Mekongbathynella jayavarthoni Park 2021, sp. n.
Creators: name: Park, Jong-Geun
Publisher: name: Zenodo
Year: 2021
Subjects: Biodiversity
Taxonomy
Animalia
Arthropoda
Malacostraca
Bathynellacea
Parabathynellidae
Mekongbathynella
Mekongbathynella jayavarthoni

description: Mekongbathynella jayavarthoni sp. n. (Figures 16–20)    Etymology   
The specific epithet is derived from Jayavarthon, the civil-name of Jayavarman VII, who is generally recognised as the most powerful king of the Khmer Empire. Gender feminine.  
 Type material   
 Holotype. Adult female, dissected on five slides. Cambodia, Kampong Cham Province, Tbong Kmom District, Pien Island, Interstitial in sand bank of Mekong River, depth 70 cm, 20 m from shore, Site 3 (11°56’29.4”, 105°27’40”), 25. April 2012 (Collector: J.-L. Cho) (NIBR IV0000266909).   
 Allotype. Adult male dissected on six slides, same data as holotype (NIBR IV00002668910).   
 Paratypes. One adult female dissected on four slides (NIBR IV0000266900), one adult male dissected on five slides (NIBR IV0000266901), five females and two males each kept as a whole specimen on separate slides (NIBR IV0000266911 – IV0000266924), same data as holotype.   
 Description of adult female (holotype)   
 Body. Length 1.04 mm (average of other females: 1.03, n = 8), approximately 15 times as long as wide. Head slightly longer than anterior two thoracic segments combined.   
 Antennule (Figure 16 (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 seta 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 16 (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 16 (f)) flat with eight median teeth flanked by two bulges carrying three spinules (inner) and one spinule (outer), respectively; median teeth decreasing in size medially. Inner surface with six rows of spinules in distal one third area, with one pair of long rows of spinules in proximal two thirds area.   
Mandible (Figure 17 (a, b)) with incisor process of four teeth. Tooth of ventral edge long and seta-like. Spine row consisting of six spines. Most distal spine distally wide and serrated. Palp of one segments and three times longer than wide, with one apical seta exceeding incisor process in length.  
 Maxillule (Figure 17 (c)) 2-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 17 (d)) 4-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 17 (e, f), 18(a–d), and 19(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 20 (a)) with two subterminal spines of equal size in oblique row on inner margin of sympod. Endopod 54% as long as sympod, with one distal spine, two dorsal plumose setae, and two terminal setae. Distal outer margin slightly protruded. Exopod 53% as long as sympod, with one basi-ventral seta and three setae on outer and terminal margin.   
 Pleotelson (Figure 20 (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.   
 Furcal rami (Figure 20 (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.   
 Description of adult male (allotype)   
 Body (Figure 16 (a)). length 0.99 mm (average of other males: 0.99, n = 7). Differing from the female in second antennular segment, flat inner margin of thoracopod VI and thoracopod VIII as well as slightly in body length.   
 Second antennular segment (Figure 16 (c, d)) bearing a prominent protrusion formed by two lobes. Ventral lobe nearly half as wide as dorsal lobe. Other setal ornamentation similar to second antennular segment of female.   
 Thoracopod VIII (Figure 19 (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 11 – 12 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 one spiculated lobe. Exopod as large as half size of basipod, semicircular in general form, inserted widely along the basipod; distal margin with one smooth lobe, one lobe carrying three distal setae and one medial seta and one group of three teeth being serrated distally. Endopod pointing backwards, having one fifth of the basipod length, with two terminal setae.
description: Published as part of Park, Jong-Geun, 2021, Four new species of Parabathynellidae (Bathynellacea, Syncarida) from the interstitial environment of the Mekong River, pp. 1075-1111 in Journal of Natural History 55 (17 - 18) on pages 1097-1100, DOI: 10.1080/00222933.2021.1936677, http://zenodo.org/record/5465801