Pethia castor, 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 : 81-90

Pethia castor , new species

( Fig. 1)

Holotype. MHNG 2785.033 , 36.9 mm SL; Myanmar: Kachin State: 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.

Paratypes. Myanmar: Kachin State: CMK 27085 , 24 , 38.4–39.7 mm SL; ZRC 61662 , 8 , 35.0– 37.5 mm SL; TCWC 20251.01 , 1 [CT voucher, M87960], 34.7 mm SL; TCWC 20251.02 , 8 , 34.5–37.9 mm SL; TCWC 20251.03 , 2 [C&S], 36.0– 38.4 mm SL; same as holotype. — CMK 28806 , 2 ,

29.7–35.5 mm SL; 2, 35.7–38.4 mm SL [DNA voucher, fixed in 95% ethanol]; 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.

Other Material. Myanmar. Sagaing Region: CMK 27130 , 1 , 39.4 mm SL ; Ayeyarwady River upstream of Hti Chaint , 23°46′23″N 96°10′22″E; M. Kottelat & Nyein Chan, 30 June 2017. — CMK 27144 , 2 , 32.4–34.5 mm SL ; Ayeyarwady River upstream of Hti Chaint, Sa Khan Kyaut Maw Inn (backwaters), 23°46′23″N 96°10′22″E; M. Kottelat & Nyein Chan, 30 June 2017. — CMK 27159 , 5 , 35.4–39.9 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 27224 , 1 , 35.4 mm SL ; Ayeyarwady River, sand bank near Shein Ma Kar village , about 40 km North of Mandalay, 22°18′20″N 95°59′12″E; M. Kottelat et al., 2 July 2017. — CMK 28787 , 113 , 26.7–42.2 mm SL; same data as holotype .

Diagnosis. Pethia castor is distinguished from all other species of Pethia , except P. pollux , 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 the distal, flexible tip of third unbranched ray.

The external differences in preserved material of P. castor and P. pollux are not striking and may not be discernible in specimens that have not been optimally preserved. Pethia castor is distinguished from P. pollux by the weak horizontal stripe along body side (most evident in males; Fig. 1A–C) (vs. absence); scales on lower half of caudal peduncle with few or no faint brown melanophores scattered over pocket and along posterior margin, when present forming a weak or barely discernible reticulate pattern (vs. with dense scattering of dark brown melanophores, forming an obvious reticulate pattern; Fig. 2A); the absence of isolated lateral-line canal ossicles on scales in lateral-line canal row on posterior part of body (vs. presence); a slightly shorter (19–25% HL vs. 23–29), more rounded (vs. pointed) snout; a slightly smaller eye (orbit diameter 29–33% HL vs. 32–36); upper lip relatively thin, of uniform thickness along lateral margin of jaw (vs. upper lip swollen posteriorly); lateral fold on snout poorly developed (vs. well developed) along anterior margin of lachrymal; rostral cap barely discernible from remainder of snout, not overlapping upper lip dorsally (vs. rostral cap swollen, obvious in lateral view, overlapping upper lip at midline).

The following osteological characters too, distinguish P. castor from P. pollux : ascending process of premaxilla poorly developed, posteriormost tip not reaching midpoint of kinethmoid with jaws closed (vs. ascending process moderately developed, posteriormost tip reaching past midpoint of kinethmoid with jaws closed); posterior part of anguloarticular elevated dorsally as large triangular process (vs. posterior part of anguloarticular with flat dorsal margin); central part of mesethmoid weakly concave (vs. with central cavernous indentation); supraorbital relatively narrow anteriorly, tapering in width gently towards posterior, firmly attached to lateral ethmoid anteriorly (vs. relatively wide anteriorly, tapering in width abruptly towards posterior, not in contact with lateral ethmoid anteriorly); anterior edge of epibranchial 1 with well-developed (vs. poorly developed), shelf-like anterior extension.

Description. See Figure 1 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 mid-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 bases, slightly concave along caudal peduncle.

Head short, laterally compressed ( Fig. 3). Eye diameter greater than snout length. Pupil spherical to subelliptical ( Fig. 3A, B). Snout rounded to moderately pointed ( Fig. 3A) in lateral aspect. Mouth small, subterminal, not reaching vertical through anterior margin of anterior nostril. Lips exposed, smooth, moderately thick; lower lip fold interrupted medially. Lateral fold on snout poorly developed, located along anterior margin of lachrymal. Rostral cap poorly developed ( Fig. 3B). Barbels absent. Skin anterior to eye thick, depigmented, opaque ( Fig. 3B), visible in dorsal view as a clear, slightly bulbous patch of opaque skin between posterior nostril and orbit ( Fig. 3C). 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. 4A), widest anteriorly, tapering in width posteriorly, firmly attached to posterodorsal part of lateral ethmoid anteriorly. Mesethmoid irregularly shaped, weakly concave at centre ( Figs. 4A, 5A), 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 imaginary vertical line 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 poorly developed ( Fig. 6A), terminating at point anterior to imaginary vertical line through midpoint of kinethmoid with jaws closed. Autopalatine process of maxilla and coronoid process of dentary both well-developed ( Fig. 6A). Dorsal margin of angulo-articular elevated posteriorly into triangular process ( Fig. 6A), overlapping ventralmost part of ectopterygoid in lateral view.

Five infraorbital bones ( Fig. 4A–C); lachrymal (IO1) an irregular, shield-shaped bone, pointed dorsally, rounded anteroventrally, with poorly ossified, yet enclosed lateral line canal, with three openings located on bone, and a fourth located at junction between IO1 and IO2; IO2 narrow, with poorly ossified, yet enclosed lateral line canal, with single opening located at middle of bone, an additional pore located at junction between IO2 and IO3; IO3 broad, anterior tip at middle of orbit, anteriorly extending ventrally to middle of cheek, posteriorly extending ventrally to overlap preopercle, with open lateral-line canal; IO4 with poorly ossified, yet enclosed lateral-line canal along anterior part, without pore on bone, single pore located at junction with both IO3 and IO5; IO5 a small ossification composed largely of poorly ossified canal bone, transfers infraorbital canal to otic canal at junction between frontal and pterotic. Supraorbital lateral line canal extending from nasal, through frontal, terminating at junction with otic canal between frontal and pterotic, fully enclosed, with 6 pores, including 2 on nasal, 1 at junction between nasal and frontal, 3 along frontal dorsal to orbit, and 1 at junction between frontal and pterotic; parietal branch of supraorbital canal absent. Preoperculo-mandibular lateral line canal ( Fig. 6A) starting on dentary, continuing through anguloarticular, preopercle, and opercle, terminating at junction with otic lateral line canal at centre of pterotic; fully enclosed, excluding open portion along preopercle, with 10 pores, including 3 on dentary, 1 at junction between dentary and anguloarticular, 1 at junction between anguloarticular and preopercle, 3 along preopercle, 1 at junction between preopercle and opercle, and 1 between junction of opercle and pterotic. Anterior (horizontal) part of preoperculo-mandibular lateral line canal on preopercle not fully enclosed; with roof of canal formed by skin. Otic lateral line canal located on pterotic, connecting with trunk lateral line canal via posttemporal and supracleithrum; fully enclosed with three pores, including 1 pore at junction with frontal and IO5, 1 pore at junction with opercle, and 1 pore at junction with posttemporal. Junction between supratemporal lateral line canal and otic lateral line canal located at posterior part of pterotic. Supratemporal lateral line canal with single pore close to origin; supratemporal commissure located along dorsal midline at junction of parietal bones, without pore, without contact to supraoccipital.

Gill rakers present on gill arches 1–5 ( Fig. 6C), with following distribution in 2 C&S specimens: arch 1, anterior row (a) 7–8 (total), 5–6 on ceratobranchial (cb)+1 on epibranchial (eb)+1 at cb/eb junction (cej)/posterior row (p) 12–13, 9–10cb+2eb+1cej; arch 2, a14–16, 10–11cb+3–4eb+1cej/ p17–18, 13–14cb+3–4eb+1cej; arch 3, a15–16, 11–12cb+3– 4eb+1cej/p15–16, 11–12cb+3–4eb+1cej; arch 4, a15–16, 11–12cb+3–4eb+1cej/p12–13cb; arch 5, a13cb. Pharyngeal teeth on ceratobranchial 5 unicuspid ( Fig. 6C, E, F), with slightly hooked tips, arranged in three rows, with formula 5,3,2. Epibranchial 1 with expanded, shelf-like process on anterior edge ( Fig. 6B). Basihyal a thin rod. Three branchiostegal rays; tip of posteriormost ray expanded, approximately 3–4 times deeper than pointed tip of anterior rays ( Fig. 6D).

Dorsal-fin rays iii.8.i (1) or iii.9 (1). Anal-fin rays iii.5.i (1) or iii.6 (1). Principal caudal-fin rays 10+9; dorsal procurrent rays 5 (1) or 6 (1), ventral procurrent rays 5 (1) or 6 (1). Pelvic-fin rays i.7.i (1) or i.8 (1). Pectoral-fin rays 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; 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 ( Fig. 7C). Anal-fin origin posterior to vertical through base of posterior dorsal-fin ray; posterior margin slightly concave. Caudal fin deeply emarginate; tip of lobes rounded. Pelvic-fin tip rounded to weakly pointed, adpressed fin reaching to vent. Pectoral-fin tip rounded, adpressed fin reaching one scale row anterior to pelvic-fin origin.

Body lateral line incomplete, continuous for 9 (1), 10 (2), 11 (3 *), 12 (1), 14 (2) or 16 (1) scales. Lateral line gently curved, lowest point located on 7 th or 8 th scale. Scales in lateral line scale row 22 (4) or 23 (5 *), plus 1 (3) or 2 (4 *) on base of caudal fin. Predorsal scales 7 (1) or 8 (10 *). Circumpeduncular scales 12 (½/5/½). Scales in transverse row starting at dorsal-fin origin ½4/1/2½. 4–5 well-developed radii on anterior and posterior field of each scale. Pseudotympanum located beneath third and fourth scales in lateral line row ( Fig. 8A, B), 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 ribs, through which anterior swimbladder chamber is visible. Myosepta anterior to pseudotympanum; musculature associated with 5 th and 6 th rib more pronounced in male ( Fig. 8A) than in female ( Fig. 8B).

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. First anal-fin pterygiophore inserted between hemal spines of vertebrae 17/18. 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. Six hypurals in caudal skeleton. Free uroneural (second) absent.

Colouration. In formalin, about 1 month after fixation ( Fig. 1A, B). Body background colour yellowish to light cream. Dorsal half of body (above horizontal septum) light brown, caused by pigmentation on scales (see below). Indistinct blackish-grey horizontal stripe along side of body, in deeper layer dorsal to thin black axial streak along horizontal septum; most distinct along posterior half of body in males. Scales above horizontal septum with dense scattering of light brown melanophores over much of scale pockets; a thin row of light brown melanophores bordering posterior margin of each scale, separated from anterior melanophores by crescentic unpigmented area. Melanophores on scales contributing to weak reticulate pattern on dorsal surface of body. Scales below horizontal septum with little pigment, restricted to sparse scattering of small faint brown melanophores on scale pocket and slightly darker brown melanophores along posterior edge, most obvious on scales covering pseudotympanum and lower part of caudal peduncle after some time in alcohol ( Figs. 1C, D, 2A). Dorsal surface of head and lateral surface of snout with dense scattering of dark brown melanophores ( Fig. 3). Sparse scattering of dark brown melanophores on skin covering upper part of opercle and around ventral margin of orbit ( Fig. 3). Dorsal fin with two black markings on anterior half ( Fig. 7A, B), 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. Dorsal fin with sparse scattering of light brown melanophores distally, most evident along branched tips of rays. Centre of dorsal fin with a faint triangular canaryyellow marking in specimens after 1 month in formalin ( Fig. 1A, B), and possibly also in life; absent in alcohol-preserved specimens in which dorsal fin, other than black markings, is hyaline ( Fig. 1C, D). Caudal fin with faint canary yellow in formalin-fixed material, hyaline in alcohol, except for light scattering of light brown melanophores over base of upper- and lowermost principal caudal-fin rays. Pectoral fin hyaline, except for sparse 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 and anal fin hyaline. Colour in life not recorded.

Sexual dimorphism. No obvious external sexual dimorphism other than slight dichromatism described above under section on colouration. Minute conical tubercles scattered over dorsal surface of head in two male specimens (CMK 27085). Hypaxial musculature surrounding pseudotympanum slightly hypertrophied in male, most obvious in hypaxial myomeres anterior to structure and hypaxial musculature surrounding 5 th and 6 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 ~ 200 eggs of diameter 0.6–0.8 mm.

Distribution. Known to date only from a few sites in the middle Ayeyarwady ( Fig. 9), between Bhamo and Mandalay. The majority of specimens were collected from the type locality, at the mouth of outlet of Indaw Inn into Ayeyarwady River. At the time of collection (June–July, beginning of floods) all sampling sites were along edges of floodplain lakes, backwaters and the Ayeyarwady proper, with murky water and mud bottom. At three sites, P. castor and P. pollux were collected together. The species were not identified in the field and no habitat preference could be noted. At two sites, P. castor was more abundant ( Fig. 10A), and at the third site ( Fig. 10B) P. pollux was more abundant. At the time of collection, this last site was a small ponded stream, separated from a lake, but which floods would soon connect; the stream was flowing from an area shaded by trees and bushes and the water was slightly clearer. This suggests that P. pollux might prefer clearer water and shade.

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