A new species of Stenasellus Dollfus, 1897 from Iran, with a key to the western Asian species (Crustacea, Isopoda, Stenasellidae)

Abstract A new stenasellid isopod is described from Tashan Cave, Khuzestan Province, south-west Iran, belonging to the genus Stenasellus Dollfus, 1897. The first recorded species of Stenasellidae from Iran, Stenasellus tashanicus sp. n., is diagnosed by the presence of antennae with a minute squama bearing paired, long, robust setae; a maxilliped endite with six coupling hooks; and slender appendix masculina with an acute apex. A revised generic diagnosis is provided with a key to the six known western Asian Stenasellus species.

As species of Stenasellus were reported from southeastern Turkmenistan to the south-eastern corner of the Arabian Peninsula and east Africa (Somalia, Kenya and Oman), it was also expected to document their presence in the intervening geographical regions, such as Iran and Yemen. As reported here, our finding of the new stygobiont isopod species from the Iranian underground environment narrows the gap between these broad geographical areas.

Materials and methods
Specimens for this study were collected from Tashan Cave, located inside a hill close to Sarjooshar Village, Tashan City, Behbahan County, Khuzestan Province, south-west Iran (Fig. 1A, B). The cave was visited seven times, but the isopods were only collected on the 13 and 27 August 2016. The specimens were preserved in 96% ethanol and deposited in the Zoological Museum, Shahrekord University, Iran. Appendages were drawn using an Olympus BX 51 compound microscope equipped with differential interference contrast and a camera lucida. Pencil drawings were scanned and electronically inked using Corel Draw X6 and were then processed using Adobe Photoshop CS5. Specimens were photographed with a Zeiss AxioCam ERc5s camera mounted on a Zeiss Stereomicroscope (Stemi 508). Appendages were dissected from specimens and stained by antibacterial glycerine-gelatine (Merck). The terminology of morphological characters follows Bruce and Buxton (2013).
Remarks. The first restrictive diagnosis to the genus was given by Dollfus (1897) when describing Stenasellus virei from the subterranean waters of France. Later, Magniez (1966) wrote a more detailed diagnosis, when comparing stenasellids of Africa. Subsequently, Magniez (1999) divided species of the genus Stenasellus of the Iberian Peninsula in two species groups: (S. breuili group and S.virei group). The species of S. breuili group being recognized by protopod of the male pleopod I with a single simple seta; pleopod II appendix masculina with cylindroid and elongated distal article, little or no twisted, bearing short setae (spine) on apical margin; and pleopods IV and V with endopod smaller than exopod with round apical margin. The species and subspecies that compose S. virei group stand out by the protopod of the male pleopod I with a single coupling hook on mesial margin, pleopod II appendix masculina fusiform distal article, more or less twisted, without apical short setae (spine); the exopods of pleopod IV and V, initially lamellar and broad subequal to apically rounded endopods.

Stenasellus tashanicus
Description of male. Body completely coral pink in the live specimen (Fig. 1C,  D), length 4.2 as greatest width, head trapezoidal, with slightly concave frontal margin, dorsal surface smooth. Pereonites II-IV subequal in length, with rounded lateral margins, pereonites V-VII with posterolateral margins projected posteriorly, pereonites VI and VII sub-equal, longest ( Fig. 2A, D). Pleonites I-II subequal in length, with projected posterolateral corner.
Pleotelson elongated, 1.4 as long as broad, posterior margin with two slight excavations; with scattered marginal setae.
Antenna (Fig. 3B) peduncle articles I and II reduced; the four others longer, increasing in length from the fourth to the sixth; article VI about 1.6 times as article V,  with long simple setae distally; article III with minute squama bearing two long RS, flagellum reaching to pereonite V posterior margin, up to 86 articles.
Left mandible (Fig. 3D, E) incisor and lacinia mobilis with four cusps, spine row of 18 serrate spines, molar with a row of long, tiny, simple setae. Palp article II longer than I, articles III distolateral margins with approximately 15 pectinated setae.
Maxillula (Fig. 3F) lateral endite apical margin with 12 serrate RS and eight tiny serrated smaller setae; mesial endite with three long, robust, comb and two short simple setae.
Maxilliped (Fig. 3H) endite mesial margin with six coupling hooks, distal margin with approximately 10 serrated and rarely plumose RS; palp article I with single RS on the inferior margin, palp articles II-V with several long simple setae on the inferior margin. Pereopod I (Fig. 3I) basis length 1.66 width, ischium superior margin with one RS on distal corner and five small RS on the medial projection; merus supradistal angle with three RS; carpus triangular, inferior margin covered with several long and short simple RS; propodus inferior margin covered with several long simple RS set in amongst some serrated RS; dactylus 9.2 times as long as basal width, inferior margin with a row of contiguous scale-like flattened setae with accessory setulae, main unguis elongate.
Pereopod II (Fig. 3J) basis about 1.8 times as long as the greatest width, superior margin with nine long distally plumose setae; ischium superior margin with five long RS; merus supradistal angle with two long RS, inferior margin with nine long simple setae; carpus superior margin with five simple setae, inferior margin covered with several long and short simple RS; propodus inferior margin covered with several short, simple, acute setae, supradistal angle with two long simple and single sensory palmate setae; dactylus shorter than main unguis, with two secondary unguis.
Pereopod III (Fig. 4A) is similar to pereopod II as illustrated.
Pereopod VII (Fig. 4B) basis about two times as long as the greatest width, superior margin with nine long distally plumose setae; Ischium length 2.2 width; merus supradistal angle with three long RS; carpus length 5.0 width, inferior margin covered with several long and short simple RS, supradistal angle with a long simple and a single sensory palmate setae; propodus length 7.3 width, inferior and superior margins covered with several short, simple, acute setae, supradistal angle with two long simple and a single sensory palmate setae; dactylus with elongated main unguis, bearing two secondary unguis.
Pleopod I (Fig. 4D) protopod length 1.2 width, mesial margin with a single coupling hook, exopod elongated, mesial margin with a row of 21 PMS and four simple setae, apical margin with row of ~18 simple fine setae, lateral margin concave.
Female. (Fig. 2B) Typically, longer than the male, apart from sexual characteristics similar to male, pleotelson is broader and antenna is longer than male.
Etymology. The name of this species comes from the type locality, the Tashan Cave, Iran.
Habitat. The isopods were collected from two pools in the dark zone of the Tashan Cave (at 20 to 200 cm depths). They were observed in all life cycle stages. They were observed crawling on the floor and hiding inside the sediment and cavities of the pools, as well as swimming in the water column. Mousavi-Sabet et al. (2016) described a blind fish from this cave (see Fig. 1D).
Remarks. Stenasellus tashanicus sp. n. can be identified by a slender and distally acute appendix masculina, and a maxilliped endite with six coupling hooks on the mesial margin. The new species is the largest known Stenasellus member: length up to 18 mm in males and 20 mm in females. The new species is similar to S. vermeuleni Magniez & Stock, 2000 (known from Wadi Halban, Oman), in having an appendix masculinum with acute apex. The shape of pleopods III-V in both species is also similar. Based on the drawings and description of S. vermeuleni, the new species differs by having a uropodal exopod smaller than the endopod (rather than subequal in length), pleopod I apical margin with a row of 18 simple fine setae (rather than six), exopodal article II of pleopod II is narrower than pleopod II in S. vermeuleni and pleopod V exopod and endopod subequal in length (rather than a smaller exopod). Based on the description and drawings of S. asiaticus by Birstein and Starostin (1949) from Turkmenistan, this species is readily distinguished from the new species by setose body dorsal surface (rather than smooth body surface) and its flattened appendix masculina (rather than a narrow with an acute apex).