Under an assumed name : the identity of Nanobagrus fuscus ( Popta , 1904 ) clarified ( Actinopterygii : Siluriformes )

Nanobagrus fuscus is redescribed on the basis of fresh material collected from the Mentaya River drainage, Borneo. The identity of material from central Sumatra, the Malay Peninsula and western Borneo previously identified as this species is reidentified here as N. torquatus.


Introduction
The diminutive bagrid catfish genus Nanobagrus is found in river drainages in the Malay Peninsula, Sumatra and Borneo.Comprising of seven species (N.armatus, N. fuscus, N. immaculatus, N. lemniscatus, N. nebulosus, N. stellatus and N. torquatus), members of the genus are typically found in swift-flowing forested streams (Ng, 2010).Nanobagrus is distinguished from other bagrid genera by their miniature adult size (maximum size under approx.45 mm SL), reduced supraoccipital process, reduced anterior nuchal-plate element, and large and prominent posterior cranial fontanel.
Nanobagrus fuscus (reported from Sumatra, the Malay Peninsula and Borneo) and N. torquatus (reported from southern Sumatra) are two very similar-looking species that are distinguished from each other only by their color pattern (Thomson et al., 2008).While identifying fish specimens collected from the Mentaya River drainage in southern central Borneo, significant differences were observed between them and material identified as N. fuscus from Sumatra, the Malay Peninsula and western Borneo.This prompted a reassessment of the identity of material identified as N. fuscus from throughout Sundaic Southeast Asia.The results of this reassess-ment are reported in this study, as is a redescription of Nanobagrus fuscus from the holotype and fresh material from the Mentaya River drainage.

Materials and methods
Measurements were made point to point with digital calipers and data recorded to tenths of a millimeter.Counts and measurements were made on the left side of specimens whenever possible, following Ng & Kottelat (2013).Subunits of the head are presented as proportions of head length (HL).Head length and measurements of body parts are given as proportions of standard length (SL).Unpaired fin-ray and vertebral counts were made from radiographs, with the latter counted following the method of Roberts (1994), i.e. with the first rib-bearing vertebra considered the fifth and the counts presented as abdominal (with hemal spines anterior to first anal-fin pterygiophore) + caudal (with hemal spines posterior to first anal-fin pterygiophore) = total vertebrae.Asterisks after a meristic value indicates the condition for the holotype.Institutional codes follow Sabaj (2016).(Popta, 1904) Fig. 1 Leiocassis fuscus Popta, 1904: 186 (type  Mouth subterminal.Oral teeth small, viliform, in irregular rows on all tooth-bearing surfaces.Premaxillary tooth band rounded, of equal width throughout.Dentary tooth band much narrower than premaxillary tooth band at symphysis, tapering laterally.Vomerine tooth band unpaired, continuous across midline; smoothly arched along anterior margin, tapering laterally; band width equal to premaxillary band at midline, narrowing slightly laterally, then tapering to a sharp point posterolaterally.

Nanobagrus fuscus
Barbels in four pairs.Maxillary barbel slender, extending to base of pectoral spine.Nasal barbel slender, extending to one third of distance between posterior orbital margin and base of pectoral spine.Inner mandibularbarbel origin close to midline; barbel thicker and longer than nasal barbel, extending to one third of distance between posterior orbital margin and base of pectoral spine.Outer mandibular barbel originating posterolateral of inner mandibular barbel, extending to midway between posterior orbital margin and base of pectoral spine.
Body subterete, slightly compressed, becoming more so toward caudal peduncle.Dorsal profile rising evenly, not steeply, from tip of snout to origin of dorsal fin, sloping gently ventrad from origin of dorsal fin to end of caudal peduncle.Ventral profile slightly convex to anal-fin base, then sloping slightly dorsally to end of caudal peduncle.Skin smooth.Lateral line complete, midlateral in position.Vertebrae 17 + 18 = 35 (1).
Dorsal fin with spinelet, spine, and 7 rays.Origin of dorsal fin at about two-fifths of body.Dorsal-fin margin convex, usually with anterior branch of fin rays longer than other branches.Dorsal-fin spine short, straight, slender, posterior edge without serrations.Nuchal plate reduced to narrow triangle, with pointed tip anteriorly.
Pectoral fin with stout spine, sharply pointed at tip, and I,6 (3) rays.Anterior margin of spine smooth; posterior margin of spine with 10 -12 large serrae along entire length.Pectoral-fin margin straight anteriorly, convex posteriorly.Cleithral process slender, extending for half of pectoral-spine length.
Pelvic-fin origin at vertical through posterior end of dorsal-fin base, with i,5 (3) rays, its distal margin slightly convex; tip of adpressed fin not reaching anal-fin origin.Anus and urogenital openings located at vertical through middle of adpressed pelvic fin.Males with an elongate conical genital papilla.
Adipose fin with convex margin for entire length, with deeply-incised posterior portion; adipose-fin base moderate, spanning one third of postdorsal distance.Anal-fin origin posterior to vertical through anterior origin of adipose fin; anal fin with iv,8 (2) or iv,9 (1) rays and convex distal margin.
Coloration.In 70% ethanol: Dorsal and lateral surfaces of head dark brown, fading to cream ventrally.Interorbital region with an irregular cream band.Body dark brown on dorsal and lateral surfaces; ventral surfaces cream.Three cream bands encircling body: first band at nape, second immediately posterior to dorsal fin and third below posterior insertion of adipose fin.Adipose fin hyaline along dorsal margin and posterior half; dark brown on anterior half.Dorsal and anal fins with dark brown bases and dark brown crescentic mark on middle third of fins.Pectoral and pelvic fins hyaline.Base of caudal fin dark brown, with three hyaline spots: one on dorsal procurrent caudal-fin rays, second at posterior extremity of lateral line and third on ventral procurrent caudal-fin rays.Caudal fin with dark brown crescentic mark on middle third of each fin lobe; rest of fin hyaline.Barbels cream.
Distribution.Known from the upper Mahakam River drainage in eastern Borneo and the Mentaya River drainage in southern central Borneo (Fig. 2).Given the disjunct nature of this distribution, it is likely that the species is also found in the river drainages between these two (i.e. the Katingan, Sebangau, Kahayan and Barito river drainages).

Discussion
Nanobagrus fuscus and N. torquatus are easily distinguishable from all other congeners by their strongly depressed (vs.moderately depressed) heads (depth 10.0 -12.5% SL vs. 14.0 -17.3) and a color pattern consisting of at least one yellow or cream band encircling the body (vs.uniformly brown coloration or with series of cream or yellow spots or patches on body).These two species are distinguished from each other only by their color pattern: N. torquatus has a single band immediately behind the head, while N. fuscus has three bands: one behind the head, another behind the dorsal fin, and the third on the caudal peduncle (Thomson et al., 2008).
A comparison between material identified as N. fuscus collected from the Mahakam and Mentaya river drainages (including the holotype from the Mahakam River drainage) and from the remainder of its known distribution (i.e.river drainages in central Sumatra, the Malay Peninsula and western Borneo) reveals differences in morphometry.The specimens from the Mahakam and Mentaya river drainages have a longer adipose-fin base (99 -103% length of anal-fin base vs. 69 -97) and a shorter dorsal-to-adipose distance (15.3 -18.5% SL vs. 18.8 -25.5), suggesting that material identified as N. fuscus from central Sumatra, the Malay Peninsula and western Borneo are not conspecific.Aside from differences in the color pattern, no other morphometric differences were found between two paratypes of N. torquatus (ANSP 187394) and specimens from central Sumatra, the Malay Peninsula and western Borneo identified as N. fuscus (see comparative material listed).Furthermore, material identified as N. fuscus from central Sumatra, the Malay Peninsula and western Borneo displays intraspecific variation in color pattern that includes the one described for N. torquatus.These color patterns range from the presence of three light yellow or cream bands on the body (Fig. 3a), to the complete or partial absence of one or more of the light yellow or cream bands (Figs.3b -d) to the almost complete absence of light yellow or cream bands on the body (Fig. 3e); furthermore, I have encountered instances where individuals with the different color patterns have been found within the same population (e.g. of the eight specimens in ZRC 28418 -28424, three possessed three pale bands, one had the middle pale band almost missing, and four had a single pale band behind the head).Thomson et al. (2008) identified specimens collected from the southern Malay Peninsula with a single band encircling the body behind the head (CAS-SU 39339) as a possibly unnamed species, citing differences in barbel development and possibly fin coloration between this material and N. torquatus.Examination of fresh material collected from the southern Malay Peninsula (ZRC 28418 -28242 and ZRC 40263) show no differences in barbel development and fin coloration between specimens with a single pale band and those with three.
Taken together, the available evidence indicates that: (1) the species from the Mahakam and Mentaya river drainages (N.fuscus s. str.) is distinct from the one found in central Sumatra, the Malay Peninsula and western Borneo; and (2) the species from central Sumatra, southern Malay Peninsula and western Borneo is indistinguishable from, and conspecific with N. torquatus.Therefore, N. torquatus is hereby rediagnosed as a species of Nanobagrus with one to three light yellow or cream bands encircling the body (very rarely without any such bands), length of adipose-fin base 69 -97% length of anal-fin base and dorsal-to-adipose distance 18.8 -25.5% SL.The length of the adipose-fin base as expressed as a ratio of SL distinguishes N. fuscus (14.9 -16.5% SL) from  (Parenti & Meisner, 1995;Rachmatika et al., 2006;Thomson et al., 2008).Locality in upper Mahakam River drainage for N. fuscus approximate.Each symbol may represent more than a single locality.N. torquatus (10.2 -14.5% SL) amongst the material examined in this study.However, Thomson et al. (2008) report the length of the adipose-fin base of N. torquatus as 9.8 -18.5% SL (although my measurements from the photographs of the type series indicate 10.2 -15.1% SL), which entirely spans the range of values observed for both species.Because of the possible unreliability of this ratio in diagnosing the two species, I have instead used the length of the adipose-fin base expressed as a ratio of the anal-fin base length (i.e. a relative measure of the sizes of the adipose and anal fins) to distinguish between N. fuscus and N. torquatus; this character is useful for diagnosing the two species, as confirmed by measurement from the photographs of the type series of N. torquatus.The distribution range of N. torquatus is now expanded to comprise river drainages in central and southern Sumatra (Kampar River drainage southwards to the Musi River drainage), Malay Peninsula (from the Songkhla Lake drainage southwards) and western Borneo (Kapuas River drainage northwards to the Belait River drainage; Fig. 2).

Fig. 2 .
Fig. 2. Map showing distributions of Nanobagrus fuscus and N. torquatus.Closed symbols indicate records from specimens examined in this study and open symbols indicate records from the literature(Parenti & Meisner, 1995;Rachmatika et al., 2006;Thomson et al., 2008).Locality in upper Mahakam River drainage for N. fuscus approximate.Each symbol may represent more than a single locality.