Oryzias mekongensis, UWA & MAGTOON, 1986

Parenti, Lynne R., 2008, A phylogenetic analysis and taxonomic revision of ricefishes, Oryzias and relatives (Beloniformes, Adrianichthyidae), Zoological Journal of the Linnean Society 154 (3), pp. 494-610 : 573-574

ORYZIAS MEKONGENSIS UWA & MAGTOON, 1986

MEKONG MEDAKA

FIGURES 6, 11E, F, 15A , 47

Oryzias sp. - Kottelat, 1985: 272 [report of an apparently new species from Tonlé Sap, Phnom Penh, Kampuchea].- Magtoon, 1986: 859–865 [report of undescribed species from Thailand].- Uwa, 1985a: 3 [photograph of live specimens; distribution].- Uwa, 1985b: 7 [distribution, cytogenetic comparisons].- Uwa, 1986: 867–875 [cytogenetic comparisons; Yang Talat, Thailand].

Oryzias mekongensis Uwa & Magtoon, 1986: 474–475 , figs 1–3 [type locality: Thailand: Mekong basin, Kalasin Prov., Yang Talat].- Iwamatsu, 1986: 99–109 [comparative anatomy].- Uwa, 1986: 867–875 [cytogenetic comparisons].- Uwa & Parenti, 1988: 159 [morphometric and cytogenetic comparisons].- Collette, Parin & Nizinski, 1992: 3 [USNM type specimens].- Hamaguchi, 1996: 757–763 [description and comparison of testis structure].- Rainboth, 1996: 172, figure [Mekong].- Roberts, 1998: 220–221 [characters, relationships, distribution].- Kottelat, 2001b: 143, fig. 405 [characters, distribution].

Differential diagnosis: Oryzias mekongensis is a miniature species (largest specimen examined 16.7 mm) of the biarmed chromosome group of Uwa (1986), along with O. luzonensis , O. latipes , O. curvinotus and the miniature O. sinensis , that have anal-fin rays of approximately the same length, forming a ‘parallelogram-shaped’ fin, and chromosome arms numbering 58 or more. Oryzias mekongensis is distinguished from all other ricefishes by a low number of anal-fin rays, ranging from 13 to 18, and bright orangish-red, rather than yellow, submarginal bands on the dorsal and ventral portions of the caudal fin in life. Oryzias mekongensis and O. sinensis both have the first pleural rib on the second, rather than the third, vertebra. They are like O. latipes and O. luzonensis , and differ from Oryzias curvinotus , by having bony processes on the pectoral-fin rays.

Description: Miniature, maximum size of specimens examined 16.7 mm SL. Body compressed laterally, slender, body depth 17–18 [17]. No pronounced abdominal concavity between pelvic fins and anal fin. Mouth terminal, jaws subequal or lower jaw projecting slightly beyond upper jaw. Dorsal body profile straight from head to dorsal-fin origin; ventral body profile straight from head to anal-fin origin. Dorsal surface of head slightly convex just anterior to orbits. Head length 24–25 [24]; snout length 4–5 [4]; eye moderate, 7–9 [8], orbits meet dorsal surface of head. Single-lobed testis on right side of body of males. Basal portion of dorsal and anal fin do not project significantly beyond primary body profile. Scales relatively large, cycloid; 29–32 in a lateral series. Somewhat filamentous dorsal- and anal-fin rays in males; anal-fin rays without bony contact organs or with a few, small contact organs on posterior anal-fin rays. Medialmost pelvic-fin ray connected to body via a membrane along its proximal portion. Caudal fin elongate and rounded. Male with a short, tubular urogenital papilla; female with small, slightly bilobed urogenital papilla.

Premaxilla short and broad with distinct ascending process; premaxilla and dentary with two irregular rows of caniniform teeth; males with two or three enlarged posterior teeth on the premaxilla and dentary; tooth tips project through lips. No preethmoid cartilage; ossified portions of mesethmoid suboval; ethmoid cartilage rectangular with anterior projection. No flanges on the ventral surface of the palatine and the quadrate. Dorsal ramus of hyomandibula not distinctly bifid, single cartilage articulates with sphenotic and pterotic. Lacrimal sensory canal carried in open bony groove. First pleural rib on parapophysis of second vertebra; first epipleural bone attaches to parapophysis of first vertebra dorsal to, and not in horizontal line with, posterior epipleural bones; lateral process of pelvic bone attaches to fourth pleural rib. Caudal skeleton with two epural bones; one ventral accessory bone and a second accessory cartilage or bone. Fifth ceratobranchial toothplates subtriangular, with teeth in irregular rows anteriorly, followed by six discrete rows of unicuspid teeth, including a small, incomplete posterior row. Basihyal bone relatively short and triangular, basihyal cartilage extremely elongate and rectangular. Epibranchial elements fully ossified; epibranchial 2 notably smaller than the other epibranchial elements.

Dorsal-fin rays 5–7 [6]. Anal-fin rays 13–18 [15]. Pelvic-fin rays 5–6. Pectoral-fin rays 6–8. Principal caudal-fin rays i,4/5,i. Procurrent fin-rays, dorsal 3–4, ventral 4–5. Vertebrae 27–31 (10–11 + 17–20). Branchiostegal rays 4–5.

Cytogenetic data: Oryzias mekongensis has a biarmed chromosome constitution. Diploid chromosomes number 48, with one metacentric, four submetacentric, 12 subtelocentric and seven acrocentric pairs. Chromosome arms number 58; genome size is 1.5 picograms of DNA per nucleus. ( Table 2).

Colour in life: Body translucent, with brownish yellow mottling overall and bright orange submarginal bands on the dorsal and ventral portions of the caudal fin. Females with a subrectangular, males with a smaller, subtriangular silvery peritoneum and both sexes with a silvery operculum.

Colour in alcohol: Ground colour pale yellow, a black line from the dorsal surface of the head to the dorsal-fin origin, a midlateral black line from the head to base of the caudal fin, a black line along the anal-fin base, a black submarginal line on dorsal and on ventral portion of the caudal fin, body covered with scattered melanophores concentrated at the pectoral-fin base. Interrupted brown horizontal bar from eye to tip of lower jaw. Females with a subrectangular, males with a smaller, subtriangular blackish peritoneum.

Distribution and habitat: Widely distributed throughout the Mekong basin in north-east Thailand, Laos and Kampuchea, in clear water swamps ( Kottelat, 2001b). This species may be associated with acidic waters ( Roberts, 1998: 220).

Remarks: Additional paratypes listed in the original description, but not examined by me, include CTNRC 42.2612-2620 (9), 42.2621 (1), 42.2622 (1) and NSMT-P 23233-38 (6), 23239-44 (6). Morphometric and meristic data are supplemented by those in Uwa & Parenti (1988).

Material examined: 99 specimens (9–16.7 mm SL).

Holotype. THAILAND. Kalasin Prov.: Yang Talat , USNM 268540 , male, 13.0 mm, W. Magtoon & H. Uwa, 19.iv.1984.

Paratypes. THAILAND. Kalasin Prov.: Yang Talat , USNM 268541 , 3 (1 male and 2 females, 11.4– 13.1 mm), W. Magtoon & H. Uwa, 19.iv.1984 .

Non-type specimens. THAILAND. Kalasin Prov.: Yang Talat : CAS 58030, 40 (10.0–14.0 mm, 4 of which have been cleared and counterstained), W. Magtoon & H. Uwa, 19.iv.1984 ; Srisaket , CAS 58027, 4 (14.0– 15.5 mm, 2 of which have been cleared and counterstained), W. Magtoon, 16.ix.1984 , pres. 6.xii.1985. Sakon Nakhon Prov.: Nong han just NE of Nakhon, USNM 246920 , 5 (10–15 mm), 7.i.1971 .

LAOS. Vientiane: AMNH 56084 , 43 (9–12 mm), C. W. Heckman, 6.i.1970 , rice paddy at kilometre 10, highway 13, NW of Vientiane, AMNH 56082 , 2 (9.5–12 mm), C. W. Heckman, 7.v.1972 , vicinity of Vientiane, aquarium specimen, AMNH 56083 , 1 (16.7 mm), C. W. Heckman,. vii.1971 .

Figures

Figure 15. A, Oryzias mekongensis, USNM 268540, holotype, male, 13.0 mm SL. B, Oryzias pectoralis, CAS 92321, paratype, male, 20.1 mm SL. Arrows point to blackish spot at pectoral-fin base in each species.

Figure 47. Oryzias mekongensis Uwa & Magtoon, 1986, Yang Talat, Thailand, USNM 268540, holotype, male, 13.0 mm SL.