Oryzias minutillus, SMITH, 1945

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 : 574-576

ORYZIAS MINUTILLUS SMITH, 1945

DWARF MEDAKA

FIGURES 11D, 20C , 48

Oryzias minutillus Smith, 1945: 424–425 , fig. 95. [type locality: Thailand: Bangkok].- Rosen, 1964: 227 [systematics].- Scheel, 1969: 5–7, fig. 1 [behaviour, reproduction, characters].- Schrey, 1978: 338 [taxonomy of Oryzias ].- Magtoon & Uwa, 1985: 157–160 [karyotype, relationships].- Magtoon, 1986: 859–865 [distribution in Thailand, relationships, karyotype].- Uwa, 1986: 867–875 [cytogenetic comparisons].- Uwa & Magtoon, 1986: 475–477 [comparison with O. mekongensis ].- Ashida & Uwa, 1987: 1003 [karyotype polymorphism].- Uwa et al., 1988: 332–340 [karyotype, distribution, China].- Uwa & Parenti, 1988: 159 [morphometric and cytogenetic comparisons].- Chen et al., 1989: 239–246 [comparison with O. latipes sinensis , and report from Yunnan Province, China].- Chen, 1990: 227–228 [distribution in Yunnan].- Collette et al., 1992: 3 [USNM type specimens].- Magtoon et al., 1992: 489–497 [karyotype and geographical variation].- Takata et al., 1993: 319–326 [genetic differentiation].- Hamaguchi, 1996: 757–763 [description and comparison of testis structure].- Seegers, 1997: 15, 21 [listed, photographs].- Roberts, 1998: 219–220 [characters, relationships, distribution].- Kottelat, 2001b: 143, fig. 406 [characters, distribution].

Oryzias sp. undet.- Roberts, 1998: 220 [specimens from Menghai, southern Yunnan].

Differential diagnosis: Oryzias minutillus is a miniature species (maximum size of specimens examined 17 mm, but with most adults known less than 14 mm) hypothesized to be most closely related to two other miniatures, Oryzias uwai and O. setnai , with which it shares the anterior portion of the anal fin with elongate rays set off from the rest of the fin; a medial extension of the ethmoid cartilage; and principal caudal-fin rays number i,3/4,i, the lowest for all ricefishes. It shares with O. setnai five, rather than six pelvic fin-rays, but lacks the many autapomorphies of that species. Four miniatures, O. minutillus , O. pectoralis , O. setnai and O. uwai , share the characters of a pigmented anal or urogenital region and an elongate, rounded caudal fin.

Description: Miniature, maximum size of specimens examined 17 mm SL. Body compressed laterally, slender, body depth 18–19 [18]. No pronounced abdominal concavity between pelvic fins and anal fin. Mouth terminal, lower jaw projecting slightly beyond upper jaw. Dorsal body profile relatively straight from head to dorsal-fin origin; ventral body profile slightly convex from head to anal-fin origin. Dorsal surface of head slightly convex just anterior to orbits. Head length 21–25 [21]; snout length 4–5 [4]; eye moderate, 7–8 [7] orbits do not project beyond dorsal surface of head. Single-lobed testis on right side of body of males. Basal portion of dorsal fin projects slightly beyond primary body profile. Scales relatively large, cycloid; 26–29 in a lateral series. Dorsal and pectoral fins elongate, anal fin slightly rounded; anal-fin rays without bony contact organs. Medialmost pelvic-fin ray connected to body via a membrane along its proximal half; pelvic fins relatively small, do not extend to anal-fin origin. Caudal fin rounded. Male with a short, tubular urogenital papilla; female with small, slightly bilobed urogenital papilla.

Premaxilla short and broad with barely distinct ascending process; premaxilla and dentary with a single irregular row of caniniform teeth; no large canine teeth on lateral ramus of the premaxilla or dentary. No preethmoid cartilage; mesethmoid cartilaginous or weakly ossified, when ossified, mesethmoid small and 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 first vertebra; lateral process of pelvic bone in close association with third or fourth pleural rib. Caudal skeleton with two epural bones; one ventral accessory bone and one accessory cartilage. Anteriormost dorsal and ventral procurrent rays hooked at base. Fifth ceratobranchial toothplate triangular, with teeth in irregular rows anteriorly, followed by two discrete rows of unicuspid teeth, and no incomplete posterior row. Basihyal bone 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 17–21 [19]. Pelvic-fin rays 5. Pectoral-fin rays 7–8. Principal caudal-fin rays i,3/4,i. Procurrent fin-rays, dorsal 4, ventral 6. Vertebrae 24–29 (8–11 + 16–18). Branchiostegal rays 4–5.

Cytogenetic data: Oryzias minutillus is a highly variable species cytogenetically as demonstrated by Magtoon et al. (1992: table I) who reported variation among 18 populations from throughout Thailand (see Table 2). Diploid chromosome number and constitution varies from 28, comprising seven large metacentric, one submetacentric and six acrocentric pairs, to 42, comprising 21 acrocentric pairs. Genome size is 1.5 pg of DNA per nucleus in all reported populations ( Magtoon & Uwa, 1985; Uwa, 1986; Table 2). Chromosome arm number (NF) varies from 42 to 44. Oryzias minutillus was grouped first with other species that have a fused chromosome constitution because populations from Bangkok and Chiang Mai have large metacentric (= fused) chromosomes (Uwa, 1986; Table 2). Examination of populations from throughout Thailand led to the recharacterization of O. minutillus as having a monoarmed chromosome constitution because of the predominance of acrocentric chromosomes in populations from Phuket and Menghai ( Uwa, 1991b). In the phylogenetic analysis, it is coded as polymorphic for chromosome constitution.

Colour in life: Body translucent, and with melanophore pattern as described below in alcohol. Dorsal surface of eye green in males, golden in females ( Scheel, 1969). Females with a subrectangular, males with a smaller, subtriangular silvery peritoneum and both sexes with a silvery operculum.

Colour in alcohol: A diffuse row of melanophores 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 that continues onto the caudal fin on the membrane just dorsal and ventral to the first ray above and below the midline, respectively. Females with a subrectangular, males with a smaller, subtriangular blackish peritoneum. A faint black line along the anal-fin base. Perianal melanophores in both sexes. Dorsal and anal fin interradial membranes with scattered melanophores. Abdomen black or silvery.

Distribution and habitat: Chao Phrya basin and Salween basins, Thailand, Mekong basin in northern Thailand and Kampuchea, and Yunnan Province, China, in clear water swamps ( Kottelat, 2001b).

Remarks: Roberts (1998: 220) considered the specimens from Menghai, Yunnan, to represent an undescribed species because they are larger than most other O. minutillus , and differ in cytogenetic and some morphological characters. I continue to refer these specimens to O. minutillus because their proportions and counts, as reported by Uwa et al. (1988: 335), are consistent with O. minutillus populations from Thailand. Furthermore, their diploid chromosome number (42) as reported by Uwa et al. (1988) is consistent with the chromosome numbers of all O. minutillus populations ( Table 2). A karyotype of 42 acrocentric chromosomes was considered ‘basic’ for this species, with reductions resulting from pericentric inversions and centric fusions (Uwa et al., 1988; Takata et al., 1993). Morphometric and meristic data are supplemented by those in Uwa & Parenti (1988). Another common name for this species is Thaimedaka ( Magtoon et al., 1992).

Material examined: 86 specimens (6.5–17 mm SL).

Holotype. THAILAND. Central Thailand : small canal in Bangkok, USNM 107958 , 1 (female, 14.0 mm), H.M. Smith, 10.v.1934.

Paratypes. THAILAND. Central Thailand: small canal in Bangkok, USNM 109789 , 4 (1 male and 3 females, 12.2–13.3 mm), collected with the holotype. Non-type specimens. CHINA. Yunnan Prov.: Menghai, Xishiangbanna , CAS 60237, 4 (13.2–17 mm), H. Uwa, R.- F. Wang & Y.- R. Chen, 24.viii.1986 .

THAILAND. GVF reg. 1540, sta. 82 , CAS 40758, 1 (16 mm) , R. R. Rofen, 24.xi.1957; Chiang Mai , CAS 58023, 5 (8.0–11.0 mm, 2 of which have been cleared and counterstained) , W. Magtoon & H. Uwa, 25.iv.1984; Bangkhen , CAS 58022, 19 (6.5–12.1 mm, 4 of which have been cleared and counterstained) , W. Magtoon, 16.i.1983; Nakhon Sithammarat Prov. at Thungsong , CAS 60741, 9 (11.5–13.0 mm) , W. Magtoon & H. Uwa, 10.xii.1984; W of Bangkok, Klong Ta Pa at Ban pong , USNM 229287 , 2 (10–12.5 mm) , T. R. Roberts & P. Wongrat, 15.iv.1973. Phuket Is., Teacher’s College , CAS 58025, 11 (10.0– 13.5 mm, 2 of which have been cleared and counterstained) , N. Tawimsunnuk, H. Uwa & W. Magtoon, 2.ii.1985; Trat Prov., Ban Pret Nok, about 17 Km SSW of Trat , ZRC 35689–35718 , 30 .

Figures

Figure 20. A, Oryzias setnai, USNM 277482, 14.5 mm SL; B, O. uwai, CAS 92310, 12 mm SL; C, O. minutillus, CAS 58022, 11 mm SL. Anterior portion of skull and jaw and jaw suspensorium. Arrow points to the medial anterior extension of the ethmoid cartilage. Scale bar = 0.5 mm.

Figure 48. Oryzias minutillus Smith, 1945, Bangkok, Thailand, USNM 107958, holotype, female, 14.0 mm SL.