Pompeii Worm feeds upon chemosynthetic bacteria, using retractable, ciliated feeding tentacles to scoop them up or absorbing their organic matter within the hydrothermal vent fluid. These tentacles are coated with 4 types of secretory cells along the entire epidermis. The tentacle’s muscle cells seem to be filled with homogenous blood cells and individual hat-shaped cells, the hat-shaped cells have a condensed nucleus and it is hypothesised that these are heavily modified sperm cells.
The gills of A. pompejana are pinnate with many thin outgrowths. This organ is ultrastructurally similar to the gills of terebellidae and the epidermis is irregularly folded inwards. This gives the blood access to a space very close to the skin of A. pompejana, thus allowing more effective oxygen diffusion. Secretory cells of the goblet type have also been observed, along with hairlike receptor cells known as bipolar ciliary receptor cells. It is important to note that the Pompeii worm’s epibiotic bacteria are absent from both the gills and tentacles.
Physiology
Alvinella pompejana’s blood is abnormally cool at 20-30 °C. This is due to their blood's high positive cooperativity at these temperatures, thus haemoglobin is more likely to gain oxygen, an essential feature in an environment with sparse oxygen levels. Additionally, A. pompejana's gills have the highest specific surface area of any polychaete and small diffusion distances between the circulatory system and external seawater further assist in oxygenation.