A new troglobitic species of Allochthonius (subgenus Urochthonius) (Pseudoscorpiones, Pseudotyrannochthoniidae) from Japan

Allochthonius (Urochthonius) yoshizawai sp. nov., found in Hiura-do Cave, a limestone cave located in the municipality of Kumakogen, Ehime Prefecture, Japan, is described. It can be distinguished from the consubgeneric species mainly by the carapacal chaetotaxy (6–2, 18), by the presence of 6 setae on the cheliceral palm, by the rallum with 11 blades, by the presence of 8 clavate coxal blades on coxae I, and by the decreased number and distinct shape of the chelal teeth. A redescription of the subgenus Urochthonius, and keys to the subgenera of Allochthonius and to the species and subspecies of Urochthonius are also provided, as well as some ecological remarks, a brief discussion on troglomorphisms for the subgenus, and potential threats for this species.

During an expedition to caves in Japan (carried out from September 5 to 15, 2017), a single pseudoscorpion was found, belonging to a new species herein described. The single male specimen belonging to the subgenus Urochthonius was found in Hiura-do Cave, a limestone cave located in Shikoku Island. The new species is considered troglobitic, and it shows a distinct combination of morphological features. It shares some characteristics with two consubgeneric species, A. (U.) ishikawai Morikawa, 1954 and A. (U.) brevitus Hu & Zhang, 2012. It is important to point out that the subgeneric division of the genus Allochthonius, which is solely based on morphological characters (e.g. the absence or number of eyes) and typical habitat type, appears unstable and thus a taxonomic revision is imperative. However, while we recognize the need of further research on this matter, the description of a new species based on a single specimen, although oftentimes discouraged by researchers, can be of crucial importance, especially when taken into consideration species conservation. Furthermore, it is noteworthy that many troglobitic species, especially predators, can be extremely rare. Due to impacts on cave systems, a species may lose its habitat and become potentially extinct before its formal taxonomic description, as observed by Ferreira et. al. (2020). Accordingly, we hereby propose the description of a newly discovered Urochthonius species.

Study area
Fieldwork was carried out in September 2017 at Hiura-do Cave, a limestone cave located in the municipality of Kumakogen, Ehime Prefecture, Shikoku Island, Japan (Figs 1, 5A-D). A visual search was conducted, and a single male specimen was found walking on a rock wall. It was captured by using a fine brush, and subsequently transferred to a small-labeled plastic vial containing 70% ethanol for preservation.
More details on habitat are covered in a separate section (habitat and threats) later in the paper.

Preparation and analysis
In order to properly observe taxonomic characters, the specimen and its dissected appendages were mounted in temporary cavity slides, using glycerin as medium. Photographs and measurements of body parts were taken with a Zeiss Axio Zoom.V16 stereomicroscope, using the software Zen 2.3. Drawings were prepared with a draw-ing tube on a Leica DM750 microscope equipped with phase contrast. For drawings, Kaiser's glycerol gelatin was used instead of glycerin. This mounting media solidifies at cold temperatures, thus allowing the dissected body parts to be kept in a fixed position.
Description of coloration was based on photographs of the living specimen, which were taken with a Cannon SX50 camera. The terminology used in the description follows Chamberlin (1931), Harvey (1992), Judson (2007), Vachon (1941aVachon ( , 1941b and Gabbutt and Vachon (1963). Measurements follow Chamberlin (1931) and represent the average of two measurements taken on different days.

Family Pseudotyrannochthoniidae Beier, 1932
Genus Allochthonius Chamberlin, 1929 Subgenus Urochthonius Morikawa, 1954 Type species. Allochthonius (Urochthonius) ishikawai Morikawa, 1954. Diagnosis (modified from Morikawa 1960). No eyes or, more rarely, two rudimentary eyes present. Epistomal process absent. Fixed chelal finger with 7-20 acute marginal teeth, movable finger with 10-17. Cheliceral palm with 5-6 setae; fixed fin- ger of chelicera generally with one basal large tooth and a few small teeth before it, with one distal large tooth and several teeth after it, or with several small teeth on the median swelling without any large tooth. Coxal blades of coxa I with a spray of 5-11 clavate processes on a mound. Palps, chelicerae and legs long and slender. Setae of the body also long and slender. Typically cave inhabitants. Etymology. The specific name is given in honor of Dr. Kazunori Yoshizawa, not only due to the assistance provided during fieldwork in Japanese caves, but also to his great contribution to the knowledge of arthropods, especially Psocodea.

Allochthonius
Diagnosis. Differing from the other members of subgenus Urochthonius by the following combination of characters: carapace with 18 setae (6 on anterior margin, 2 on posterior margin); cheliceral palm with 6 setae, fixed cheliceral finger with large basal tooth, rallum with 11 blades (each with fine barbules, the basal-most blade shorter than the others); coxa I with a spray of 8 clavate coxal blades (subequal in length) on a mound, bisetose intercoxal tubercle present between coxae III and IV; on the fixed chelal finger, 7 (8 on the right chela) acute, narrow, large, widely-spaced teeth; on the movable chelal finger, 10 acute, small, widely-spaced teeth; chelal teeth varying in size.
Genital operculum of male (Fig. 4A, C): Anterior genital operculum with 6 anterior setae, and one posterior seta; genital opening with 8 setae on the right side, and 10 on the left.  3D). Trichobothria ib and isb located on a small dorsal hump. Trichobothrial pattern (Fig. 3B): trichobothrium sb distinctly nearer b than sb; it distad to est; trichobothria ib-isb-eb-esb-ist clustered at the base of fixed finger. On the left chela trichobothrium est is missing. Fingers distinctly curved, movable finger shorter than fixed finger. Fixed finger with 7 (8 on the right chela) acute, large, narrow, widely-spaced, irregular marginal teeth; on the left chela, the fourth distal tooth distinctly larger than the others, two small basal tubercles present; the right fixed finger marginal teeth larger compared with those of the left chela. Movable finger with 10 acute, small, widely-spaced, irregular teeth.
Leg IV (Fig. 3E): Subterminal setae long and acuminate. Arolia shorter than claws, latter slender and smooth. Two tactile setae present, one on the metatarsus and another on the tarsus.

Habitat and threats
Hiura-do Cave is a limestone cave with approximately 160 meters of horizontal extent and two entrances (Fig. 5B). The secondary entrance (Fig. 5B, SE), although wider than the main entrance (Fig. 5B, ME), is considerably low (<1 m in height), in addition to being located on a rock escarpment, which makes access to the cave interior quite difficult. From the main entrance (Fig. 5A), the conduit presents a descending slope, until reaching a vertical pit, from which is possible to access a lower level. At the deepest part of the cave there is a drainage (Fig. 5D), which springs at the end of the cave and sinks a few meters further. This drainage springs out at the external environment some dozen meters down from the main cave entrance, forming a stream. Most of the cave conduits are formed by exposed limestone, being devoid of sediments (Fig. 5C). The single specimen of A. (U.) yoshizawai sp. nov. was found freely walking on a limestone surface, on the side of the wall, actively crawling in an aphotic area located around 50 meters from the nearest entrance (Fig. 5B). The cave is highly oligotrophic, and only some scarce organic debris deriving from vegetation was observed when it was visited. Neither bat colonies nor guano deposits were observed. Nonetheless, it is important to point out that there may be seasonal variation (i.e. the influx of organic matter may be higher during certain periods of the year). Potential prey for the pseudoscorpion are mainly springtails (Entomobryomorpha and Onychiuridae), which are relatively abundant in the cave. Other troglobitic species observed in the cave during our visit included, besides the Collembola, the highly troglomorphic carabid beetle Nipponaphaenops erraticus Ueno, 1971, the staphylinid beetle Quedius sp., the Grylloblattodea Galloisiana (an undescribed species), and a Rhagidiidae mite.
The cave presents obvious signs of human visitation (there is an iron ladder installed from the upper to the lower level), but such visitors seem to be mostly speleologists, so no severe impacts were observed in the cave. The external environment is also well preserved, with a forest covering most of the landscape. Considering the well-preserved status of both the cave and the external landscape surrounding the cave, the species seems not to be seriously threatened at the moment.

Troglomorphisms and taxonomic traits
Concerning Urochthonius spp., it is difficult to state with certainty whether some characteristics represent typical troglomorphisms found in other pseudoscorpions. Two spe- cies, A. (U.) biocularis Morikawa, 1956 and A. (U.) ishikawai, are cave-dwelling, found in Japan. One of the main differences regarding external morphology between A. (U.) biocularis and A. (U.) ishikawai is that the former bears two anterior rudimentary eyes (Morikawa 1956), whereas the latter is anophthalmic (Morikawa 1954(Morikawa , 1956(Morikawa , 1960. The only epigean species within the subgenus, A. (U.) brevitus, which is recorded from China, is also characterized by the absence of eyes (Hu and Zhang 2012;Zhang and Zhang 2014). Contrastingly, the cave-dwelling representative of the nominal subgenus from Japan, A. (A.) opticus troglophilus Morikawa, 1956, bears four eyes on the carapace -the main diagnostic character for the subgenus (Morikawa 1960). It is clear, therefore, that the genus Allochthonius should undergo a major taxonomic revision, including phylogenetic analyses and thorough examination of type material, not only for providing a better understanding of troglomorphisms in the group, but also for assessing the validity of the subgenera (which could eventually be synonymized). In conclusion, the lack of phylogenetic studies precludes any tests whether the absence of eyes represents a morphological specialization to the subterranean environment within the genus.
Nonetheless, A. (U.) brevitus exhibits an array of characteristics that sets it apart from the hypogean species. Allochthonius (U.) brevitus is characterized by having generally stouter appendages. Particularly in reference to its palpal femur ratio (3.9 times longer than broad) (Hu and Zhang 2012;Zhang and Zhang 2014), it is significantly smaller when taken into account the range shown by the consubgeneric species (5.1-6.5 times longer than broad) (Morikawa 1954(Morikawa , 1956(Morikawa , 1960. Additionally, in terms of coloration, the body is mostly light yellowish (except for the carapace and tergites, which are strong yellowish brown), and the chelicerae and palps are reddish (Hu and Zhang 2012;Zhang and Zhang 2014). On the other hand, cave-dwelling Urochthonius species show a pronounced reduction in body color as can be inferred from the descriptions of A. (U.) ishikawai uenoi Morikawa, 1956, A. (U.) biocularis, and A. (U.) ishikawai ishikawai Morikawa, 1954(Morikawa 1954, 1956. Paleness and more elongate, slender appendages represent common troglomorphic traits found in pseudoscorpions (Heurtault 1994). Therefore, we argue that the aforementioned characters could be pointed out as the main morphological specializations presented by cavernicolous Urochthonius species.
Furthermore, although it may simply represent a dispersal-aiding trait, the considerably high level of chelal finger curvature could potentially represent a troglomorphism for the cave-dwelling species in the subgenus. When comparing the chelal fingers of hypogean species and the single epigean representative, A. (U.) brevitus, the former present generally curved fingers (Morikawa 1954(Morikawa , 1956(Morikawa , 1960, whereas the latter shows straight fingers, only slightly curved distally (Hu and Zhang 2012;Zhang and Zhang 2014). This higher curvature (see in Figure 2B the wide gap between the curved fingers of the chelae, especially in the right chela) may enable the troglobitic species to capture both bigger and smaller prey. When considering the usually low population densities of cave invertebrates in general, such a trait could be adaptive, allowing them to feed on a wider range of potential prey, which may be important in an oligotrophic environment.
We note some inconsistencies in the measurements and ratios in the descriptions of A. (U.) biocularis and A. (U.) ishikawai kyushuensis Morikawa, 1960, regarding the palpal femur (Morikawa 1956(Morikawa , 1960. In reference to A. (U.) biocularis, a range of 5.3-5.7 times longer than broad is mentioned, but measurements are only specified for the holotype (Morikawa 1956). Concerning A. (U.) ishikawai kyushuensis, the values for length and breadth provided for the palpal femur of the holotype are, respectively, 1.13 mm and 0.07 mm. Comparing to the dimensions given for the remaining types (female allotype, 1.29/0.20; male paratype, 1.02/0.18; female paratype, 1.35/0.22), a breadth of 0.07 mm appears to be considerably narrow. Additionally, the ratios that can be obtained by using the previous values, which make up a range of 5.7-16.1 times longer than broad, are different from those found in the description: "palpal femur 5.7-6.7 times (in male) and 6.1-7.5 times (in female)" (Morikawa 1960 Morikawa, 1954 and A. (U.) brevitus include measurement data for leg IV. Hence, we opted for not comparing dimensions extensively between Urochthonius species and subspecies.
Allochthonius (U.) yoshizawai sp. nov. is markedly pale, as evidenced by the mostly translucent cuticle of the holotype, and also presents slender appendages (e.g. palpal femur 6.5 times longer than broad). The new species presents a combination of characters based on which distinction from the consubgeneric species can be easily made. Differences related to the carapacal chaetotaxy, number of setae on the cheliceral palm, number of rallum blades, and number of palp chela marginal teeth can be indicated.
Number of chelal teeth: Allochthonius (U.) ishikawai subspecies have a range of 9-17 teeth on the fixed chelal finger, and 11-17 teeth on the movable finger (Morikawa 1954(Morikawa , 1956(Morikawa , 1960. Regarding A. (U.) brevitus, 20 teeth can be found on the fixed finger, 17 on the movable (Hu and Zhang 2012). Finally, the new species has 7 (8 on the right chela) teeth on the fixed finger, 10 on the movable. A similar number (fixed finger: 9, movable finger: 11) was identified in A. (U.) ishikawai shiragatakiensis Morikawa, 1954. Accordingly, both taxa bear less than half the number of teeth generally shown by the congeners.

On the troglobitic status of Urochthonius species
With regard to the subgenus Urochthonius, the troglobitic status of its representatives has not been considered in previous works. As outlined earlier, important morphological specializations to the subterranean environment (e.g. paleness) can be recognized in Urochthonius cavernicolous species. Hence, we argue that A. (U.) yoshizawai sp. nov. and the consubgeneric cave-dwellers are troglobitic.
Even when taking into account that A. (U.) ishikawai kyushuensis was recorded from six caves located in Kyushu and Honshu islands, inference of the troglobitic status for the subgenus as a whole is still plausible. Sendra et al. (2018) described a troglobitic species of campodeid dipluran collected from caves located in Shikoku and Kyushu islands. Pacificampa nipponica Sendra, 2018, was found in two caves, each one located in a different island. It is known that these islands, currently separate, used to be connected during the last glacial age (Sendra et al. 2018). In this regard, we can conclude that although a certain species inhabits more than one cave, even when distant from each other, it can still be assigned as troglobitic.