Belongs within: Arrenuridae.
South American water mites
Published 4 February 2025
Though most people might not think of arachnids as aquatic animals, water mites are one of the most diverse and taxonomically best-studied of all mite lineages. These animals begin their lives as external parasites on other invertebrates (commonly insects) before metamorphosing into aquatic predators. Among the more diverse genera of water mites is Arrenurus, including about a thousand species worldwide divided between several subgenera. One of the lesser known of these subgenera is the subject of this post, Dadayella.
Dadayella includes about a dozen known species to date, all of which are found in the Neotropical realm. Originally described on the basis of females only in 1907, it was treated as a distinct genus until males were definitively recognised by Cramer & Cook (1992). The features that had previously distinguished Dadayella from the remainder of Arrenurus are less marked in males than females, leading to the group’s demotion. Palps of Dadayella have a fairly simple arrangement of setae and the terminal segment of the palp is typically rotated relative to the proceeding segment. The hind coxae are narrowed medially, so the inner margin of the fourth coxae is reduced to a point. Females have a distinct pair of glandular openings directly behind the acetabular plates (part of the genital field); in males, these openings may be shifted laterally or absent. Males of many Arrenurus species have the posterior part of the body modified into remarkable projections but in Dadayella males such protrusions are little developed.
I have not found any direct commentary on whether these morphological distinctions are related to differences in lifestyle from other Arrenurus but we might make some conjectures. Species of Dadayella seem to be recorded from fairly still waters such as ponds and marshes. The hind legs bear long setae that mark the mites as reasonably strong swimmers. The rotated terminal segment of the palp may close against the proceeding segment like a pincer, and might perhaps be used in grasping some object such as aquatic vegetation, prey or other mites. The elaborate projections of the male body in other species function in spermatophore transfer; their absence in Dadayella suggests that the mating behaviour of this subgenus might be distinctive. Perhaps Dadayella males prefer to hold their mates closer than their congeners.
Systematics of Arrenurus (Dadayella)
<==Arrenurus (Dadayella Koenike 1907)RFF05
|--A. (D.) clavipes (Lundblad 1938)RFF05
|--A. (D.) guarani Rosso de Ferradás 2005 [=A. (D.) boettgeri Viets & Böttger 1986 non Viets 1968]RFF05
|--A. (D.) guttifera (Viets 1954)RFF05
|--A. (D.) hirtipalpis (Besseling 1949)RFF05
|--A. (D.) minima (Daday 1905)RFF05
|--A. (D.) projecta (Lundblad 1938)RFF05
|--A. (D.) rotunda (Daday 1905)RFF05
| |--A. r. rotundaRFF05
| `--A. r. subrotundata (Viets 1954)RFF05
`--A. (D.) walteri (Lundblad 1953)RFF05
*Type species of generic name indicated
References
Cramer, C., & D. R. Cook. 1992. New species of Arrenurus (Dadayella) (Acari: Arrenuridae) from Mexico, with a discussion of the latter’s relationships. International Journal of Acarology 18 (3): 221–229.
[RFF05] Rosso de Ferradás, B., & H. R. Fernández. 2005. Elenco y biogeografía de los ácaros acuáticos (Acari, Parasitengona, Hydrachnidia) de Sudamérica. Graellsia 61 (2): 181–224.