Boraras naevus, Conway, Kevin W. & Kottelat, And Maurice, 2011

Conway, Kevin W. & Kottelat, And Maurice, 2011, Boraras naevus, a new species of miniature and sexually dichromatic freshwater fish from peninsular Thailand (Ostariophysi: Cyprinidae), Zootaxa 3002, pp. 45-51 : 46-49

Boraras naevus , new species

Figure 1

Holotype. ZRC 53120, male, 10.6 mm SL; Thailand: Surat Thani Province: swamp, East of road North of Amphoe Tha Chana, 83 km before Surat Thani on road from Lang Suan; M. Kottelat et al., 4 March 2001.

Paratypes. CMK 16459, 255; ZRC 53121, 50; BMNH 2011.8.3.1-20, 20; NRM 61765, 20; TCWC 15185.01, 4(c&s), 10.2–12.7 mm SL; same data as holotype.

Diagnosis. Boraras naevus is distinguished from its congeners with a similar blotched color pattern (viz. B. maculatus and B. micros ) by pronounced sexual dimorphism of the anteriormost blotch situated on the body side, which is a small circular marking of roughly orbit size or smaller in females and a large dorso-ventrally orientated oval-shaped marking larger than the orbit in males (vs. anteriormost blotch similar in size and shape in both sexes) and by its lower number of principal caudal-fin rays (9+8 vs. 9+ 9 in B. micros , 9–10+ 9 in B. maculatus ). It is further distinguished from B. maculatus by its lower number of body scales in the midlateral row (24–26 vs. 26–29), and its lower number of pelvic-fin rays (i.5. i vs. i.6. i), and from B. micros by its higher number of body scales in the midlateral row (24–26 vs. 22–23), higher number of branched dorsal-fin rays (7 vs. 5–6), the presence of red and black pigment along the anterior edge of the dorsal and anal fins of males in life (vs. all fins transparent without red or black pigmentation in life in both sexes), the presence of infraorbital 4 (vs. infraorbital 4 absent), infraorbital 2 contacting both infraorbital 1 and infraorbital 3 (vs. infraorbital 2 greatly reduced in size, without contact with adjacent infraorbital bones), and the presence of the mesocoracoid (vs. mesocoracoid absent). Boraras naevus can be distinguished from the remaining species of Boraras ( B. brigittae , B. merah and B. urophthalmoides ) by the absence of a midlateral stripe (vs. broad, uninterrupted midlateral stripe from upper extremity of gill opening to middle of caudal-fin base in B. brigittae and B. urophthalmoides ; or narrow, frequently interrupted midlateral stripe from above anal-fin origin to middle of caudal-fin base in B. merah ).

Description. General body shape as in Figure 1 . Morphometric and meristic data are listed in Table 1. Miniature species, largest specimen examined 12.7 mm SL (range 10.2–12.7 mm). Head and eye large, snout rounded, mouth small, terminal. Anterior nostril small, oval-shaped, separated from larger posterior nostril by a narrow, low strip of skin. Body compressed, deepest midway between occiput and dorsal-fin origin. Caudal peduncle slender. Dorsal fin origin situated roughly at mid-body, insertion of posteriormost dorsal-fin ray slightly anterior to vertical through anal-fin origin. Pelvic-fin origin situated anterior to vertical through dorsal-fin origin. Pectoral fin insertion situated low on flank. Caudal fin forked, upper and lower lobes rounded, upper lobe slightly longer than lower.

holotype (ZRC 53120) and 10 paratypes (CMK 16459). Values for B. maculatus obtained from CMK 20696, BMNH 1985.12.18:10–

18, 1995.5.17.112–126, USNM 101267 and 229241. Morphometric values for B. micros taken from Kottelat & Vidthayanon (1993)

with additional meristic data obtained from BMNH 2004.4.29.1–3.

Boraras naevus Boraras micros Boraras maculatus Holotype Range Mean St. Dev. Range Range Mean St. Dev.

Standard length 10.6 10.2–12.7 8.7–13.3 11.4–18.5

In percentage of standard length

Branchiostegal rays 3. Pharyngeal teeth arranged in two or three rows, formula 2,4–5,2,1(1) or 1,2,4–4,2,1(2). Infraorbital series composed of infraorbital 1 to infraorbital 4. Infraorbital 1 irregular in shape, its posterior edge rimming anterior margin of orbit. Infraorbital 2 smaller than infraorbital 1, rimming anteroventral margin of orbit ventral to posterior edge of infraorbital 1 and anterior edge of infraorbital 3. Infraorbital 3 largest of series, roughly boomerang-shaped, its dorsal margin rimming much of posteroventral margin of orbit. Infraorbital 4 smallest of series, represented by poorly ossified sliver of dermal bone. Cephalic lateral line sensory system greatly reduced, composed only of short, open segment of canal ossification representing preopercular portion of preopercular-mandibular canal along ventral arm of preopercle. Otic, infraorbital, supraorbital, supratemporal and mandibular portion of preopercular-mandibular canal absent.

Dorsal-fin rays ii.7. i. Anal-fin rays iii.5. i. Principal caudal-fin rays 9+8. Pectoral-fin rays i.6. ii, pelvic-fin rays i.5. i. Dorsal procurrent rays 4(1) or 5(3), ventral procurrent rays 5(4). Total number of vertebrae 29–30, comprising 14 abdominal and 15(1) or 16(3) caudal vertebrae. Caudal-fin skeleton composed of 5 hypural elements, comprising fused parhypural+hypural 1 and hypurals 2–5. Free (second) uroneural absent. Three supraneurals posterior to supraneural 3, representing supraneurals 5–7.

Scales relatively large, cycloid, lacking radii. Scales in midlateral row from upper extremity of gill opening to caudal flexure 24–26. Predorsal scales 11 or 12. Transverse scale rows ½5½ or ½6½. Circumpeduncular scale rows 12.

Coloration in preservative. Body background colour ranging from light cream to light yellow. Body with three black blotches, one situated mid-height of body side, anterior to origin of pelvic fins, one situated at base of branched anal-fin rays (equivalent to the supraanal pigment of Brittan, 1954), and one at center of caudal-fin base. Black blotches made of pigments located in deeper layers of epidermis. Anteriormost blotch in males oval-shaped and larger than orbit, remaining blotches roughly circular in shape and smaller than orbit. All blotches of similar size and shape in females. Dorsal surface of body with weak reticulate pattern formed by heavy scattering of darkbrown pigment along posterior edge of scales. Dorsal reticulation more pronounced in males. Posterior edge of scales situated along body posterior to anteriormost blotch in males with weak scattering of dark-brown melanophores giving appearance of a weak reticulate pattern over posterior half of body side. Dense scattering of darkbrown melanophores concentrated along body side below scales directly anterior and posterior to anteriormost blotch in males, separated from blotch by an area without pigment. Equivalent area in females with weak scattering of dark melanophores. Axial streak pronounced along posterior half of body, not visible on side anterior to dorsal fin origin. Occiput dark brown. All fins with light scattering of dark-brown melanophores across fin membranes between rays. Anterior edge of dorsal and anal fin and base of pelvic fin with intense scattering of dark brownblack melanophores in males. Anterior edge of dorsal fin weakly marked with dark brown-black melanophores in females.

Distribution and habitat. We have examined material from the type locality, in a swampy area north of Surat Thani. The species is reported to have a wider distribution in the lower Tapi drainage, on the Gulf of Thailand slope of peninsular Thailand. It is suspected that most of its natural habitats (swamps) has been transformed into paddy fields. Additional populations, or another, similar species is known from the Andaman Sea slope of the Malay Peninsula near Trang.

Etymology. From the Latin naevus , a spot, a mark on skin, a blemish, in allusion to the large sexually dichromatic blotch on the side of the body. A noun in apposition.

Figures

FIGURE 1. Boraras naevus, Thailand: Surat Thani Province. Above. ZRC 53120, holotype, male, 10.6 mm SL. Below. CMK 16459, paratype, female, 11.7 mm SL.