Celestichthys margaritatus, Roberts, 2007

Roberts, Tyson R., 2007, The “ Celestial Pearl Danio ”, A New Genus And Species Of Colourful Minute Cyprinid Fish From Myanmar (Pisces: Cypriniformes), Raffles Bulletin of Zoology 55 (1), pp. 131-140 : 132-135

Celestichthys margaritatus new species

( Figs. 1–3 )

Material examined. – Holotype: male, 21.2 mm ( ZRC 50706), Myanmar, isolated small, heavily-vegetated pools at the foot of a mountain near Hopong town 30 km east of Taunggyi, elevation 1,040 m (about 3,420 feet), coll. Kyaw Toe, Oct.2006.

Paratypes: Female, 20.5 mm ( ZRC 50707), same collection data as holotype ; 303 specimens: 14.0– 19.6 mm ( ZRC 50708), same collection data as holotype ; 40 specimens: 14.1–19.3 mm ( CAS 224434 ), same collection data as holotype ; 40 specimens: 13.9–20.5 mm ( USNM 388753 ), same collection data as holotype .

Description. – Dorsal fin rays ii 71 / 2 – 81 / 2, anal fin rays iii 81 / 2 - 101 / 2, caudal fin rays v-i9/8v, pectoral fin rays 11–12, pelvic fin rays 7. Trailing edge of second dorsal fin ray smooth, without serrations. Dorsal and anal fins with convex margins. Caudal fin moderately forked; upper and lower lobes nearly equal, broadly rounded. Danioins have branched dorsal fin ray counts of 6–7 and 8–16, with no previously-described species spanning the interval 7–8 ( Fang, 2003: 722). Dorsal fin origin far anterior to a vertical drawn through anal fin origin and distinctly posterior to a vertical line drawn through pelvic fin origin.

Scales in lateral series 24–25. No scales with lateral line pores. Dorsum scale count from head to dorsal fin origin 14–15 (first 3–4 scales bilateral, rest dorsomedian). Scale rows between dorsal and anal fin origin 7. Circumpeduncular scales 8. Scaly sheath of anal fin with a single row of 5–6 scales. The anterior and posterior circuli and the radii are similar in M. rubescens , “ M.” erythromicron and Celestichthys , but in the latter two the shape of the scale is vertically more elongate than in M. rubescens ( Fig. 2 ).

Gill rakers absent. Pharyngeal arches small but stout. Pharyngeal teeth in three rows with 2/3/5 teeth, as in many

Asian Cyprinidae , including M. rubescens and “ M. ” erythromicron. The teeth are conical.

Head rounded, its length 3.9–4.1 times in standard length. Snout very short, its length only one-half of eye diameter. Eye large, in anterior half of head. Horizontal eye diameter 10 times in standard length. Barbels absent (as in most Danioinae ).

Mouth very small, jaws entirely in front of orbit ( Fig. 3 ). When mouth is closed jaws are oriented at an angle of 45º upward. See discussion for remarks on “danioin notch.”

Nostrils normally developed, anterior nostril with low rounded rim from the posterior margin of which arises the nasal flap ( Fig. 2a ). Area surrounding posterior nostril and immediately posterior to it largely covered with exposed elongate cilia (apparently olfactory cilia) ( Fig. 2 c-d). Such cilia are usually found only on the lamellae of the olfactory rosette inside the nasal capsule (largely covered over by the nostrils and nasal flap).

Body deep and strongly compressed, much more so than in M. rubescens but similar to “ M. ” erythromicron. Greatest body depth (just anterior to dorsal fin) 3.0–3.1 times in SL. Caudal peduncle sexually dimorphic, length 4.8–5.2 times in SL in both sexes, height 6.5 times in SL in males but 7.3 times in SL in females.

Total vertebrae 30–32, with a strong mode at 31 (N=40). Frequencies 30(7), 31(26), 32(7). Abdominal and postabdominal vertebrae often equal or nearly equal in number, with a modal count of 15+16 (n=13). Counts of abdominal vertebrae range from 13 to 16 and of post-abdominal vertebrae from 15 to 17. Post-abdominal vertebrae more numerous than abdominal by 2–3; abdominal vertebrae more numerous than post-abdominal by 1–2 in several specimens.

Tuberculation. – Multicellular keratinous tubercles occur on the head, jaws, dorsal surface of the paired fins and elsewhere on many cyprinids (especially bottom dwellers). Tubercles occur on the head and especially on the jaws including the mandibular flap ventrolateral margin of the mandibles in many Danioinae . These may occur in both sexes (e.g. in Danio rerio ), or only in males (e.g. in Sundadanio axelrodi ) ( Roberts, 1989: 69-70, Fig. 48). Tubercles have not been observed on the head or elsewhere in either sex of Celestichthys margaritatus . No tubercles were found on the mandibular flap any of the male or female specimens examined with SEM ( Fig. 2b ). Taste buds are often present in the sites occupied by tubercles in larger danioins and other cyprinids ( Fig. 2c ).

Unculi. – Most Cyprinidae (probably including many Danioinae ) have unicellular keratinous projections or unculi on the ventral surfaces of the paired fins and on the lips and/ or horny sheaths of both jaws. Unculi also often project from the surfaces of cyprinid multicellular tubercles. No unculi were found on any of the male or female specimens of C. margaritatus examined with SEM.

Eggs. – Dissection of several 18 to 19 mm females revealed eggs of different size classes. The largest eggs are 1.0– 1.3 mm in diameter, slightly smaller than the pearl spots on the flanks of the fish. The number of eggs of the largest size is very few, about a half-dozen. If this is typical of wild fish in spawning condition, it indicates that spawning events are partial and sporadic or partial, with very few eggs fertilized at a time, and reproductive activity extends over a long period of time, perhaps throughout the year.

Vertebral column. – As elsewhere in the skeleton, stress on the vertebral column apparently results in significantly higher levels of calcification, which show up in radiographs as brightened areas. As usual in cyprinids, the anterior half of the centrum of each abdominal vertebra where it articulates with the ribs is particularly well mineralized (pers. obs.). Of more interest is that the centrum of the third post-abdominal vertebrae is often strongly calcified. This sample of 40 specimens includes six in which the radiograph shows that the third abdominal vertebrae is much denser than the rest of the vertebrae. One specimen has the posterior half of the centrum of the second abdominal vertebra denser, and one has the centrum of the seventh abdominal vertebra denser. None of the 40 specimens have the centrum of any other vertebrae similarly affected. Similarly modified postabdominal vertebrae at about the same position occur in specimens examined of “ M.” erythromicron but at lower frequencies. For vertebral counts in the latter two taxa see Discussion.

Colouration. – This new species is distinguishable from all other known species by its extraordinary colouration. Most of the body is deep blue, steely blue or greenish blue with numerous white, cream-coloured, pearly pink, or goldeniridescent oval spots. The dorsum, covered by the dorsomedian scale row and the first scale row immediately below it on either side, is bronze or greenish from the head to the end of the body. The ventrum, covered by the ventro-median scale row and one or two scale rows on either side of it, is rosy or reddish from the head to the end of the body. In males, the white-spotted bluish colour of the flanks covers the sides of the anterior part of the abdomen. In females, this part of the external body wall is white or cream. The dorsal, anal, and caudal fins in both sexes, and also the pelvic and to a lesser degree the pectoral fins of the males, have broad cinnabar red and black slashes. See Discussion for comparisons of colouration in other danioins.

Variation in the disposition of the spots, especially in somewhat smaller fish, indicates that they develop from (and therefore presumably evolved from) continuous or nearlycontinuous longitudinal pale stripes of the sort seen in many danioins. In some of these danioins, the longitudinal stripes occasionally break up into smaller units suggestive of the numerous small oval spots in Celestichthys (but far fewer).

In reproductively active males the abdomen reddens. The dorsum of males has a pale stripe, and the prominence of this is enhanced when males are showing courtship colours. It is reminiscent of the “blaze” shown in courting male rainbowfishes, Melanotaenia spp. , and appears to be used in a similar fashion during the “head-down” display, although whereas in Melanotaenia it is the “blaze” which becomes stronger in appearance, in the C. margaritatus it is the flanks which darken and therefore increase the contrast with this area (Peter Liptrot, pers. comm.). Even small males show intense breeding colour and court females.

Sexual dimorphism. – In addition to the differences in colouration, fin shapes, and caudal peduncle depth mentioned in the species description above, adult males have a darklypigmented “mandibular pad” with a rounded, weakly crenulate distal margin on the anteroventral surface of each mandible ( Fig. 3 ). This apparently is homologous with a similar mandibular structure, sometimes bearing multicellular keratinous tubercles, present in both sexes or in males only of some danioins (e.g., Danio rerio , Sundadanio axelrodi ). The structure is absent or much reduced and unpigmented in female Celestichthys . Scanning electron microscopy observations of the mandibular pad in a 16.4 mm male revealed that its free margin is non-tuberculate. Examination of a female revealed that the pad is either absent or considerably reduced.

Etymology. – The generic name Celestichthys , gender masculine, is from the Latin caelestis, “heavenly”; and Greek ichthys, masculine, “fish”. The species or trivial name margaritatus is Latin for “adorned with pearls.” Used as a noun in apposition.

Figures

Fig. 1. Celestichthys margaritatus, new species. Above: holotype, 21.2 mm mature male (ZRC 50706); below: paratype, 20.5 mm mature female (ZRC 50707). Images: Kamphol Udomritthiruj.

Fig. 2. Celestichthys margaritatus, male, 16.4 mm (scanning electron micrographs): a, head in side view showing nostrils, eye, and jaws (scale bar = 500 μm); b, left half of lower jaw with mandibular flap; c, row of taste buds on lower jaw (scale bar = 50 μm) (similar taste buds present on lips, barbels, and widely scattered on the head); c and d, exposed olfactory cilia in shallow depression posterior to nostrils (scale bars = 20 μm and 5 μm).

Fig. 3. Scale from mid-body slightly anterior to dorsal-fin origin: a, Celestichthys margaritatus, 18.5 mm; b, Microrasbora rubescens, 22.8 mm. Scales of “Microrasbora” erythromicron are similar to those of C. margaritatus.