Four new species of Parabathynellidae (Bathynellacea, Syncarida) from the interstitial environment of the Mekong River Author Park, Jong-Geun text Journal of Natural History 2021 2021-07-21 55 17 - 18 1075 1111 http://dx.doi.org/10.1080/00222933.2021.1936677 journal article 10.1080/00222933.2021.1936677 1464-5262 5465801 Mekongbathynella jayavarmani sp. n . ( Figures 11–15 ) Etymology The specific epithet is derived from Jayavarman II who is generally recognised as the founder of the Khmer Empire, the dominant civilisation on the Southeast Asian mainland until the mid 15th century. Gender feminine. Figure 9. Mekongbathynella pienensis sp. n. (Holotype: ♀; allotype: ♂). (a), thoracopod VII ♀ (frontal); (b), thoracopod VIII ♂ (inner lateral); (c), right and left thoracopods VIII ♂ (ventral). Scale bars = 0.05 mm. Type material Holotype . Adult female, dissected on five slides. Cambodia , Stung Treng Province , Siem Bok District , Stung treng Town , interstitial habitat in sand bank of the Mekong River , depth 30 cm , 10 m from shore, site 24 ( 13°32’26.5”N , 106°00’23.2”E ), 27. April 2012 ( Collector : J.-L. Cho ) ( NIBR IV0000266943 ). Allotype . Adult male dissected on five slides, same data as holotype ( NIBR IV0000266944 ). Figure 10. Mekongbathynella pienensis sp. n. (Holotype: ♀). (a), uropod (dorsal); (b), pleotelson and furcal rami (dorsal); (c), pleotelson, uropod and furcal ramus (lateral). Scale bars = 0.05 mm. Paratypes . One adult female dissected on four slides ( NIBR IV0000266945 ), one adult male dissected on six slides ( NIBR IV0000266946 ), five females and seven males each kept as a whole specimen on separate slides ( NIBR IV0000266947 – IV0000266958 ), same data as holotype . Description of adult female ( holotype ) Body . Length 1.05 mm (average of other females: 1.05, n = 6), approximately 15 times as long as wide. Head slightly longer than anterior two thoracic segments combined. Antennule ( Figure 11 (b)) 6-segmented. First segment with one seta on inner distal margin, one simple dorsal seta, and one dorsal, two lateral plumose setae. Second segment with one group of four plumose setae and one simple setae on inner distal margin. Third segment with two lateral setae (inner one tiny) and one inner ventral seta. Inner flagellum with three simple setae. Fourth segment with one stub seta and one plumose seta on dorsal margin, and two plumose setae on outer distal apophysis. Fifth segment with three setae on inner margin, two aesthetascs, one simple seta and one aesthetasc dorsally. Sixth segment with three subterminal aesthetascs and four simple setae. Antenna ( Figure 11 (e)) six-segmented, as long as antennular segments 1–4. Setal formula: 0 + 0/0 + 0/1 + 0/1 + 1/0 + 0/4(1). Labrum ( Figure 11 (f)) flat with eight median teeth flanked by two bulges carrying five spinules (inner) and one spinule (outer), respectively; median teeth decreasing in size. Inner surface with four rows of spinules, two pairs of teats in distal area, with one pair of long rows of spinules in proximal area. Mandible ( Figure 11 (g, h)) with incisor process of four teeth. Tooth of ventral edge long and distally bifurcate. Spine row consisting of six spines. Distal spine distally serrated, carrying two additional subapical teeth. Palp one segmented, three times longer than wide, with one apical seta exceeding incisor process in length. Maxillule ( Figure 12 (a)) two-segmented. Proximal segment with four claw-like setae and two subterminal spinules on inner distal margin. Distal segment distally with two spiculated spines, three spines and one flat round proximal projection on inner margin, and three simple setae on outer distal margin. All spines of inner margin spiculated. Maxilla ( Figure 12 (b)) four-segmented, setal formula 1-3-8-6. Second segment without medial seta. Distal seta of third segment claw-like, as large as claw-like seta of fourth segment. Thoracopods I–VII ( Figures 12 (c, d, e), 13(a, b, c), 14(a)). Thoracopods I–IV increasing in size posteriorly. Thoracopods IV–VII similar in size. Thoracopods III–VII each bearing one epipod on protopod. Protopod of thoracopod VI with protruded inner margin. Thoracopods I–VII each with one basipodal seta. Exopod two-segmented throughout. Ventral seta of proximal segment shorter than length of distal segment, dorsal seta tiny. Dorsal seta of distal segment shorter than basis of ventral seta. Endopod four-segmented, setal formulae: Th I: 1 + 0/0 + 1/1 + 0/3(1); Th II–VII: 0 + 0/0 + 1/0 + 0/3(1). Thoracopod VIII ( Figure 12 (d)) in form of flat and round projection being basally fused. First pleopod (cf. Figure 11 (a)) absent. Uropod ( Figure 15 (a)) with two subterminal spines of equal size in oblique row on inner margin of sympod. Endopod 45% as long as sympod, with one distal spine, two dorsal plumose setae, and two terminal setae. Distal outer margin slightly protruded. Exopod 49% as long as sympod, with one basi-ventral seta and three setae on outer and terminal margin. Pleotelson ( Figure 15 (b, c)) with one lateral seta near base of furcal rami on both sides. Anal operculum ( Figure 15 (b, c)) slightly protruded with round margin barely exceeding basis of furcal rami. Figure 11. Mekongbathynella jayavarmani sp. n. (Holotype: ♀; allotype: ♂). (a), general habitus ♂ (lateral); (b), antennule ♀ (dorsal); (c), second antennular segment ♂ (lateral); (d), second antennular segment ♂ (ventral); (e), antenna ♀ (dorsal); (f), labrum ♀ (ventral); (g), mandible ♀ (dorsal); (h), mandible ♀ (inner lateral). Scale bars = 0.05 mm, unless otherwise specified. Furcal rami ( Figure 15 (b, c)) 1.2 times as long as wide, with two large distal spines and one smaller spine on inner margin and with one simple seta and one plumose setae dorsally. Figure 12. Mekongbathynella jayavarmani sp. n. (Holotype: ♀). (a), maxillule (dorsal); (b). maxilla (dorsal); (c), thoracopod I (frontal); (d), thoracopod II (frontal); (e), thoracopod III (frontal). Scale bars = 0.05 mm. Description of adult male ( allotype ) Body ( Figure 11 (a)). length 0.99 mm (average of other males: 0.99, n = 8). Differing from the female in second antennular segment, flat inner margin of thoracopod VI and thoracopod VIII as well as slightly in body length. Figure 13. Mekongbathynella jayavarmani sp. n. (Holotype: ♀). (a), thoracopod IV (frontal); (b), thoracopod V (frontal); (c), thoracopod VI (frontal); (d), thoracopod VIII (ventral). Scale bar = 0.05 mm. Second antennular segment ( Figure 11 (c, d)) bearing a prominent protrusion formed by two lobes of similar thickness on the ventro-distal side, ventral lobe smooth, dorsal lobe bifurcated distally. Other setal ornamentation similar to second antennular segment of female. Figure 14. Mekongbathynella jayavarmani sp. n. (Holotype: ♀; allotype: ♂). (a), thoracopod VII ♀ (frontal); (b), right and left thoracopods VIII ♂ (ventral); (c), thoracopods VIII ♂ (inner lateral). Scale bars = 0.05 mm. Thoracopod VIII ( Figure 14 (b, c)) massive in relation to body size, as long as 8th thoracomer, 1.3 times longer than wide in lateral view. Protopod dominating anterior part, massive and globular in lateral view, not exceeding height of exopod in lateral view, with dentate lobe carrying 9 – 10 dentils. Epipod flat and round. Basipod inserting behind penial lobe, having almost same size of protopod in lateral view, main axis parallel to that of 8th thoracomere, without setae, disto-middle part of inner margin of basipod with two lobes being serrated distally. Exopod as large as half size of basipod, semicircular in general form, inserting widely along the basipod; distal margin with one smooth lobe, one lobe carrying three distal setae and one medial seta, one group of three teeth being serrated distally and 3 spinules. Endopod pointing backwards, having one fifth of the basipod length, with two terminal setae.