Phallostethus cuulong, Shibukawa, Koichi, Tran, Dinh Dac & Tran, Loi Xuan, 2012

Shibukawa, Koichi, Tran, Dinh Dac & Tran, Loi Xuan, 2012, Phallostethus cuulong, a new species of priapiumfish (Actinopterygii: Atheriniformes: Phallostethidae) from the Vietnamese Mekong, Zootaxa 3363, pp. 45-51 : 46-51

Phallostethus cuulong , new species

New Vietnamese name: Cá bụng đầu ( Figures 1–4 )

Holotype. ZRC 53233, male, 24.2 mm SL, a branch of Hau River (a distributary of Mekong), Cu Lao Dung, Soc Trang Province, Vietnam (9°30.8’ N, 106°13.7’ E), 0–0.5 m depth, 31 July 2009, collected by K. Shibukawa.

Paratypes. Total eight specimens (five males and three females), 20.0– 24.5 mm SL: CTU-P 2327, 1 specimen (female), 23.7 mm SL, Duyen Hai, Tra Vinh Province, Vietnam (9°40.9’ N, 106°30.7’ E), 0.3–0.8 m depths, 5 April 2009, collected by K. Shibukawa, V.V. Tran and L.X. Tran; CTU-P 2494, 1 specimen (male), 22.5 mm SL, My Thanh River (a distributary of Mekong), Vinh Chau, Soc Trang Province, Vietnam (9°22.7’ N, 106°0.7’ E), 0.5–1.2 m depths, 1 August 2009, collected by H.P. Ha, V.V. Tran and L.X. Tran; CTU-P 5020, 1 specimen (male, cleared and stained), 23.5 mm SL, Cho Lach, Ben Tre, Vietnam (10°10.5’ N, 106°.8.9’ E), 0.5 m depth, 3 February 2010, collected by L.X. Tran; NSMT-P 106664, 1 specimen (male), 20.0 mm SL, collected with CTU-P 2494; NSMT-P 106665, 1 specimen (female), 22.0 mm SL, collected with CTU-P 2494; USNM 404477, 1 specimen (female), 23.8 mm SL, collected with CTU-P 2494; USNM 404478, 1 specimen (male), 20.3 mm SL, Cau Ke, Tra Vinh Province, Vietnam (9°57.1’ N, 106°1.8’ E), 0.5–3.5 m depths, 28 May 2010, collected by L.X. Tran; USNM 404479, 1 specimen (male, cleared and stained), 24.5 mm SL, collected with CTU-P 5020.

Diagnosis. Phallostethus cuulong is distinguished from congeners in having following characters: seven serrae on the second ctenactinium in adult males (vs. five and eight in P. dunckeri and P. l e h i, respectively); 11–13 pectoral-fin rays (vs. 9–10 and 12 in P. dunckeri and P. l e h i, respectively); 11–14+25-26=37–40 vertebrae (vs. 13+27=40 and 12+28= 40 in P. dunckeri and P. l e h i); approximately 5–19 teeth on paradentary (vs. 15–20 and 28 or more in P. dunckeri and P. le h i, respectively). All six examined males are dextral (vs. one and two known males of P. dunckeri are sinistral and dextral respectively, and all four know males of P. l e h i are sinistral).

Description. Counts of the holotype are asterisked, and the frequency of each count is given in parentheses following the relevant count. Dorsal-fin rays 7 (2), 8* (6) or 9 (1); anal-fin rays 24* (4), 25 (1), 26 (3) or 27 (1); pectoral-fin rays 11 (2), 12* (7) or 13* (9); scales in lateral series 34 (5), 35* (10) or 36 (3); predorsal scales 1+25 (1), 2+25 (1), 2+26 (4) or 2+27* (3); transverse scales 6 (3), 7* (12) or 8* (3); circumpeduncular scales 12* (8) or 13 (1); paradentary teeth approximately 5 (1), 6 (3), 7 (3), 8 (3), 10 (1), 13* (1), 14 (2), 15* (1), 18 (2) or 19 (1).

The following measurements are % of SL: head length 22.1–24.1; snout length 7.0–8.5; eye diameter 6.7–7.3; interorbital width 3.3–5.2; length of jaw 8.0–9.4; predorsal length 78.0–82.6; preanal length 46.4–48.7; maximum body depth 15.0–18.7; body depth at anal-fin origin 12.4–15.0; body width 9.4–14.0; caudal-peduncle length 18.6–20.7; caudal-peduncle depth 5.6–7.6; length of dorsal-fin base 9.1–10.5; length of anal-fin base 32.3–36.7; pectoral-fin length 14.6–19.1; caudal-fin length 20.1–22.4.

Head depressed anteriorly, with flat or barely concave interorbital space. Dorsal surface of head with membranous dome when alive or freshly collected (which can be seen in the cleared and stained specimens, e.g., Fig. 4 ), but shrunken and not apparent in alcohol specimen. Snout rather pointed. Eyes lateral on head, large, diameter slightly less than snout length. Mouth terminal or subterminal; jaws small, barely extend to a level of anterior margin of eye; upper jaws highly protractile. Body compressed, moderately deep. Anus and urogenital openings anterior, ventral to pectoral-fin base. A slightly frayed, fleshy membranous mid-ventral keel between urogenital opening and anal-fin origin. In males, a distinct mid-ventral groove, deepened and widened anteriorly, supports the priapium and mid-ventral keel. Pectoral fin falcate, the uppermost branched ray longest in most specimens; pectoral-fin rays branched, except for the uppermost 1 (uppermost nubby ossicle not included here; see “Materials and Methods” above) and lowermost 1–2 rays unbranched. Pelvic fin absent in males, present but rudimentary in females ( Fig. 3 ). First dorsal fin absent; origin of second dorsal fin at, or slightly before, a level of posterior end of anal-fin base; anterior 2–3 rays simple, whereas the other rays branched. Anal fin with a long base, commencing well before midlength; anterior 2–4 rays simple, whereas the other rays branched. Caudal fin emarginate, symmetrical dorsoventrally.

Male bilaterally asymmetric, dextral; namely, seminal papilla offset to right side of body (= aproctal side), and anus offset to left side of body (= proctal side); a long rod-like toxactinium curved from left to right; a large fleshy pad, the pulvinulus, covers articulation point of toxactinium and aproctal axial bone ( Fig. 3 ).

Scales on body cycloid, moderately large and deciduous; scales on abdomen largest; body entirely scaled, except for pectoral-fin base, mid-ventral groove before anal-fin origin, and mid-predorsal narrow naked space slightly behind occipital region; head and fins naked, except for posterior part of occipital region and basal part of caudal fin with some scales.

Teeth on premaxilla and dentary unicuspid, slightly curved inward. Paradentary slender (as in Phallostethus dunckeri illustrated by Parenti, 1984: 4, fig. 1), with 5–19 minute teeth laterally; teeth on paradentary form a uniserial row or, in some larger specimens, biserial rows; teeth on inner row, if present, much smaller than those on outer row.

Cephalic sensory canals reduced, comprising: two short infraorbital canals (each with terminal pores only) anteriorly and anteroventrally to eye; preopercular canal (with 6–7 pores).

Main external bones in males including a long, curved toxactinium and a short stout ctenactinium with seven serrae dorsally (not including a hook-like distal tip); two smallest males examined (CTU-P 2493 and USNM 404478, 20.0– 20.3 mm SL) bearing 5–6 serrae on ctenactinium, assumued to represent the immature condition (and not included in the diagnosis, above). First pleural rib attached to fifth vertebra in males, fourth in females; first pleural rib in female much shorter than in male. Branchiostegal rays 4.

Color when alive or freshly collected. Body subtranslucent in life, but whitish immediately after death ( Fig. 1 ); a bright white blotch over brain when alive (assumed to fade just after death); iris silvery; minute melanophores scattered on snout, cheek and jaws; a melanophore at angle of lower jaw; a large reddish yellow blotch, slightly smaller than eye, at mid-lateral caudal fin base; male priapium with several large and small melanophores, particularly on the aproctal side (= right side in the new species) just anterior to the base of second ctenactinium; a series of minute black dots along mid-lateral septum of body musculature at least on caudal part of body; inner side of pectoral fin with many melanophores at least dorsally (the area with melanophores much more broader in females than in males); a series of mid-ventral black dots from anal-fin origin to caudal-fin base; other fins transparent.

Color in alcohol. Head and body pale straw-colored; a series of irregular-sized melanophores (several of them dash-like) along midlateral septum of body musculature (at least on caudal part); paired patches of melanophores on anterior part of snout dorsally; many melanophores scattered on head above neurocranium, those posterior distinctly larger than those anterior; a melanophore at angle of lower jaw; a patch of minute gular melanophores; two patches of melanophores at throat and just behind urogenital opening in females; male priapium with several large and small melanophores, particularly on the aproctal side (= right side in this species) just anterior to the base of second ctenactinium; a mid-ventral series of black dots from anal- to caudal-fin bases, each along anal-fin base on interspace between fin rays (continuous and forming a irregular blackish gray line in some specimens); inner side of pectoral fin with many melanophores dorsally (the area with melanophores much broader in females than in males); caudal fin covered by numerous minute melanophores; other fins transparent.

Distribution, habitat and the other notes. Phallostethus cuulong is known from nine specimens, six males and three females, collected from shallow waters (<1.2m depth) around banks of slow-flowing turbid canals and rivers with soft muddy bottoms in Soc Trang and Tra Vinh Provinces, Vietnam.

The first author (KS) observed that a fish, latter designated as one of the paratypes of Phallostethus cuulong (NSMT-P 106665), swam slowly at the water surface around a bank of the slow-flowing tidal canal with dense semi-aquatic vegetation. A bright white blotch on dorsal surface of head was clearly confirmed in the field, but less vivid than that of the sympatric aplocheilid, Aplocheilus panchax (well-known for its reflective “pineal” spot on the top of the head). When the fish was disturbed, it quickly swam a short distance away from the original position; it was subsequently scooped up carefully using a hand net by KS. The species was usually solitary, and collected by hand nets or seine nets.

Like the other phallostethids in the Vietnamese Mekong, this species has never been seen in the fish markets. As far as we aware, all fishes of the family Phallostethidae have no vernacular names in the Vietnamese Mekong (except for the new species herein named), since they are usually overlooked.

Etymology. The specific name, cuulong , is the Vietnamese name of the Mekong delta (Cưu Long), where the type series of the new species was collected. The name, here applied as a noun in apposition, means “nine dragons,” in reference to nine distributaries of the Mekong basin in Vietnam.

Remarks. Following the key to genera of phallostethid fishes by Parenti (1989), the new species is clearly assigned to Phallostethus by having the combination of, e.g., shield-like pulvinulus, large seminal papilla, long toxactinium, membranous dome on dorsal surface of head, 24–27 anal-fin rays, 37–40 vertebrae, serrated ctenactinium, non-projecting lower jaw beyond upper jaw, no first dorsal fin, and 7–9 second dorsal-fin rays. In particular, no other phallostethid genera are known that bear 24 or more anal-fin rays (vs. 22 or less anal-fin rays in the other phallostethids). Within the genus, the new species resembles the Bornean species Phallostethus lehi in sharing 11–13 pectoral-fin rays, but differs in having seven serrae on second ctenactinium in adult (vs. eight in P. l e h i), 25–26 caudal vertebrae (vs. 28), and 6–19 paradentary teeth (vs. 28 or more). All six examined males of the new species are dextral, immediately distinguishing them from sinistral males in P. l e h i. The new species is also distinguished from Phallostethus dunckeri , known only from Johor, Malay Peninsula but presumed to be extinct ( Parenti, 1996), by having seven serrae on second ctenactinium in adults (vs. five in P. d u n c k e r i), 11–13 pectoral-fin rays (vs. 9–10), and 25–26 caudal vertebrae (vs. 27).

Sexual dimorphism in the pleural ribs was reported from Phallostethus lehi by Parenti (1996); according to her, the species has the first pair of pleural ribs on the fifth vertebrae in males, the fourth vertebrae in females. This dimorphism is also found in Phallostethus cuulong . Furthermore, P. cuulong appears to show sexual dimorphism in the number of precaudal vertebrae: all six males examined have 13–14 precaudal vertebrae, as against 11–12 in all three females examined. Although Parenti & Louie (1998) reported similar sexual dimorphism in vertebral counts from four species of Neostethus , hitherto it has never been known from the other species of Phallostethus .

Figures

FIGURE 1. Freshly-collected specimens of Phallostethus cuulong. A) CTU-P 2494, male, 22.5 mm SL, Tra Vinh Province, Vietnam; B) CTU-P 2327, female, 23.7 mm SL, Sóc Träng Province, Vietnam. Photographed and retouched by L. X. Tran and K. Shibukawa, respectively.

FIGURE 2. Preserved specimen of Phallostethus cuulong, showing dorsal (A), lateral (B) and ventral (C) views. ZRC 53233, holotype (male), 24.2 mm SL, Sóc Träng Province, Vietnam. Photographed and retouched by L. X. Tran.

FIGURE 3. Head and anterior part of body of Phallostethus cuulong. A) lateral view of left side of head and body of male, ZRC 53233, holotype, 24.2 mm SL; B) lateral view of right side of head and body of male (same specimen with A); C) lateral view of head and body of female, USNM 404477; D) ventral view of head and body of female (same specimen with C). Specimens are temporally stained with cyanine blue. Photographed and retouched by L. X. Tran and K. Shibukawa, respectively.

FIGURE 4. Lateral view of head and anterior part of body of cleared and stained specimen of Phallostethus cuulong. CTU-P 5020, male, 23.5 mm SL. Photographed by L. X. Tran.