Gammarus arasensis sp. nov.urn:lsid:zoobank.org:act: 744736DA-D19C-485A-B108-9B7DC43AE40B(Figs. 2–7)Holotype: Male, 12, 5 mm. (RTEÜ-FFR20002) Erzurum Province, Türkiye (39.969121 N 41.883690 E); collected by H. Baytaşoğlu.Paratypes: 24 males and 12 females, same data as the holotypeGenetic material: RTEÜ-FFR-DNA K254, K179, K165 same data as the holotype (GenBank accession numbers: PQ458061, PQ458062, PQ458063 for COI and PQ456280, PQ456281, PQ456282 for 28S, respectively)Diagnosis: Antenna 1 with 17–20 flagellar articles. Antennal gland cone reaching the distal end of peduncular article 3 of antenna 2. Coxae 1–4 with long setae on inner ventral margins. Pereopods 3–4 with short posterior setae. Uropod 3: endopod 0.64 times the length of exopod.Description of male holotypeHead: Eyes kidney-shaped; slightly shorter than the diameter of the first peduncle segment of antenna 1 (Fig. 2).Antennae: Antenna 1 (Fig. 4A) is distinctly shorter than half the body length; peduncle segments do not bear long setae; several sets of setae present distally on all three segments; segments gradually decrease in length (ratio: 1:0.75:0.5); flagellum short, bearing 16–17 segments; each flagellum segment bearing a few thin setae distally and an aesthetasc from the second segment; accessory flagellum 3-segmented, the last segment 1/3 the length of the previous one. Antenna 2 (Fig. 4B) slightly shorter than antenna 1 (ratio 1:0.86); antennal gland cone straight, almost reaching the distal end of the third peduncle segment; the last peduncle segment is slightly longer than the previous one (ratio: 1.12:1); the fourth and fifth peduncle segments bear bundles of setae in 3–4 groups in three rows; setae are not longer than the diameter of the peduncle segment; the 10 non-inflated flagellar segments are not densely setose and each has calceoli.Mouthparts: Upper lip (Figure 3A) has many small setules in the distal part; left mandible has 5-toothed incisors and 4-toothed lacina mobilis; between triturative molar and lacina mobilis there are 6–7 finger-like structures; one long plumose seta is present in the base part of the molar; while there is no seta in the first segment of the mandibular palp, there are 8 setae gradually extending distally in the second segment; third segment bears 24 D-seta, 4–5 E-seta, one group A-seta and one group B-seta; C-seta is absent (Figure 3H, H’); right mandible has 4-toothed incisors and multi-toothed lacina mobilis; first and last palp segments, molar and surrounding structures are the same as those in the left mandible; 15 setae are present in the second segment of the palp (Figure 3G, G’); right and left maxilla have 1 asymmetric structure; right maxilla 1 (Figure 3F, F’) bears 13 plumose setae on the distal inner lobe; there are 11 serrated teeth on the distal outer lobe, of which the serrations of the inner lobe side are more numerous and finer; there is no setation on the first segment of the palp; the second segment of the palp is shovel-shaped and bears four thick spines and two setae (one thicker) on the distal side; left maxilla 1 (Figure 3D, D’) inner and outer lobes are similar to the right; the first segment of the palp bears no setae; the second segment is slightly elongated sickle-shaped and bears 8 spines on the distal end; there are no lateral setae on the palp segments of right and left maxilla 1. There is no inner lobe on the lower lip (Figure 3C); it bears numerous tiny distal and disto-lateral setae; maxilla 2 (Figure 3E) the outer lobe is slightly longer than the inner one (ratio: 1:1.1); the convex distal margins of both lobes bear 20–30 and 10–15 simple setae, respectively; similarly, the outer margins of both lobes contain numerous tiny setae; the inner margin of the inner lobe bears 14–15 plumose setae in a diagonal row.Coxal Plates: Coxal plates 1–4 bear one relatively long simple seta located near the anterodistal margin of their inner surfaces (Figs. 1; 4C, D; 5A, B). Coxal plate 1 is rectangular and slightly widened in distal part (length/ width ratio: 1:1.28); it bears 2 and 1 anteriorly and posteriorly positioned setae along the convex ventral margin, respectively (Fig. 4C); coxal plate 2 has a slightly convex margin that is slightly narrowed distally (length/width ratio: 1:1.6); in addition to one long seta on the anterodistal inner surface as in coxal plate 1, it bears 2 short setae anteriorly and 1 posteriorly on the ventral margin (Fig. 4D); coxal plate 3 is rectangular (length/width ratio: 1:1.76) and the ventral margin is convex and bears 2 anterior and 1 posterior distal setae, respectively; there is one long setae on the posterodistal side of the inner surface (Figure 5A); the ventral margin of coxal plate 4 is slightly convex with a length/width ratio of about 1:1.25; there is 1 long setae on the inner surface, located near the anterodistal side; in addition, there are 2 short setae on the anterodistal corner and 4 short setae on the posterior margin (Figure 5B); coxal plate 5 is bilobed and is wider than it is long (length/width ratio: 1:1.83); anterior lobe 1 with very convex margins; the posterior lobe bears 2 short setae (Figure 6A); coxal plate 6 is bilobed, with lobes slightly separated and without very convex margins; The posterior margin of the posterior lobe bears 2 short setae (Fig. 6B); coxal plate 7 is similar in structure to 6 and bears 6 short setae in the posterior part (Fig. 6C).Gnathopods: The basal segment of gnathopod 1 has 5 and 18–20 setae on the anterior and posterior margins, respectively (Fig. 4C); setae along the posterior margin are 1.8 times longer than the segment diameter, and those on the anterior margin are 1.5 times longer; in addition, there are 2 groups of setae on the posterodistal, some of which are more rigid; ischium and merus bear setae distoposteriorly; triangular carpus is slightly shorter than the propodus (ratio: 1:1.38); it has 3 groups of setae on the anterior margin in addition to numerous setae on the distoposterior part; the length/width ratio of the pyriform propodus is 1:0.55; there are two groups of setae on the anterior margin, bearing 2–3 groups of short setae on the inner and outer surfaces; medial palmar spine present; posterodistal angle surrounded by 7–8 small spines and 7–8 long simple setae; sickle-shaped dactylus bears 1 seta on outer margin and barely reaches posterodistal corner. Gnathopod 2 is stronger and more setose than 1; basal segment anterior and posterior margins have 7–8 and 28–30 long setae respectively and the length of the setae on the posterior margin reaches twice the segment diameter; ischium and merus are similar to those of gnathopod 1 in shape and setae; triangular carpus is shorter than propodus (ratio: 1:1.29) and bears two groups of setae along the anterior margin; there are 5 groups of setae in a transverse row on the posterior margin; propodus length/width ratio is around 1:1.65 and bears setae in 4 groups along the anterior margin; there are 7–8 transverse setae groups on the posterior margin and 3–4 setae groups on the inner and outer surfaces; medial palmar spines are present; there are 3 strong spines and a few small spines on the posterodistal corner; The sickle-shaped dactylus extends between the spines at the posterosdital corner and bears a single seta on its outer margin.Pereopod 3: Basal segment of pereopod 3 bears numerous long (up to 1.8 times the segment diameter) setae along its anterior and posterior margins; a group of setae is present on the posterodistal of the ischium; merus bears two groups of spines and setae (and a single short setae) on its anterior margin, while 7 groups of setae are carried along its posterior margin, and the length of these setae can reach 1.5 times the segment diameter; carpus is shorter than merus (ratio: 1:1.41) and has fewer setae; it bears 1 group of spines+seta anterodistally and 4–5 groups of setae along its posterior margin; the lengths of these setae are shorter than those on merus; propodus is almost the same length as carpus, with one group anterodistally and 5–6 groups of armament on its posterior margin; dactylus is about 3 times longer than wide, with 1 plumose seta on outer margin and 2 small setae on inner margin at base of nail (Fig. 5APereopod 4: Pereopod 4 is slightly shorter than 3; the shape and setation of basis, ischium, propodus, and dactylus are similar to pereopod 3; the setation on the posterior margins of merus and carpus is shorter and less than that of pereopod 3 (Fig. 5B).Pereopod 5: Basis in rectangular shape and bears 8–9 short setae along the posterior margin; anterior margin with 7–8 small spines in addition to two groups of setae in the proximal part; inner surface with no setae; no spine exists in the postero-ventral corner; Ischium anterodistal corner with 2 spines; merus has two groups of spines in addition to a group of short setae along the anterior margin, posterior margin with two groups of spines; Carpus has three groups of spines both on the anterior and posterior margins; anterior margin of the propodus with 5 pairs of spines while there are two groups of setae along its posterior margin; dactylus with a plumose setule and two simple setules along its outer and inner margins, respectively (Fig. 6A).Pereopod 6: Basis in an elongated rectangular shape; posterior margin of the basal segment is concave and bears 10 setules; anterior margin with 4 small spines in addition to an anterodistal spine group; inner surface without setae; Ischium is similar to that of pereopod 5 in terms of shape and armament; Merus bears two groups of spines in both distal corners in addition to three and a group of spines along the anterior and posterior margins, respectively; Carpus bears no setae on both margins but has four and two groups of spines along its anterior and posterior margins, respectively; Propodus and dactylus are similar to that of pereopod 5 in terms of the armament (Fig. 6B).Pereopod 7: Basis has 10 setules along its concave posterior margin; its anterior margin with 5 small spines in addition to a group of anterodistal spines; inner surface without setae; posterodistal corner bears a seta in the inner part; Ischium, propodus and dactylus are similar to that of pereopod 6 in terms of shape and armament; Merus and Carpus have no setae on both margins; both of them have two groups of spines along their posterior margins while they bear three and four spine groups along their anterior margins, respectively (Fig. 6C).Uropod 1: The peduncle is longer than the rami (the length ratio is about 1.0.66) and has 2 small spines at the base; two and five spines occur along the inner and outer margins, respectively; inner ramus with 2 spines on the outer margin and no spine on the inner margin; outer ramus bears two spines on both margins; both rami have 5 distal spines (Fig. 5C).Uropod 2: Uropod 2 is smaller than the first one; the length ratio is about 1:0.7; the peduncle segment is as long as the outer ramus; it bears a spine in both distal corners in addition to a spine along the inner margin; the inner ramus is slightly shorter than the outer one; both rami have a just spine along their margins and bears 5 distal spines (Fig. 5D).Uropod 3: The peduncle segment is much shorter than the two-articulated outer ramus and the length ratio is about 1:0.37; the length ratio of the inner/outer ramus is about 1:0.63; the inner ramus bears both simple and plumose setae on both margins while there are only simple setae in the distal tip; it bears two spines along the outer margin; outer ramus with plumose setae along the inner margin, its outer margin bears 4 groups of spines accompanying with longer simple setae groups; the second article is longer than the surrounding spines and has long simple setae (Fig. 5E).Urosomites: Not elevated. Each segment bears one median and two dorsolateral groups of armaments; each consists of 1–2 spines and 1–3 accompanying short setae.Epimeral plates and Pleopods: Epimeral plate 1 (Fig. 5F) is slightly pointed in the postero distal corner and bears 8–9 long setae along the anteroventral margin, no spine is present; pleopod 1–3 are similar to each other in terms of shape and setation; each one bears 2 retinacula (Figs. 5F, G, H); Epimeral plate 2 moderately pointed; the ventral margin is armed with 3 spines, and the posterior margin with 2–3 setae (Fig. 5G); Epimeral plate 3 slightly pointed at the posterior corner and bears 4 spines along the ventral side (Fig. 5H).Telson: Telson lobes cleft, each lobe bears 2–3 groups of setae on its outer surfaces in addition to 2–3 spines accompanied by 2–3 setae in their distal part. The length/width ratio of each lobe is about 1:0.45 (Fig. 5I).Molecular data analysesThe genetic distances based on COI ranged from 5.24% (between Gammarus stankokaramani Karaman, 1976 and Gammarus salemaai Karaman, 1985) to 28.10% (between Gammarus pljakici Karaman, 1964 and Gammarus komareki Schaferna, 1923 (Iranian population). The species most closely related to Gammarus arasensis sp. nov. is Gammarus kischineffensis Schellenberg, 1937, with a genetic distance of 16.23%. This value is approximately three times greater than the minimum genetic distance observed (5.24%). For the 28S, pairwise genetic distances ranged from 0.11% (between Gammarus halilicae Karaman, 1969 and Gammarus pljakici) to 7.21% (between Gammarus roeselii Gervais, 1835 and Gammarus stojicevici (Karaman, 1929)). Gammarus kischineffensis again appeared as the species most closely related to Gammarus arasensis sp. nov., with a genetic distance of 1.44%, which is approximately thirteen times greater than the minimum genetic distance observed (0.11%). All interspecific genetic distance values are provided in Table S1 (Supplementary Material).Phylogenies constructed using ML and BI have largely been similar topologies. In both topologies, the closest taxon of Gammarus arasensis sp. nov., is Gammarus kischineffensis. Gammarus arasensis sp. nov. was resolved from the closest clade with strong support (0.92) in the BI analysis but remained unresolved with weak support (45%) in the ML analysis. The new species has high branch support (ML: 100%; BI: 1.0) in phylogenies constructed by both methods and is monophyletic (Figure 9). G. arasensis sp. nov. indicates a new lineage phylogenetically distinct from its congeners.According to the ASAP result, 22 MOTUs belonging to 22 species were determined. The best ASAP score was 1.5 (p = 0.01), with a threshold distance of 0.079053. Using the PTP method, 23 MOTUs were identified among the 22 species. The analysis yielded a p -value of 0.001, with a Null-model score of 51.953957. The best score under the single coalescent rate was 56.081402. Notably, Gammarus arasensis sp. nov. was identified as a distinct MOTU separate from other species in both methods.