Mekongbathynella pienensis, Park, 2021

Park, Jong-Geun, 2021, Four new species of Parabathynellidae (Bathynellacea, Syncarida) from the interstitial environment of the Mekong River, Journal of Natural History 55 (17 - 18), pp. 1075-1111 : 1084-1087

Mekongbathynella pienensis sp. n. ( Figures 5–10 )

Etymology

The specific epithet is derived from Pien Island, where the material is collected. Gender feminine.

Type material

Holotype. Adult female, dissected on six 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 IV0000266898 ).

Allotype. Adult male dissected on five slides, same data as holotype ( NIBR IV0000266899 ).

Paratypes. One adult female dissected on seven slides ( NIBR IV0000266900 ), one adult male dissected on six slides ( NIBR IV0000266901 ), five females and two males each kept as a whole specimen on separate slides ( NIBR IV0000266902 – IV0000266908 ), same data as holotype .

Description of adult female (holotype)

Body. Length 1.41 mm (average of other females: 1.41, n = 6), approximately 12 times as long as wide. Head somewhat shorter than anterior three thoracic segments combined.

Antennule ( Figure 6 (b)) 6-segmented. First segment with one seta on inner distal margin, with one simple dorsal seta, and one dorsal and two lateral plumose setae. Second segment with one group of four plumose setae and with 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, three aesthetascs and one simple seta dorsally. Sixth segment with three subterminal aesthetascs and four simple setae.

Antenna ( Figure 6 (e)) 6-segmented, as long as antennular segments 1–4. Setal formula: 0 + 0/0 + 0/1 + 0/1 + 1/0 + 0/4(1).

Labrum ( Figure 6 (f)) flat with eight median teeth flanked by one bulge carrying two spinules and two additional spinules on both sides. Inner surface with one tiny median protuberance surrounded by six rows of spinules, two pairs of teats in distal one third area, with 14 oblique rows of spinules in proximal two thirds area.

Mandible ( Figure 6 (g, h)) with incisor process of four teeth. Tooth of ventral edge long and seta-like. Spine row consisting of six serrated spines. Palp one-segmented and three times longer than wide, with one apical seta exceeding incisor process in length.

Maxillule ( Figure 7 (a)) 2-segmented. Proximal segment with four claw-like setae on inner distal margin. Distal segment distally with two smooth spines, three spines and one flat round proximal projection on inner margin, and three simple setae on the outer distal margin. Distal spine of inner margin serrated in rows, middle spine spiculated, proximal spine smooth.

Maxilla ( Figure 7 (b)) 4-segmented, setal formula 1-3-8-6. Second segment without medial seta. Most distal seta of third segment claw-like, as large as claw-like seta of fourth segment.

Thoracopods I–VII ( Figure 7 (e, f, g), 8(a, b, c), and 9(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 not shorter than 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 8 (d, e)) in form of flat and round projection being basally fused.

First pleopod absent.

Uropod ( Figure 10 (a)) with two subterminal spines of equal size slightly oblique on inner margin of sympod. Endopod 48% as long as sympod, with one distal spine, two dorsal plumose setae, and two terminal setae. Distal outer margin protruded prominently. Exopod 47% as long as sympod with one basi-ventral seta and three setae on outer and terminal margin.

Pleotelson ( Figure 10 (b, c)) with one lateral seta near base of furcal rami on both sides.

Anal operculum ( Figure 10 (b, c)) protruded with round margin nearly reaching distal end of furcal rami.

Furcal rami ( Figure 5 (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 6 (a)). length 1.39 mm (average of other males: 1.39, n = 3). 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 6 (c, d)) bearing a prominent protrusion formed by two lobes of similar thickness on the ventro-distal side, inner lobe with four teeth, outer lobe smooth. Other setal ornamentation similar to second antennular segment of female. Thoracopod VIII ( Figure 9 (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; penial region prominent, exceeding height of exopod in lateral view, with dentate lobe carrying 10 dentils. Epipod flat and round. Basipod inserting behind penial lobe, almost same size of protopod in lateral view, main axis parallel to that of 8th thoracomere, without setae, middle part of inner margin of basipod drawn out into two smooth lobes directing towards the endopod. Exopod half size the basipod, semicircular in general form, inserted widely along basipod; distal margin with one smooth lobe, one lobe carrying three distal setae and one medial seta and one group of three distally serrated teeth on bulge. Endopod pointing backwards, one fifth of the basipod and bearing two terminal setae.

Figures

Figure 5. Mekongbathynella kampucheana sp. n. (Holotype: ♀). (a), uropod (dorsal); (b), pleotelson and furcal rami (dorsal); (c), pleotelson, uropod and furcal ramus (lateral). Scale bars = 0.05 mm.

Figure 6. Mekongbathynella pienensis sp. n. (Holotype: ♀; allotype: ♂). (a), general habitus ♂ (lateral); (b), antennule ♀ (dorsal); (c), second antennular segment ♂ (ventral); (d), second antennular segment ♂ (lateral); €, antenna ♀ (dorsal); (f), labrum ♀ (ventral); (g), mandible ♀ (dorsal); (h), mandibular incisor process and molar process ♀ (inner lateral). Scale bars = 0.05 mm, unless otherwise specified.

Figure 7. Mekongbathynella pienensis 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.

Figure 8. Mekongbathynella pienensis sp. n. (Holotype: ♀). (a), thoracopod IV (frontal); (b), thoracopod V (frontal); (c), thoracopod VI (frontal); (d), thoracopod VIII (lateral); (e), thoracopod VI (ventral). Scale bar = 0.05 mm.

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.

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.