Meiofaunal Richness in Highly Acidic Hot Springs in Unzen-Amakusa National Park, Japan, Including the First Rediscovery Attempt for Mesotardigrada.

Extreme environmentssometimes support surprisingly high meiofaunal diversity. We sampled runoff from the acidic hot springsof Unzen, Japan. This is the type localityof Thermozodium esakii Rahm, 1937, the only tardigradein the class Mesotardigrada, which remains contentious in the absence of corroboration or supporting specimens. Our sampling revealed at least three species of arthropods, four rotifers, and five nematodes living in the hot (ca. 40°C) and acidic (ca. pH 2.5) water, but no tardigrades.