Belongs within: Hygrobatoidea.
The Omartacaridae are a group of interstitial water mites with a disjunct distribution in southern Asia, Australia and the Americas. Species have a soft integument, somewhat elongated idiosoma, all coxal plates grouped together with the medial edges of anterior coxal group noticeably longer than those of the posterior group, and suture lines between coxal plates III–IV extending at right angles to the midline or slightly posterolaterally (Walter et al. 2009).
Mites of southern sediments
Published 10 December 2020
Water mites of the clade Hydrachnidiae are one of the few groups of arachnids that have not only adopted an aquatic lifestyle but have thrived and diversified there. Over fifty families are currently recognised within this clade, some of which can be found in almost every body of fresh water worldwide. Others, however, are notable for their restricted ranges. One of these latter examples is the Omartacaridae.
Omartacaridae is a small family currently recognised as including only two genera, Omartacarus and Maharashtracarus. They have a somewhat elongated body with a soft integument, contrasting with the more globular form of many other water mites. They are also distinguished by the arrangement of the coxae (the basal segments of the legs on the underside of the body) which are clustered together with the medial edges of the anterior pairs much longer than those of the posterior pairs (Walter et al. 2009) so the third pair of coxae are triangular in shape. As far as is known, omartacarids are restricted to interstitial habitats or the hyporheic zone of sediment beneath and alongside stream beds. I am unaware of any direct observations of omartacarid behaviour but they are presumably predators like other water mites. Most of the (rather limited) attention that has been given to omartacarids has focused on discussions of their distribution. Species of Omartacarus are found in South and southern North America, as well as in Australia. Maharashtracarus species are known from India and Costa Rica. It has been presumed that this reflects an ancestral Gondwanan distribution, spreading into North America from South America as the continents joined.
The larval stage of omartacarids is, to date, unknown. Larvae of other water mites live as parasites of water-associated insects such as midges and omartacarid larvae are presumably also parasitic. But in what capacity? Do mature omartacarids emerge from their subterranean habitats at some particular time of year in search of a host for their eggs? Do they somehow manage to find a host while remaining safely sequestered underground? The secret remains to be uncovered.
Systematics of Omartacaridae
OmartacaridaeRFF05
|--Maharashtracarus Cook 1967 [Maharashtracarinae]S86
| |--M. neotropicus Cook 1980S86
| `--M. phreaticus Cook 1967S86
`--Omartacarus Cook 1963 [Omartacarinae]RFF05
| i. s.: O. australicus Viets 1978H98
| O. blythi Cook 1986H98
| O. bobidus Cook 1986H98
| O. ferradasae Cook 1988RFF05
| O. lomedus Cook 1986H98
| O. paraelongatus Fernández & Grosso 1991RFF05
| O. pauciporus Cook 1986H98
| O. tucumanensis Fernández 1992RFF05
|--O. (Omartacarus)S86
| |--O. (O.) elongatus Cook 1963S86
| `--O. (O.) motasi Cook 1980S86
`--O. (Omartacarellus Cook 1974)S86
`--O. (O.) brevipalpis Cook 1974S86
*Type species of generic name indicated
References
[H98] Halliday, R. B. 1998. Mites of Australia: A checklist and bibliography. CSIRO Publishing: Collingwood.
[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.
[S86] Schwoerbel, J. 1986. Acari: “Hydrachnellae”. In: Botosaneanu, L. (ed.) Stygofauna Mundi: A Faunistic, Distributional, and Ecological Synthesis of the World Fauna inhabiting Subterranean Waters (including the Marine Interstitial) pp. 652–696. E. J. Brill/Dr W. Backhuys: Leiden.
Walter, D. E., E. E. Lindquist, I. M. Smith, D. R. Cook & G. W. Krantz. 2009. Order Trombidiformes. In: Krantz, G. W., & D. E. Walter (eds) A Manual of Acarology 3rd ed. pp. 233-420. Texas Tech University Press.