Pethia pollux, Conway & & Kottelat & Maurice, 2021

Conway, Kevin W., , Amanda K. Pinion, Kottelat, & Maurice, 2021, Two new species of Pethia (Teleostei: Cyprinidae), representing a sympatric species pair, from the Ayeyarwady drainage, Myanmar, Raffles Bulletin of Zoology 69, pp. 80-101 : 92-96

Pethia pollux , new species

( Fig. 11)

Holotype. MHNG 2785.034 , 40.4 mm SL; Myanmar: Kachin State: Tan Pway Kone Chaung at northern edge of Thila Pha Inn, Indaw Inn , 24°18′15″N 96°48′42″E [near Shwegu]; M. Kottelat & Nyein Chan, 27 June 2017.

Paratypes. All Myanmar. Kachin State: CMK 27070 , 3 , 36.1–39.3 mm SL; 2, 38.2–40.9 mm SL [DNA voucher, fixed in 95% ethanol]; same as holotype. — CMK 28804 , 12 , 38.6–43.2 mm SL; ZRC 61663 , 5 , 36.3–39.9 mm SL; TCWC 20250.01 , 1 [CT voucher, M87992], 37.1 mm SL; TCWC 20250.02 , 4 , 35.8–41.7 mm SL; TCWC 20250.03 , 3 [C&S], 35.4 –40.0 mm SL; mouth of outlet of Indaw Inn into Ayeyarwady River, 24°13′08″N 96°49′25″E [near Shwegu]; M. Kottelat & Nyein Chan, 27 June 2017 .

Sagaing Region: CMK 28807 , 2 , 26.1–28.3 mm SL ; Ayeyarwady River near Hti Chaint Myit Yoe , 23°43′59″N 96°09′50″E; M. Kottelat & Nyein Chan, 30 June 2017 . — CMK 27937 , 1 , 39.3 mm SL [DNA voucher, fixed in 95% ethanol] ; Ayeyarwady River near Hti Kone village , about 55 km North of Mandalay, 22°26′41″N 96°00′18″E; M. Kottelat et al., 2 July 2017 .

Additional material (non-types). CMK 28805 , 56 , 33.6 – 41.6 ; Myanmar: Kachin State: mouth of outlet of Indaw Inn into Ayeyarwady River , 24°13′08″N 96°49′25″E; M. Kottelat & Nyein Chan, 27 June 2017 . — UF 191631 , 10 [2 C&S], 29.8–35.8 mm SL; Myanmar: Kachin State: Ayeyarwady drainage, Bhamo Market (24°15′00″N 97°12′36″E), purchased from fishermen by T. R. Roberts, April 2002 . — CMK 26822 , 2 , 26.4–33.8 mm SL; Myanmar: Sagaing Region: oxbow lake of Ayeyarwady River, about 37 km downriver of Shwegu , 92 masl, 24°17′11″N 96°28′30″E; Nyein Chan, 9 February 2017 .

Diagnosis. Pethia pollux is distinguished from all congeners, except P. castor , by the absence of black blotches (round or vertically elongate) on the body and by the presence of two black markings on anterior half of dorsal fin, including a larger marking over base of second unbranched to third branched ray and intervening fin membranes, and a smaller marking covering distal, flexible tip of third unbranched ray.

The external differences in preserved material of P. pollux and P. castor are not striking and may not be discernible in not optimally preserved specimens. Pethia pollux is distinguished from P. castor by: lacking a horizontal stripe along body side (vs. a weak stripe present, most evident in males); scales located on lower half of caudal peduncle, with dense scattering of dark brown melanophores, forming distinct reticulate pattern ( Fig. 2B; vs. with few faint brown melanophores forming weak or barely discernible reticulate pattern); presence of isolated lateral-line canal ossicles on scales in lateral line canal row on posterior part of body ( Fig. 12; vs. absence); a slightly larger eye (orbit diameter 32–36% HL vs. 29–33); a slightly longer (23–29% HL vs. 19–25), more pointed (vs. rounded) snout; upper lip swollen posteriorly (vs. upper lip uniform thickness along lateral margin of jaw); lateral fold on snout well developed (vs. poorly developed) along anterior margin of lachrymal; rostral cap swollen, obvious in lateral view, overlapping upper lip at midline (vs. rostral cap barely discernible from remainder of snout, not overlapping upper lip dorsally).

The following osteological characters too, distinguish P. pollux from P. castor : ascending process of premaxilla moderately developed, posteriormost tip reaching past midpoint of kinethmoid when jaws closed (vs. poorly developed, posteriormost tip not reaching to midpoint of kinethmoid when jaws closed); posterior part of anguloarticular with flat dorsal margin (vs. posterior part of anguloarticular elevated dorsally as large triangular process); central part of mesethmoid with cavernous indentation (vs. weakly concave); supraorbital relatively wide anteriorly, tapering in width abruptly towards posterior, without contact to lateral ethmoid (vs. relatively narrow anteriorly, tapering in width gently towards posterior, firmly attached to lateral ethmoid anteriorly); anterior edge of epibranchial 1 with poorly developed (vs. well-developed), shelf-like anterior extension.

Description. See Figure 11 for general appearance and Table 1 for morphometric data obtained from holotype and

9 paratypes. Body laterally compressed, relatively deep, greatest depth at dorsal-fin origin. Caudal peduncle depth approximately equal to head depth through orbit. Dorsal profile continuous between head and body, with slight hump at nape; ascending, almost straight, prior to dorsal fin, descending, almost straight, from dorsal-fin origin to just anterior of base of caudal fin, slightly concave at base of caudal fin. Ventral profile of head and body continuous, rounded through pelvic- and anal-fin base, slightly concave along caudal peduncle.

Head short, laterally compressed ( Fig. 13). Orbit diameter greater than snout length. Pupil oval, with apex located within anteroventral part of eye in lateral view ( Fig. 13A, B). Snout moderately pointed. Mouth small, subterminal, reaching vertical line through anterior margin of anterior nostril. Lips exposed, smooth, moderately thick; lower lip fold interrupted medially; upper lip slightly expanded posteriorly, starting at point opposite lateral fold on snout. Lateral fold on snout located along anterior margin of lachrymal (IO1), well-developed, continuous with shallow groove over dorsal surface of snout. Rostral cap conspicuous, visible in lateral view as swelling at anterior tip of snout; anteriormost part of rostral cap overlapping dorsal part of upper lip along midline ( Fig. 13B). Barbels absent. Skin anterior to eye thick, depigmented, opaque ( Fig. 13B), visible in dorsal view as a clear, slightly bulbous patch of skin between posterior nostril and orbit ( Fig. 13C). Opaque skin overlaying a dense aggregation of connective tissue, closely associated with anterolateral extension of lateral ethmoid and dorsal edge and upper lateral face of the lachrymal (IO1).

Supraorbital well-ossified ( Fig. 4D), widest at point approximately ⅓ from anterior tip, tapering in width abruptly towards posterior; separate from lateral ethmoid anteriorly. Mesethmoid irregularly shaped; centre with cavernous indentation ( Figs. 4F, 5B), extended dorsolaterally into poorly ossified flanges of membrane bone. Skull roof complete, without post-epiphyseal fontanelle. Masticatory plate broad, oval in ventral view; centre of plate slightly concave. Pharyngeal process of basioccipital process well-developed, extending beyond vertical through centre of compound centrum 2+3. General appearance of neurocranium otherwise similar to that of P. ticto (see Katwate et al., 2015). Ascending process of premaxilla moderately developed ( Fig. 14A), terminating at point posterior to imaginary vertical line through midpoint of kinethmoid with jaws closed. Autopalatine process of maxilla and coronoid process of dentary both well developed ( Fig. 14A). Dorsal margin of posterior part of anguloarticular horizontal ( Fig. 14A).

Cephalic lateral line canal system as described for P. castor , except preoperculo-mandibular lateral line canal open (roof formed by skin only) along entire horizontal part of preopercle ( Fig. 14A).

Gill rakers present on gill arches 1–5 ( Fig. 14C), with following distribution in 3 C&S specimens: arch 1, a7, 5cb+1eb+1cej/p13–14, 10–11cb+2eb+1cej; arch 2, a13–15, 9–10cb+3–4eb+1cej/p14–16, 10–11cb+3–4eb+1cej; arch 3, a14–16, 10–11cb+3–4eb+1cej/p15–16, 11–12cb+3– 4eb+1cej; arch 4, a15–16, 11cb+3–4eb+1cej/p11–12cb; arch 5, a10–11cb. Pharyngeal teeth on ceratobranchial 5 unicuspid, arranged in three rows, with formula 5,3,2 ( Fig. 14C, E, F); tip of crown strongly hooked with weakly serrated surface medial to hooked portion visible on some teeth ( Fig. 14F). Anterior edge of epibranchial 1 weakly convex ( Fig. 14B). Three branchiostegal rays; tip of posteriormost ray expanded, approximately 3–4 times deeper than pointed tip of anterior rays ( Fig. 14D).

Dorsal-fin rays iii.8.i (3) or iii.9 (2). Anal-fin rays iii.5.i (3) or iii.6 (2). Principal caudal-fin rays 10+9; dorsal procurrent rays 6 (4) or 7 (1), ventral procurrent rays 5 (4) or 6 (1). Pelvic-fin rays i.7.i (2), i.8 (1) or i.8.i (2). Pectoral-fin rays i.10.i (1), i.11.i (2), i.11.ii (1) or i.12 (1). Dorsal-fin origin slightly anterior to pelvic-fin origin; posterior margin slightly concave. Last unbranched ray almost as long as first branched ray ( Fig. 7F); proximal ⅔ compact, approximately twice as thick as first branched ray, rigid, strongly serrated, with 14–15 pairs of serrae on distal ⅔; apical ⅓ flexible, segmented, without serrae, or with poorly developed serrae on one or two proximal segments; apical ⅓ of ray (developing portion) fragile, missing from majority of specimens (including preserved holotype, but present at time of capture; Fig. 11A). Anal-fin origin posterior to vertical through base of posteriormost dorsal-fin ray; posterior margin straight to slightly concave. Caudal fin deeply emarginate; tip of lobes weakly pointed. Pelvic-fin tip pointed, adpressed fin reaching vent. Pectoral-fin tip rounded, adpressed fin reaching one scale row anterior to pelvic-fin origin.

Body lateral line incomplete, continuous for 8 (1), 9 (3), 10 (1), 11 (1 *), 12 (1), 13 (1), 14 (1) or 16 (1) scales, followed in majority of specimens by several (most commonly 1–2, up to 5) isolated lateral line canal ossicles on scales in lateral line row along caudal peduncle ( Fig. 12). Lateral line gently curved, lowest point located on 7 th or 8 th scale. Scales in lateral-line scale row 23 (4) or 24 (5 *), plus 1 (4) or 2 (5 *) on base of caudal fin. Predorsal scales 8. Circumpeduncular scales 12 (½/5/½). Scales in transverse row starting at dorsal-fin origin ½4/1/2½. Four to five well-developed radii on anterior and posterior field of each scale. Pseudotympanum located beneath third and fourth scales in lateral-line row ( Fig. 8C, D), comprising three openings in hypaxial body musculature; two small, round openings located anterior to 5 th rib, larger elongate opening located between 5 th and 6 th rib through which anterior swimbladder chamber is visible. Myosepta anterior to pseudotympanum; musculature associated with 5 th rib more pronounced in male ( Fig. 8C) than in female ( Fig. 8D).

Total number of vertebrae 30, with 16 abdominal+14 caudal. Total number of ribs 12, on vertebrae 5–16. First dorsal-fin pterygiophore inserted between neural spines of vertebrae 8/9 (4) or 9/10 (1). First anal-fin pterygiophore inserted between hemal spines of vertebrae 17/18 (4) or 18/10 (1). Free supraneurals 3, well developed, inserted between neural spines of vertebrae 4/5, 6/7, 8/9. Outer arm of os suspensorium elongate, reaching past imaginary horizontal line through dorsalmost tip of postcleithrum. 6 hypurals in caudal skeleton. Free uroneural (second) absent.

Colouration. In formalin, about 1 month after fixation ( Fig. 11B). Body background colour yellowish to light cream. Scales (excluding those of lowermost abdomen) with variable brown pigment (see below). Thin black axial streak along horizontal septum, most evident on posterior ⅔ of body, obscured by scale pigment anteriorly. Scales above horizontal septum generally with dense scattering of light brown melanophores over most of scale surface, excluding an anterior depigmented area posterior to anterior scale base and a posterior, crescent-shaped depigmented area close to posterior margin. Scales below horizontal septum (excluding those on lower abdomen) with cluster of dark brown melanophores on scale pocket, forming a regular pattern of small vertically elongate markings, most obvious on lateral line canal bearing scales and those scales in row immediately below, indistinct on lower part of caudal peduncle. Scales on lower half of caudal peduncle with single row of brown melanophores along posterior margin, creating weak reticulate pattern ( Fig. 2B). Dorsal surface of head and lateral surface of snout with dense scattering of dark brown melanophores ( Fig. 13). Sparse scattering of dark brown melanophores on skin covering upper part of opercle and around ventral margin of orbit ( Fig. 13A). Dorsal fin with two black markings on anterior half ( Fig. 7D, E), including a larger marking over base of second unbranched to third branched ray and intervening fin membranes, and a smaller, indistinct marking covering distal, flexible tip of third unbranched ray. Dorsal fin with light scattering of brown or black melanophores distally, most obvious along branched tips of rays. Caudal fin hyaline in alcohol, except for light scattering of brown melanophores over base of upper- and lowermost principal caudal-fin rays. Anal fin with light scattering of brown melanophores, most obvious over branched tips of rays. Pectoral fin hyaline, except for light scattering of dark brown to black melanophores over four anteriormost rays; melanophores associated with anteriormost pectoral-fin ray darker than on other rays, creating faint black to dark brown stripe, better developed in males. Pelvic fin hyaline.

In life. Upper part of body and caudal peduncle blueishsilver ( Fig.11A). Abdominal region silvery. Infraorbital and opercular bones with large amounts of guanine and reflective, appearing silvery. Larger marking over anterior dorsal-fin base black. Smaller marking at tip of dorsal fin and markings on other fins indistinct.

Sexual dimorphism. No obvious external sexual dimorphism. Hypaxial musculature surrounding pseudotympanum slightly hypertrophied in male, most obvious in hypaxial myomeres anterior to structure and hypaxial musculature surrounding 5 th rib. Mature females with swollen abdomens; ripe ovaries visible through body side in preserved specimens, light cream in colour. One dissected female with ripe ovary containing ~ 250 eggs of diameter 0.6–0.8 mm.

Distribution. Known to date from only a few sites in the middle Ayeyarwady ( Fig. 9), between Bhamo and Mandalay. The type locality, Tan Pway Kone Chaung at northern edge of Thila Pha Inn, was a pond in a very slow-flowing stream at the edge of the floodplain ( Fig. 10B), with moderately clear water (as compared to the murky adjacent marsh and lake). Numerous submerged aquatic plants were present (in contrast to the adjacent flooded lakes). See under P. castor for more information.

Etymology. Pollux, divine twin half-brother of Castor in Greek mythology, alluded also in the name of a star of the constellation and astrological sign of Gemini (twins). A noun in apposition.

Genetic Distances. The mean genetic distance (uncorrected p -distance) between the COI sequences obtained for P. pollux (n=3; see Appendix 1 for GenBank numbers and museum voucher numbers) differed by 0.1% (range 0–0.1%). Sequences obtained for P. pollux differed from those of P. castor (n=2) by 8.2%.