Paralvinella sulfincola, also known as the sulfide worm, is a species of polychaete worm of the Alvinellidae family that thrives on undersea hot-water vents. It dwells within tubes in waters surrounding hydrothermal vents, in close proximity to super-heated fluids reaching over . The upper thermal limit for this polychaete is unknown; however, it is unlikely they can survive in constant temperatures over . It may tentatively be named a metazoan extremophile or, more specifically, a thermophile. Their unique abilities to withstand high temperatures close to hydrothermal fluids enables them to prey upon sulphur-oxidising bacterial mats which grow close to the metal rich vent plume. Morphology Paralvinella sulfincola colonizes smoker chimneys found near deep sea ridges in aggregations or singly upon vents. They grow up to a length of about 20-70 mm and secrete mucous in the form of a tube with two openings. This tube serves as a home, not only for the worm, but also for a whole host of microbial organisms on which the worm feeds. The worm has a crown of branchial filaments on the anterior side of its body that it uses for gas exchange. Among polychaetes, P. sulfincola’s crown has the largest surface area. The branchial crown usually peeks out when the worm is inside its tube in what is thought to be its resting state. The worm also has buccal tentacles, which it uses for feeding. Behavior While the worms spend most of their time inside of their tubes with their branchial crowns peeking out, they do engage in various behaviors such as the exploration of the environment surrounding their tube and occasional contact with surrounding worms. Exploration of the environment surrounding their tube involves the worm partially inside its tube. It utilizes its branchial filaments to brush across the surface of the chimney, and sometimes extends the upper parts of its body to sweep out across the tube entrance. Other times, the worm can be almost fully extended, with only its tail-end hidden inside the tube. This exploration behavior is mainly for the purpose of finding food. During these explorations, the worm’s buccal tentacles are extended over the surface of the chimney or the worm’s tube. The sweeping motion the worm exhibits is thought to free up particles from these surfaces. Direct contact with other worms comes in the form of either one worm brushing its branchial filaments against another worm or an