The first report of Xenillus salamoni Mahunka 1996 (Acari: Oribatida) in Poland, with the key to European Xenillus

During ecological studies carried out in the Drawa National Park, an oribatid mite species, Xenillus salamoni Mahunka 1996 (Liacaridae), new for Poland was found. This species is known only from Hungary and from the Central Alps and it is considered as xerophilous and a forest taxon. Additionally, a key to the identification of European Xenillus species was prepared.


Introduction
Despite several surveys, the Polish fauna of Oribatida is not fully known and new taxa are still being found (for example, Konecka and Olszanowski 2019). During ecological studies carried out in the Drawa National Park, an oribatid mite species new for Poland (Xenillus salamoni Mahunka, 1996), was found in a heavily transformed habitat below a former cormorant colony.

Materials and methods
The study was performed on two islands: Lech (LI) (53°04 ′ 42 ″ N, 15°57 ′ 47 ″ E) and Okrzeja (OI) (53°05 ′ 10 ″ N, 15°58 ′ 44 ″ E) on the Lake Ostrowiec in the Drawa National Park (Poland). The islands are small in surface (<5 ha), domed and primarily overgrown by acidophilus oak forests with pine, and they are 1.3 km away from each other. Since 1950, a colony of cormorants has been present on LI. Over the past 60 years, the activity of the cormorants led to the deforestation of a large part of the island and the disappearance of herbal plants. Occasionally the cormorants visited OI, but never bred there. In 2015, the cormorants abandoned the island and the slow recolonisation of plants (mainly nitrophilous species) began. Nowadays, a significant part of the island is still bare. The topsoil horizon is rich in wooden detritus and well exposed to sunlight. The soil chemistry is also a distinguishing feature of the studied island. According to Klimaszyk et al. (2015), the nitrogen and phosphorus concentrations in the soils beneath the colony LI compared to OI are several dozen and several hundred times higher, respectively.
On both islands, 20 soil samples were collected from steep slopes. After extraction with a Tullgren apparatus (which lasted five days), all specimens were preserved in 85% ethanol and cleared on slides with 80% lactic acid. Microscopic slides were prepared with Hoyer's mounting medium. Pictures were taken with a Nikon DSRi2 microscope camera and obtained with the aid of Nikon NISElements D software (Nikon Corporation) and rendered with the Helicon Focus 7 program (Kozub et al. 2008).

Results and discussion
In total, 44 specimens of X. salamoni were found, of which 40 were from LI and 4 from OI. The average frequency of X. salamoni on OI was 0.2 specimens per sample, while on LI, it was tenfold higher.
In Poland, only two species of Xenillus have been recorded so far: X. clypeator Robineau Desvoidy, 1839 and X. tegeocranus (Hermann, 1804) (Niedbała and Olszanowski 2008). Xenillus salamoni differs from these species primarily by the morphology of its prodorsum ( Figs. 1 and 2). The lamellar cusps of X. salamoni are fused at their bases, without intercuspidal mucro, which is present in many Xenillus species ( Fig. 2A). Lamellar setae are ciliate and directed anteromedially, similar to the setae of X. clypeator, while interlamellar setae are short (but longer than in X. clypeator) (Fig. 2B, C). Sensillus is clavate with a long stalk. Another characteristic feature of X. salamoni is the reduced exobothridial setae, represented only by small alveoli and lack of interlamellar tubercules (which are present in X. tegeocranus). The body length of X. salamoni is 695-932 μm (Mahunka 1996; Schatz 2004. Xenillus salamoni is considered to be a xerophilous and silvicolous species, which was first recorded in Hungary. Mahunka (1996) described this species from dry TilioSorbetum in the Bükk National Park, and later, it was also recorded in a commercial beech forest stand in the Mátra Mountains (Kreszivnik and Mahunka 2000). X. salamoni was also noted in dry grassland and in downy scree slope oaks of the Italian and Austrian parts of the Tyrol region, which are located in the Eastern Alps Fischer 2015; Schatz 2016). Subías (2004Subías ( , updated 2020 reported X. salamoni in material from Abkhazia (Caucasus), but this information was not published (L. S. Subías, personal communication). Thus, X. salamoni is only known from a few locations in Europe and the Caucasus. Islands on Lake Ostrowiec are the first location outside mountain areas (Alps, Bükk and Caucasus mountains) on which this species was found.
Likely, the range of occurrence of X. salamoni in Poland is not limited to the islands on the Lake Ostrowiec, and this species might be present in other parts of the country. Because of their narrow ecology niche, X. salamoni and their habitats could have been overlooked during previous research. A higher abundance of this mite on LI might have been caused by previous cormorant activity that led to the deforestation of the island and changed soil chemistry (Klimaszyk et al. 2015). It could create better conditions for xerophilous species like X. salamoni.
Cormorants can also be hypothesised as an expansion vector for this species. Xenillus tegeocranus was recorded in cormorant feathers by Lebedeva et al. (2004), so the presence of X. salamoni in cormorant plumage is probable.
To verify these assumptions, further surveys on dry environments and cormorants (their colonies and feathers) should be carried out.  Mahunka, 1996