Nemacheilus cacao, Bohlen & Kottelat & Šlechtová, 2022

Bohlen, Jörg, Kottelat, Maurice & Šlechtová, Vendula, 2022, Nemacheilus cacao, a new species of loach (Teleostei: Nemacheilidae) from the middle Mekong basin in Laos, Raffles Bulletin of Zoology 70, pp. 511-518 : 512-515

Nemacheilus cacao , new species

( Figs. 1–5 , Table 1 )

Holotype. ZRC 62553, 56.9 mm SL; Laos: Khammouan province: Thakhek district: small tributary of Nam Thorn at cave Tham Nang Eng , 17°26′39″N 104°56′55″E; collected 09 June 2013, reared until October 2013 in aquaria.

Paratypes. Laos: Khammouan province: ZRC 62554, 1 , 61.5 mm SL, collected and reared together with holotype ; ZRC 62555‒62558 , 7 , 27.7‒50.4 mm SL; same locality as holotype, March 2018. — CMK 19403 , 1, 32.2 mm SL; Southwest of Gnommalat, Tham Pong cave, near Phou Phathoung quarry, inside cave, 17°32′30″N 105°09′58″E; M. Kottelat et al., 7 May 2006. — CMK 23170 , 1, 36.9 mm SL; Xe Bangfai drainage: Houay Kalo, a small tributary of Nam Oula near Ban Phonexay , 17°20′56″N 105°20′07″E; M. Kottelat et al., 22 May 2012 .

Diagnosis. Nemacheilus cacao is distinguished from all other species of the genus except N. platiceps in having an incomplete lateral line reaching between verticals of pelvicfin origin and of anus, with 33–49 pores (vs. complete). Nemacheilus cacao is most easily distinguished from N. platiceps by its colour pattern, with a uniform dark brown body in adult males. In contrast, N. platiceps has 12–16 narrow bars on the flank, clearly distinct at all sizes and both sexes. In addition, N. cacao is distinguished from N. platiceps by males having: a conspicuous suborbital flap (vs. poorly developed); dorsal surface of first 3–7 pectoral-fin rays covered by densely-set small tubercles; flank with a patch of scales each with a small tubercle (vs. absence of tubercles on pectoral fin and flanks).

Description. See Figs. 1 and 2 for general appearance and Table 1 for morphometric data of holotype and eight paratypes. A moderately elongated nemacheilid loach with body depth about 5–7 times in SL. Body depth gradually increasing until dorsal-fin origin, then decreasing until posterior extremity of anal-fin base, uniform from there to caudal-fin base. Dorsal outline between nape and dorsalfin origin slightly convex. Head slightly depressed; body moderately compressed behind head, increasingly compressed until caudal-fin base. Interorbital area flat or nearly flat. In lateral view, upper margin of eye flush with dorsal profile of head. Cheeks not swollen; snout rounded. Depth of caudal peduncle 1.2–1.5 times in its length. Very short and low ventral crest on posterior end of caudal peduncle, absent or nearly absent dorsal crest. Largest recorded size 61.5 mm SL.

Dorsal fin with 4 unbranched and 9½ branched rays, distal margin straight to slightly convex; second branched ray longest. Anal fin with 3 unbranched and 5½ branched rays, distal margin straight, second branched ray longest. Pectoral fin with 1 unbranched and 11‒12 branched rays, reaching halfway to pelvic-fin base; no axillary lobe. Pelvic fin with 1 unbranched and 7 (6 in 1 specimen) branched rays, reaching half-way to anal-fin base, not or just reaching anus, which is located 1½–2 eye diameter in front of anal-fin origin: axillary lobe small and on whole length attached to body. Caudal fin with 9+8 (8+ 8 in 1 specimen) branched rays, slightly forked, lobes rounded, both lobes same length or upper lobe up to 1.1 times length of lower lobe.

Body entirely scaled. Lateral line incomplete, ending usually below or shortly behind dorsal-fin base, occasionally reaching level of anal-fin origin; visible pores (33–49) only until vertical of anus. Cephalic lateral line system with 5 supraorbital, 4+10–11 infraorbital, 9–10 preoperculomandibular and 3 supratemporal pores.

Anterior nare pierced at tip of an obliquely truncate tube. Posterior nare adjacent to anterior one. Mouth moderately arched, gape about twice wider than long ( Fig. 3 ). Upper lip moderately thick, without median notch, with numerous wrinkles. Processus dentiformis present. Lower lip with narrow median interruption; median part with very weakly marked sulci; thinner towards rictus. Barbels long; inner rostral barbel reaching beyond vertical through middle of eye, outer one and mandibular barbels reaching beyond eye.

Sexual dimorphism. Adult males with suborbital flap ( Fig. 4 ) and with uniform brown flank (vs. females and juveniles with narrow brown bars). Adult males (= males with suborbital flap; 48–62 mm SL) with patch of tubercles on flank under dorsal-fin base, on upper surface of pectoralfin rays, and scattered tubercles on postorbital area of head and nape ( Fig. 5 ).

Extent of patches of tubercles on flank variable, reaching longitudinally from tip of adpressed pectoral fin to end of anal-fin base, single tubercles even until caudal-fin base. Shape of patch irregular, usually elongated oval ( Fig. 5a ). In dorsoventral direction, patch stretching across 5 rows of scales above lateral line and up to 10 rows below, highest under dorsal fin and shortly behind. In this patch, each scale with a single large, white and triangular tubercle ( Fig. 5b, c ). Tubercles on pectoral fin much smaller than those on body, white, conical, in 1–5 rows on dorsal side of rays and on unculiferous pad behind; stretching to tip of ray on unbranched and first branched ray, with decreasing length on following branched rays until only on base of third-last ray ( Fig. 5d, e ). Last two branched rays without tubercles. Tubercles on sides of head small, white, conical, only tip pointing out of skin. Tubercles on body, pectoral fin, and head either all present or all absent.

Colouration. Nemacheilus cacao undergoes a remarkable ontogenetic and sexual change of colouration. A juvenile of 27.7 mm SL had 22 dark brown bars on body meeting their contralaterals on back, their width at least twice width of interspaces, some vertically split. In females, most bars split and fade with increasing size, resulting in a pattern of numerous narrow brown bars along body and broader dorsal saddles. In adult males, bars and saddles absent and whole fish with uniform chocolate brown body and head, only ventral side of head and body white. A faint dark stripe from snout to eye in most specimens, no black bar on caudal-fin base. Phylogenetic position. The topologies of the single gene trees were congruent; therefore, the data have been combined.

The analysis of the concatenated dataset estimated the phylogenetic position of N. cacao as a member of the genus Nemacheilus , member of the Selangoricus clade and sister species to N. platiceps ( Fig. 6 ). Genbank accession numbers for the sequences of the holotype (ZRC 62553) and paratype (ZRC 62554) are: ON720270 and ON720269 for cytochrome b, ON720272 and ON720271 for RAG-1.

Etymology. From Theobroma cacao , the cacao tree whose seeds are used to produce chocolate; itself derived from kakawa in some ancient Mesoamerican language ( Kaufman & Justeson, 2007). A reference to the chocolate brown colour of large males. A noun in apposition, indeclinable.

Figures

Fig. 1. Live specimen of Nemacheilus cacao, new species, ZRC 62554, paratype, male, 61.5 mm SL; Laos: Khamouane province: Thakkhet district: small tributary of Nam Thorn at cave Tham Nang Eng (Photo: J. Kühne).

Fig. 2. Nemacheilus cacao, new species, Laos: Khammouane prov: Thaket district: small tributary of Nam Thorn at cave Tham Nang Eng. a, ZRC 62553, holotype, 56.9 mm SL, adult male; b, ZRC 62558, paratype, 50.4 mm SL, adult female; c, ZRC 62556, 27.7 mm SL, juvenile.

Fig. 3. Nemacheilus cacao, new species, ZRC 62553, holotype, 56.9 mm SL; mouth in ventral view.

Fig. 4. Nemacheilus cacao, new species, ZRC 62553, holotype, 56.9 mm SL; left suborbital flap in lateral view.

Fig. 5. Tubercles on body side and pectoral fin of large male Nemacheilus cacao, new species, ZRC 62553, holotype, 56.9 mm SL. A, red outline depicts field of tubercles on body side; B–C, area of main tuberculation enlarged; D, dorsal view on right pectoral fin; E, tubercles on pectoral fin enlarged.

Fig. 6. Bayesian tree of the genus Nemacheilus based on the concatenated datasets of mitochondrial cytochrome b and nuclear RAG-1 marker. Datasets from Šlechtová et al. (2021) and Sember et al. (2015) plus two specimens of Nemacheilus cacao, new species. The tree shows N. cacao to be the sister species to N. platiceps and both to be located at the base of the Selangoricus clade within Nemacheilus. The outgroup contains 25 nemacheilid species from genera other than Nemacheilus and one species from the family Cobitidae.