The first record of Acherontacarus (Acari, Hydrachnidia) in continental France, with a key to the described species of the genus

A new species of Acherontacarus (Acari, Hydrachnidia) is described from a well in the south of France. The description is made with the help of images taken with the Environmental Scanning Electronic Microscope. A key to all described species is included. The chaetotaxy of terminal segments of IV‐Leg in male, general morphology of genital field and related platelets of male and female are diagnostic in this species. This is the first time that the genus is found in continental France, and the distributional data of known species is discussed.


Introduction
The superfamily Hydrovolzioidea has been recently revised by Tuzovskij et al. (2001) and the subfamily Acherontacarinae Cook, 1967 raised by them to family rank. In total there are nine species of the genus Acherontacarus Viets, 1932 known from interstitial and superficial waters of southern Europe and northern Africa that are typically sexually dimorphic.
A new species has been found in a well from southern France. This is the first time that an Acherontacarus species has been found in continental France.

Material and methods
The new species was collected in a well, using a Cvetkov phreatobiological net, in the Perpignan region, south-western France ( Figure 10). All previous findings of Acherontacarus have been in the interstitial environment or near to it. This region is characterized by a Mediterranean climate with dry periods and periods with strong rainfall. The mean annual precipitation is between 550 and 2200 mm per year.
For the description of the species we have used the Environmental Scanning Electron Microscope (ESEM) (Valdecasas and Camacho 2005) that allows the acquisition of highresolution images (up to 359863301 pixels) without the need for coating the specimens, allowing them to be further studied by transmission light microscopy or molecular analysis, if necessary. However, in some cases the details in the high resolution images are lost when sizing for journal publication. Only those images that retain enough detail for taxonomic purposes at journal image resolution (around 6006553 pixels) are presented, in all other cases images have been substituted by drawings delineated with the help of ESEM images.
We follow Cook's (1974) terminology for the description of the morphology. It is simple and intuitive, and is more accessible to the interested reader who it is not a specialist. More precise terminology such as that of Tuzovskij et al. (2001) is best left for comparative and phylogenetic studies. All measurements are in micrometres and the abbreviation MNCN is the Museo Nacional de Ciencias Naturales, Madrid.

Female (Figures 1-3)
Typical of genus, morphologically similar to male but differing in structure of genital field and IV-Leg structure. Glands of glandularia and eye capsules absent. Body length 1352, width 843. Dorsum with large posterior plate 965 long, 525 wide, surrounded by 10 pairs of small platelets, five pairs with setae, alternating, beginning with the anterior platelet and a larger anterior plate 180 long, 380 wide; posterior margin of anterior plate concave and paired with a slight convexity of the anterior margin of the posterior plate; four pairs of setae located on anterior plate and four pairs of setae on posterior plate.
Venter with fused coxal plates of I-Leg and II-Leg separate medially; two pairs of similarly sized genital platelets flank the genital pore between two larger trapezoidal anteromedial platelets, a small pair of anterolateral platelets and coxal plates of III-Leg; a large posteromedial excretory plate 375 long, 265 wide, is flanked by a pair of large triangular posterolateral platelets between the coxal plates of IV-Legs behind the genital field. Length genital field 205 long, 115 wide (across genital platelets). The anteromedial platelets are considerably smaller than the posterolateral platelets. Capitulum 335 long with a pair of fine setae located at the distal tip. Palps typical, dorsal lengths: P-I 25, P-II 225 (two setae), P-III 112 (one seta), P-IV 125 (one ventral seta), P-V 58. Legs without (five long setae in ventral side), IV-Leg-5 250 (six short ventral), IV-Leg-6 130 (11 ventral setae, the first six decreasing in thickness plus 10 dorsal setae, plus distal setae).

Nymph (Figure 9)
Similar to the adult, but the dorsal and ventral plates smaller and separated more widely. Body length 800, width 600.

Etymology
The species is named to honour Dr Nicole Coineau, for her dedication to the study of interstitial crustacean species, especially of microcrustaceans, all her life and even after her retirement, in the Laboratoire Arago, Banyuls-sur-Mer, France.

Discussion
Acherontacarus nicoleana seems closely related to those Acherontacarus species with stout setae on IV-L-6 segment of male: A. dividuus, A. vietsi, A. bicornis, and A. tuberculatus. Acherontacarus vietsi is easily distinguished because it has only two thick setae on IV-L-6 (''2 dents chitineuses''; Angelier 1954) and A. bicornis has the thick setae on the expanded distal half of IV-L-6. Acherontacarus nicoleiana has six to seven stout setae and IV-L-6 is not distally expanded. IV-L-5 is approximately the same size as IV-L-6 in A. dividuus but longer in A. nicoleiana. IV-L-6 of A. tuberculatus has more a ventral line of fine setae and more thickened setae than A. nicoleiana.

On the habitat
So far, this new species of Hydrachnidia is known only from the type locality, though many wells were prospected in the same region. The site is located within the hydrological basin of the Têt River (Figure 10) in the Roussillon Quaternary alluvial flood plain. The area is largely used for agriculture, mainly of vines and orchards. The well is dug in alluvial unconsolidated sediments, in the border of an orchard, a few metres away from the bed of The mite specimens collected represent 19% of all aquatic organisms collected in the well in 2002, the other 81% being mainly Oligochaeta and Amphipoda (mainly  Table I. The two sampling operations gave rather similar results for the overall fauna of the well, except for two crustaceans, an isopod Microcharon sp. (four specimens) of the family Microparasellidae and the amphipod Salentinella petiti Coineau, 1968 (three specimens) which were collected during the second operation only. Several terrestrial taxa were also accidentally present in the well water (pseudoscorpionids, opilionids, Homoptera and Diptera larvae). Niphargus amphipods appeared to be more numerous (83) the second time.
To this distribution we should add the new species and another record (Valdecasas 1981) of an Acherontacarus nymph (probably A. bicornis Cook, 1974) in central Spain, the most inland record of an Acherontacarus, a taxon that seems primarily to have close littoral affinities. Except for the species found in the Canary Islands, A. cedro, which was collected in the surface waters of a stream, all the other records came from typical subterranean (s.l.) environments: interstitial water or wells. It is known that many ''interstitial'' water mites appear with regularity in samples of surface waters of streams (Valdecasas 1984) and it is tempting to hypothesize that A. cedro (represented only by a female and a nymph in Lundblad's records and not found again) could be more abundant in the interstitial waters of the stream where it was found. In this sense, a study by Santucci (1975) in Corsica shows that extensive study of interstitial waters may result in numerous records and in this case, two different Acherontacarus species: A. vietsi and A. rutilans. At the same time, he pointed out the simultaneous occurrence of A. vietsi in superficial and interstitial waters.
The predominant marine littoral distribution of Acherontacarus species makes it tempting to hypothesize that this genus is a Tethyan relict and that the family Acherontacaridae is of   thalassoid origin. The occurrence of A. cedro in the Canary Islands, now considered as fully oceanic islands (permanently without any land connection with the continent), points strongly toward this origin. However, it is now known that the larvae of Acherontacarus are phoretic on species of the beetle genus Deronectes. At least three species previously assigned to Deronectes are found in the Canary Islands: Potamonectes cerisyi (Aubé, 1836), Potamonectes clarkii (Wollaston, 1862), and Nebrioporus canariensis (Bedel, 1881) (Machado and Oromi 2000). The last species is found in Gomera, from where A. cedro was described. An ancestral oceanic invasion from northern Africa cannot be excluded. It is clear that an analysis of phylogenetic relationships is urgently needed in this intriguing subterranean taxon.
Key to the species of Acherontacarus This is a preliminary key for the males and females of all described species of Acherontacarus, based on published descriptions.