Cyanide toxicity in xerophilic tardigrade Macrobiotus naginae in comparison with another eutardigrade species

This study investigates potassium cyanide (KCN) resistance in the xerophilic eutardigrade Macrobiotus naginae and compares it with limno-terrestrial Paramacrobiotus experimentalis. Tardigrades were exposed to 100, 200, and 300 mM KCN for 10 min, and their mean recovery times were recorded. Both species showed no significant differences in mean recovery time after exposure to 100 and 200 mM KCN. However, whilst Pam. experimentalis survived all concentrations, all specimens of Mac. naginae died after treatment with 300 mM KCN. Furthermore, ultrastructural changes in tardigrade cells were analysed using transmission electron microscopy (TEM) and compared between species. A complete set of analyses, both directly and 24 h following treatment for each concentration, was conducted for Mac. naginae. For Pam. experimentalis, already completed analyses were taken from available literature, whilst additional analyses for previously untested conditions were conducted to enable comparison between both species. Ultrastructural analysis of midgut digestive cells and storage cells, conducted immediately after KCN treatment and 24 h afterwards, showed that Mac. naginae suffered more rapid and severe changes than Pam. experimentalis. These results suggest that KCN resistance, whilst probably widely distributed amongst eutardigrades, varies significantly between tardigrade species, potentially correlating with habitat-specific adaptations, feeding behaviour, and/or anhydrobiotic capacity.