First record of two slug mite species of the genus Riccardoella Berlese (Acari: Ereynetidae) in Serbia

SUMMARY The paper presents the first data on slug mites Riccardoella (Riccardoella) limacum (Schrank) and R. (Proriccardoella) oudemansi Thor (Acari: Ereynetidae) in Serbia. The two parasitic species were detected on seven species of snails and slugs (Gastropoda, Pulmonata) in Serbia. Riccardoella limacum , which is a specialized snail parasite, was found in Helix pomatia L. and H. lucorum L species of the family Helicidae. Riccardoella oudemansi was found exclusively on slug species: Arion lusitanicus Mabille (Arionidae), Tandonia budapestensis (Hazay) (Milacidae), Limacus flavus (L.), Limax maximus L. and L. cinereoniger Wolf (Limacidae).


INTRODUCTION
Parasitic slug mites belong to the family Ereynetidae Oudemans 1931, subfamily Pseudotydeinae Baker 1974, which mostly consists of mite species free-living in detritus, moss, underneath tree bark, and similar sites, but includes also some parasitic species.Only several species of the genera Riccardoella Berlese 1923 and Austreynetes Fain & Barker 2003 are obligate parasites in the pallial cavity of terrestrial snails and slugs (Fain, 2004).
The two best-studied species show a clear distinction regarding their hosts.Riccardoella (Riccardoella) limacum (Schrank 1776) parasitize snails, while Riccardoella (Proriccardoella) oudemansi Thor 1932 is a slug parasite (Fain & van Goethem, 1986).The other biological and ecological characteristics are very similar -both species move freely on the slimy host body surface, penetrate the pneumostome and settle inside the pallial cavity.They pierce the host's thin respiratory epithelium, feed on hemolymph, and then lay eggs (Baker, 1970).Heavy infestation weakens the host, inhibits its growth and development, ultimately leading to histopathological changes and shell softening.R. limacum has been reported to cause significant damage in edible snail farms (Flechtmann & Baggio, 1985;Graham et al., 1993;Segade et al., 2013).Besides, both of these cosmopolitan species evidently have a potential for playing a role in biological control of snail and slug pests (Cagan & Shoaib, 2003;Ueckermann & Tiedt, 2003).
No targeted research of parasitic slug mites has been organized in Serbia so far but they have been observed and collected sporadically as part of various applied malacology studies, so that a volume of data has accumulated and provided a core of the present report.

MATERIAL AND METHODS
All parasitic slug mites were collected from snails and slugs in anthropogenic habitats, i.e. the most frequent synanthropic species, from early June until early September.Mites were sampled from several species of land molluscs at different locations in Serbia, mostly within urban communities.Synanthropic snails and slugs are easy to find when their density is high, which results in wide infestation with parasitic mites in the later part of the vegetation season.The intensity of infestation within snail and slug populations was not measured.
An electric magnifying glass (10x) was used for examining the foot integument and pneumostome edges in active and resting positions of snails and slugs at two minute intervals.An electronic scale and calipers were used for measuring all snails (g body weight / shell diameter) and slugs (g body weight only).Mites were sampled from the host integument by the sticky acarological pin dipped in Hoyer's medium, and then left to rest in a fixative.For fixation and transparency, a mixture of 70% ethanol and lactic acid was used at 5:1 ratio (Evans & Browing, 1955).Permanent microscopy specimens were mounted in Hoyer's medium (Baker & Wharton, 1964).Permanent specimens were examined under a phase-contrast light microscope, using the immersion oil system (Leica DMLS).Appropriate taxonomic literature was consulted, and keys for identification of mites of the genus Riccardoella (Fain & van Goethem, 1986;Fain & Klompen, 1990;André & Fain, 2000;Fain & Barker, 2003, 2004), as well as snail and slug hosts (Wiktor, 1982(Wiktor, , 1989;;Kozłowski, 2010).
The voucher specimens mounted in Hoyer's medium were deposited at the Department of Entomology and Agricultural Zoology of the Faculty of Agriculture, University of Belgrade.

RESULTS
Two species of slug mites were identified by microscopic examination: Riccardoella (Riccardoella) limacum (Schrank 1776) (Figures 1, 2, 3) and Riccardoella  (Proriccardoella) oudemansi Thor 1932 (Figures 4,  5, 6).There is a significant taxonomic distinction between the two, primarily regarding their leg and pedipalp chaetotaxy, especially at subgenus level (Fain & van Goethem, 1986).Biological differences were also confirmed, i.e. the fact that the two mite species parasitize exclusively either snails or slugs, i.e. two ecologically and morphologically different groups of molluscs.The species R. limacum was detected on the two largest snail species, Helix pomatia L. and H. lucorum L., while inspection for the smaller Cepea spp. in the same locations led to no confirming data.R. limacum was found:

DISCUSSION
In the course of this research, we collected two most frequent species of parasitic mites from molluscs that are already known as the host species of these parasites.The revealed hosts in Serbia are consistent with an earlier conclusion (Fain & van Goethem, 1986) that R. limacum, which is a member of the more advanced subgenus Riccardoella, is a highly specific parasite of snals from several genera of the family Helicidae, while R. oudemansi belongs to the more primitive genus Proriccardoella, so that its feeding is limited to primitive molluscs, i.e. slugs of the families Arionidae, Limacidae and Milacidae.
Information about the distribution of Riccardoella mites in the region of Serbia is insufficient.Our findings are partly consistent with data reported from Slovakia (Shoaib & Cagáň, 2004), where R. oudemansi was also found on the species L. flavus, L. maximus and A. lusitanicus, but it was also detected on the species Arion ater L. and Deroceras reticulatum (O.F.Müller), which were not found in our present study.We found massive mite infestations within some populations of A. lusitanicus and in H. pomatia -growing farms because both species, the invasive slug and the reared snail, have a tendency of aggregation, which facilitates mite transfer by contact, and the resulting spread of infestation (Schüpbach & Baur, 2010).In Slovakia, the highest infestation was detected in the species L. maximus, i.e. 6-12 R. oudemansi mites per animal (Shoaib & Cagáň, 2004), even though the slug is not known as a species normally forming large aggregations, but the next most infested species was also A. lusitanicus.