Aposematism in water mites (Acari: Hydracarina): A predator defense mechanism, a phylogenetic hold-over and protection from damaging light

In temperate freshwater systems, dull and inconspicuous colouration of invertebrates is commonplace. An exception is water mites which are often characterized by bright colours such as reds and oranges. Their conspicuousness is often paired with distastefulness to predators. The current explanation for aposematism in water mites is that it was selected for by fish predation. If this is the case, water mite assemblages should be more conspicuously coloured in fish-present bodies of water as compared to assemblages in fish-absent bodies of water. I censused (N = 46) bodies of water in Southern Ontario and found the opposite trend; fish-absent bodies of water had higher proportions of conspicuously coloured water mites than did fish-present bodies of water.;My alternative hypothesis for the evolution of bright colours and distastefulness in water mites investigates water mites and their terrestrial relatives. Carotenoids are present in red water mites and in their red terrestrial relatives where their respective habitats subject mites to high amounts of potentially damaging light. I suggest that terrestrial mites incorporated carotenoids into their diets and to offset the increased risk of predation, evolved distastefulness. When the ancestral water mite invaded the new aquatic habitat, it retained both characters. A combination of water mite life history traits and the physical nature of their environment determines the degree of exposure to light radiation and can explain my observations of water mite colouration patterns.