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Outside a zoological context, the word “arachnid” generally refers to spiders. However, the arthropod Class Arachnida includes 9 taxa, of which the Order Araneae (spiders) is only one. The other arachnid groups are the orders Scorpiones (scorpions), Uropygi (whip scorpions), Amblypygi (tail-less whip scorpions), Ricinulei (hooded tickspiders), Pseudoscorpiones (pseudoscorpions), Solifugae (wind scorpions), and Opiliones (daddy long-legs), and the Subclass Acari (mites and ticks). There are more than 70,000 species of arachnids, and most are terrestrial. The truly freshwater arachnids include a single spider and the rest are freshwater mites (Acari).

The most significant radiation of mites into fresh waters is the clade Hydrachnidia (= Hydracarina), the water mites. Although more than 6000 species have been described worldwide, this tally likely under-estimates the true global diversity. In North America, there are at least 1025 species in 131 genera (44 families) (Di Sabatino et al., 2008).

In addition to the hydrachnids, two other acarine lineages occur in aquatic habitats. The soil mites of the Suborder Oribatida are a diverse assemblage of about 10,000 described species, although the actual species richness may be 10x that. Roughly 90 spp. of oribatids are freshwater inhabitants, and the remainder -- as their common name suggests -- are terrestrial. In North America, there 31 species of aquatic soil mites representing 6 families (Schatz & Behan-Pelletier, 2008). The Family Halacaridae is best known from marine environments, with over 1000 described species. However, 56 species are known from freshwater habitats worldwide. Twelve of these occur in North America (Bartsch, 2008).

While many spiders are known from aquatic-adjacent terrestrial habitats, only Argyroneta aquatica (the diving bell spider) is truly freshwater. It does not occur in North America.

The Class Arachnida represents the arthropod Subphylum Chelicerata in fresh waters. The chelicerates typically have two body tagmata, the anterior prosoma and the posterior opisthosoma. As seen in the spiders, the prosoma bears only 6 pairs of appendages: chelicerae, pedipalps, and four pairs of walking legs. The opisthosomal appendages are either internalized as book lungs or are absent. The arrangement of appendages and body regions is sufficient to distinguish chelicerates from the crustaceans and insects.

The Acari fit the general chelicerate scheme, except for the fusion of the prosoma and opisthosoma into a single tagma with little evidence of arthropod segmentation beyond the presence of 6 paired appendages. Freshwater mites are generally minute to small (μm-3 mm), and hydrachnids are often brightly colored. Although some are adept at swimming, most mites use their legs to crawl over vegetation, etc. Most species are predators that use their chelicerae ("mandibles") to suck fluids. The sexes are generally separate, and the typical life cycle involves a larval stage that is an ectoparasite of an aquatic insect, followed by a free-living deutonymph instar before the adult. Modifications to the life cycle include parasitism of sponges and freshwater mussels.

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Bibliography

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