Polynemus bidentatus, Tsukawaki, 2006

Tsukawaki, Hiroyuki Motomura Shinji, 2006, New Species Of The Threadfin Genus Polynemus (Teleostei: Polynemidae) From The Mekong River Basin, Vietnam, With Comments On The Mekong Species Of Polynemus, Raffles Bulletin of Zoology 54 (2), pp. 459-464 : 460-463

Polynemus bidentatus , new species

( Figs. 1 , 2B )

Material examined. – Holotype. UMMZ 213346 , male, 153.4 mm SL; Vietnam: My Tho Province : Cho Gao Canal in Mekong River drainage; Mekong research team; otter trawl; 20 Jun.1974.

Diagnosis. – A species of Polynemus with the following combination of characters: 8 spines in first dorsal fin; 16 soft dorsal-fin rays; 7 pectoral-fin filaments (sixth filament longest); 17 pectoral-fin rays; 3 anal-fin spines; 70 pored lateral-line scales; 32 gill rakers; vomerine teeth separated at midline, forming two toothed patches; swimbladder absent; depth at posterior margin of maxilla greater than orbit diameter; relatively long snout length (6.8% of SL), upper-jaw length (13.8% of SL), first pelvic-fin soft ray length (17.2% of SL), and upper caudal-fin lobe length (38.1% of SL); relatively short pectoral-fin length (32.6% of SL), longest pectoral-fin filament length (158.7% of SL), and third anal-fin soft ray length (15.8% of SL); relatively small orbit diameter (2.2% of SL); ventroposterior portion of the pectoral fin black.

Description. – First dorsal fin with 8 spines. Second dorsal fin with 1 spine and 16 soft rays. Anal fin with 3 spines and 12 soft rays. Pectoral fin (exclusive of filaments) with 17 rays on each side of body. Pectoral-fin filaments 7 on each side of body. Pelvic fin with 1 spine and 5 soft rays. Pored lateral-line scales (exclusive of scales on caudal fin) 70. Scales rows above lateral line 6; scales below lateral line 13. Upper limb gill rakers 13; lower limb 19; total 32. Formula for configuration of supraneural bones, anterior neural spines and anterior dorsal pterygiophores /0/0+2/1+1/1/1/1/1/. Vertebrae 10 + 15. Swimbladder absent.

Morphometrics expressed as percentage of SL, followed in parentheses by times in SL: head length 26.7% (3.7); body depth at first dorsal-fin origin 22.0 (4.6); body depth at second dorsal-fin origin 22.7 (4.4); body width at pectoral-fin base 13.9 (7.2); snout length 6.8 (14.6); eye diameter 2.1 (47.9); orbit diameter 2.2 (45.1); interorbital width 7.9 (12.7); postorbital length 18.0 (5.6); upper-jaw length 13.8 (7.3); depth at posterior margin of maxilla 4.3 (23.2); pre-first dorsal-fin length 33.3 (3.0); pre-second dorsal-fin length 56.8 (1.8); pre-anal-fin length 58.3 (1.7); direct distance from pelvic-fin origin to anal-fin origin 26.9 (3.7); second dorsal-fin base length 20.2 (4.9); anal-fin base length 15.6 (6.4); pectoral-fin length 32.6 (3.1); pectoral-fin base, including pectoral-fin filaments base 13.0 (7.7); longest pectoralfilament (sixth) length 158.7 (0.6); longest pelvic-fin ray (first) length 17.2 (5.8); longest first dorsal-fin spine (third) length 18.2 (5.5); second dorsal-fin spine length 8.0 (12.5); longest second dorsal-fin ray (third) length 16.9 (5.9); longest anal-fin spine (third) length 8.7 (11.5); longest anal-fin ray (second) length 15.8 (6.3); caudal-peduncle length 26.1 (3.8); caudal-peduncle depth 10.3 (9.7); upper caudal-fin lobe length 38.1 (2.6); lower caudal-fin lobe length 33.6 (3.0).

Body oblong, moderately compressed anteriorly, progressively more compressed posteriorly. Nape and anterior body not highly arched, body depth shallow. Head large, length slightly greater than body depth. Scales covering entire body and head, except for lips and adipose eyelid, and body; body scales extending onto rays and membranes of all fins. Lateral line simple, extending from upper end of gill opening to mid-distal margin of caudal-fin membrane. Eye very small, orbit diameter 12.1 in head length, covered with thick adipose eyelid; longitudinal length of adipose eyelid slightly longer than depth of posterior margin of maxilla. Anterior nostril much smaller than posterior nostril; posterior margin of posterior nostril in contact with anterior margin of adipose eyelid. Lateral surface of maxilla covered with scales; posterior margin of maxilla extending well beyond eye. Mouth inferior, large, nearly horizontal, forming an angle of about 10 degrees to horizontal axis of head and body. Upper lip poorly developed; lower lip well developed posteriorly; anterior portion of lower jaw with small teeth extending onto lateral surface, adjacent portion of lip poorly developed. Villiform teeth in broad bands on premaxilla, dentary, vomer, palatines and ectopterygoids; tooth band of upper jaw wider than that of lower jaw; tooth band of upper jaw approximately equal to width of space separating tooth bands on opposing premaxilla; length of palatine plate longer than that of ectopterygoid plate; vomerine teeth separated at midline, forming 2 tooth patches. Posterior margin of preopercle serrated.

First spine of first dorsal fin tiny, its length less than anterior nostril diameter; second spine strong, about 1.5 times as width as third spine; bases of other spines with similar thickness; posterior tip of depressed first dorsal fin not reaching second dorsal-fin origin. Second dorsal-fin base length much longer than first dorsal-fin base length; length of longest second dorsal-fin ray slightly shorter than that of longest first dorsal-fin spine; origin of second dorsal fin anterior to that of anal fin. All pectoral-fin rays unbranched; fin inserted near midline of body; posterior tip of pectoral fin just reaching a vertical at anal-fin origin; second pectoral-fin ray longest, third to seventeenth rays progressively shorter. First pectoral filament shortest; first and second filament tips not reaching posterior tip of depressed pelvic fin; third filament tip not reaching anal-fin origin; fourth filament tip extending slightly beyond middle of anal-fin base; fifth filament tip extending slightly beyond caudal-fin base; sixth filament longest, longer than seventh filament; sixth and seventh filament tips extending well beyond posterior tips of caudal-fin lobes (filament tips being easily damaged). Posterior tip of depressed pelvic fin extending well beyond anus. First anal-fin spine tiny, mostly embedded; length of second spine more than half length of third spine; length of longest anal-fin soft ray slightly shorter than that of longest dorsal-fin soft ray. All soft rays, except for pectoral-fin rays, branched.

Colour of preserved specimen. – Head and body blackish dorsally, brownish ventrally. Adipose eyelid translucent white. Opercle brown with a poorly defined blackish blotch centrally. Pectoral fin brown with melanophores scattered on posteroventral part of fin. First and second dorsal, pelvic, anal and caudal fins brown without distinct melanophores. Base of pectoral filaments whitish, becoming brownish posteriorly. Life colour unknown.

Distribution. – Currently known only from the type locality (Mekong River basin, My Tho Province, Vietnam).

Etymology. – The name bidentatus is derived from a combination of the Latin bis (two) and dentatus (toothed) in allusion to the two tooth patches on the vomer. Suggested English name: Toothy Blackhand Paradise Fish.

Comparisons. – The genus Polynemus , distributed only in South and Southeast Asia, consists of the following eight taxa (viz. Motomura & van Oijen, 2003; Motomura, 2004b): P. aquilonaris Motomura , P. dubius Bleeker , P. hornadayi Myers , P. kapuasensis Motomura & van Oijen , P. melanochir dulcis Motomura & Sabaj , P. melanochir melanochir Valenciennes , P. multifilis Temminck & Schlegel , and P. paradiseus Linnaeus. Polynemus bidentatus can be readily distinguished from these congeners by having vomerine teeth separate at the midline, forming two toothed patches (vs. a single toothed patch in P. aquilonaris , P. dubius , P. hornadayi , P. kapuasensis , P. multifilis , and P. paradiseus or teeth absent from the vomer in P. melanochir dulcis and P. melanochir melanochir ; see Fig. 2 ). Polynemus bidentatus is thus the only species in the family Polynemidae that has separate vomerine tooth patches ( Motomura, 2004b).

In addition to the different vomer morphology, P. bidentatus differs from P. kapuasensis (distributed in the Kapuas River, western Kalimantan) and P. multifilis ( Thailand and Indonesia) in having fewer pectoral-fin filament counts (seven vs. 13–16 in the latter; Motomura & van Oijen, 2003) but shares with other congeners in having fewer filaments. Polynemus bidentatus differs from P. hornadayi (western Sarawak, Kalimantan, Malaysia) and P. paradiseus ( India to Thailand) in having eight spines in the first dorsal fin (vs. seven spines in the latter two species), three anal-fin spines (vs. two in P. paradiseus ), 70 pored lateral-line scales (vs. 90–103 in P. hornadayi ), and 32 gill rakers (vs. 26–28 in P. hornadayi ), and lacking a swimbladder (vs. present in P. hornadayi ). Data for P. hornadayi and P. paradiseus are based on 26 specimens (including holotype and nine paratypes) and 42 specimens (including neotype), respectively (listed in Motomura et al., 2002a, b).

Depth at the posterior margin of the maxilla (4.3% SL) in P. bidentatus is significantly greater than orbit diameter (2.2% SL), whereas the depths of the posterior margin of the maxilla in P. aquilonaris (2.5–3.2%, mean 2.8%) and P. dubius (2.3– 3.8%, mean 3.2%) are less than orbit diameter (3.5–4.3%, mean 4.0% and 3.3–4.5%, mean 3.8%, respectively). In addition, pectoral-fin (32.6%) and longest pectoral-fin filament (158.7% SL) lengths in P. bidentatus are relatively shorter than those in P. aquilonaris (34.1–40.8%, mean 36.9% and 265.9–371.3%, mean 308.2%, respectively) and P. dubius (34.2–40.2%, mean 37.3% and 263.9–312.3%, mean 293.0%, respectively). Conversely, upper-jaw length (13.8% SL) in the former is relatively longer than that in P. aquilonaris (10.1–11.8%, mean 10.8%) and P. dubius (9.7–13.1%, mean 11.4%). Moreover, P. bidentatus differs from P. aquilonaris in having lower counts of pored lateral-line scales (70 vs. 80–86 in the latter) and higher counts of gill rakers (32 vs. 25–29). Data for P. aquilonaris and P. dubius are based on 45 specimens (including holotype and 12 paratypes) and 15 specimens (including lectotype and seven paralectotypes), respectively (listed in Motomura, 2003, and listed in Materials and methods earlier).

Polynemus bidentatus is similar to P. melanochir dulcis (Lake Tonle Sap) and P. m. melanochir (Mekong River basin and Kalimantan) in overall body appearance (see Motomura & Sabaj, 2002: Figs. 1 , 3). In addition, they share an unusual character for Polynemus , i.e. black ventroposterior portion of the pectoral fin (no melanophores in other congeners, except for P. paradiseus which is occasionally tinged with black), and their meristic characters completely overlap (see Motomura & Sabaj, 2002: table 3). However, in addition to the shape of the vomerine patch, P. bidentatus differs from the latter two subspecies in having teeth on the vomer (vs. teeth absent from the vomer in the latter two subspecies), longer snout length (6.8% SL vs. 5.2–6.0% SL, mean 5.8% SL in P. m. melanochir ), first pelvic-fin soft ray length (17.2% SL vs. 13.5–16.9% SL, mean 15.9% SL in P. m. melanochir ), and upper caudal-fin lobe length (38.1% SL vs. 34.0–35.3% SL, mean 34.8% SL in P. m. dulcis), shorter third anal-fin soft ray length (15.8% SL vs. 17.1–19.5% SL, mean 18.3% SL in P. m. dulcis), and smaller orbit diameter (2.2% SL vs. 3.2–3.6% SL, mean 3.3% SL in P. m. dulcis). Data for P. m. dulcis and P. m. melanochir are based on 3 specimens (including holotype and two paratypes) and 40 specimens (including neotype), respectively (listed in Motomura & Sabaj, 2002).

Figures

Fig. 1. Polynemus bidentatus, UMMZ 213346, holotype, 153.4 mm SL, My Tho Province, Vietnam.

Fig. 2. Ventral view of dentition of premaxilla and roof of oral cavity in species of Polynemus. A, P. aquilonaris, URM–P 13930, holotype, 136.1 mm SL, Thailand; B, P. bidentatus, UMMZ 213346, holotype, 153.4 mm SL, Vietnam; C, P. dubius, RMNH 6014, lectotype, 125.8 mm SL, Indonesia; D, P. hornadayi, USNM 100632, holotype, 193.1 mm SL, Malaysia; E, P. kapuasensis, CAS 47198, holotype, 151.1 mm SL, Indonesia; F, P. melanochir dulcis, ANSP 178011, holotype, 134.6 mm SL, Cambodia; G, P. melanochir melanochir, ZRC 37829, neotype, 178.5 mm SL, Malaysia; H, P. multifilis, RMNH 436, holotype, 136.1 mm SL, Indonesia; I, P. paradiseus, NRM 47529, neotype, 197.8 mm SL, India. EC, ectopterygoid; PA, palatine; PR, premaxilla; VO, vomer. Small dots indicate villiform teeth. Scale bars = 5 mm.