Measurement is the problem. The worm occupies a tube on a chimney wall where temperature changes sharply over centimetres and fluctuates over seconds as vent flow shifts. Probes placed near tubes have registered brief readings above 80 degrees Celsius, while the plume end of the animal sits in water closer to 20 degrees, giving a gradient across a single animal unmatched among known metazoans.
What the animal tolerates is a separate question from what the water does. Pressurised laboratory work, in which worms were retrieved without decompression and held under controlled conditions, indicated an upper limit closer to 50 to 55 degrees Celsius for sustained survival, and that transient exposure to hotter water is not the same as living in it.
The bacterial fleece on its back is a genuine feature. Dense filamentous epibionts colonise the dorsal surface and are thought to be involved in nutrition and possibly in detoxification, though the exchange has been hard to quantify. Because the worm dies during ordinary recovery to the surface, much of its physiology has been inferred rather than directly measured.