Schistura systomos, Maurice Kottelat, 2017

Maurice Kottelat, 2017, Schistura systomos ı a new species of loach from southern Laos (Teleostei: Nemacheilidae), Ichthyol. Explor. Freshwaters 28 (1), pp. 9-16 : 10-15

Schistura systomos , new species

( Figs. 1-2 )

Holotype. MHNG 2767.056 ı 35.0 mm SL; Laos: Champasak Province: Bolaven Plateau: Tad Set on Houay Set ( Xe Setı tributary of Xe Don )ı a b near Ban Nonghinkhaoı 15°18'03" N 106°18'23" E; 1136 m asl; M. Kottelat et al. ı 19 January 2013.

Paratypes. CMK 23401ı 6ı 21.7-35.4 mm SL; 2ı ethanol-fixedı 28.6-31.6 mm SL; same data as holotype.

Diagnosis. Schistura systomos is distinguished from the other species of the genus in Southeast Asia by having a small mouth (gape about 2.9- 3.1 times in maximum head width; Fig. 3 a)ı occupying only the median half of head width at its position (vs less than about two timesı lateral extremity of lips reaching or almost reaching sides of head in ventral view; Fig. 3 b); a blunt snout (vs. rounded or pointed) resulting in a relatively deep infraorbital area; head about circular in crosssection (vs. depressed)ı lower surface flat only in middle (vs. whole surface flat); infraorbital area roundedı not expanded laterally and leaving the eyes partly or almost visible in ventral view (vs. infraorbital area vertical or expanded laterally or swollenı eyes visible only in dorsal view).

Additional diagnostic charactersı not unique to the species are: small size at maturity (35 mm SL and possibly less); wide interorbital area (interorbital distance about 1.7-2.1 times eye diameter); depth of caudal peduncle 1.5-1.7 times in its length; 8 1 / 2 branched dorsal-fin rays; pelvic-fin origin below or slightly in front of dorsal-fin origin; pelvic axillary lobe varying from rudimentary and fused to small and free; 9 +8 branched caudal-fin rays; no suborbital flap; 14-17 bars (4-6 predorsalı 3-4 subdorsalı 6-9 postdorsal) on bodyı of regular width and shapeı wider than interspacesı continuous over back with contralateral bars under dorsal fin and on caudal peduncle; on predorsal areaı bars irregularly shaped and setı dissociated in blotches and short stripes. Pattern at caudal-fin base made of a black vertically elongated blotch occupying median 1 / 3 of caudal baseı not reaching dorsal and ventral midlines; overimposed to itı a dark brown bar extending ventrally beyond the base of the lower unbranched principal rayı and extending dorsally to base of branched ray 5; and a small blotch from above the base of the upper unbranched principal rayı reaching or not the dorsal midline.

Description. See Figures 1-2 for general appearance and Table 1 for morphometric data of holotype and 2 largest paratypes. A moderately elongate nemacheilid with body depth slowly increasing up to slightly in front of dorsal-fin origin. Behind dorsal finı body depth almost uniform until caudal-fin base. Dorsal profile continuous between head and body. Head about circular in cross sectionı lower surface flat only in middle; infraorbital area roundedı not expanded laterally. Body slightly compressed anteriorly to compressed posteriorly. Interorbital area arched. Eye dorso-lateralı at edge of cranium and partly or almost visible from below because infraorbital area not expanded laterally; diameter 1.7-2.1 times in interorbital width; in lateral viewı eye located lower than profile of head. Cheeks not swollen. Snout roundedı blunt. Depth of caudal peduncle 1.5-1.7 times in its lengthı uniform; low dorsal keel on posterior fourth of post-dorsal area; no ventral keel. Dorsal keel continuous with upper margin of caudal fin. Largest recorded size 35.4 mm SL.

Dorsal fin with 4 unbranched and 8 1 / 2 (7) branched rays; distal margin slightly convex or straight. Second branched ray longest. Pectoral fin with 1 unbranched and 8 (1) or 9 (6*) branched raysı roundedı reaching about halfway of distance to pelvic-fin base. No axillary pectoral lobe. Pelvic fin with 1 unbranched and 7 (7) branched raysı reaching to anusı rounded; posterior margin convex; origin below or slightly in front of dorsalfin origin; axillary lobe from rudimentary and fused to small and free. Anus situated about 1.5 eye diameter in front of anal fin. Anal fin with 3 unbranched and 5 1 / 2 (7) branched rays; distal margin convex. Caudal fin with 9 + 8 (7) branched raysı emarginateı lobes roundedı of equal length.

Body covered by embedded scalesı except on predorsal area and on belly in front of anus. Lateral line completeı pores difficult to count with accuracyı about 80- 90ı anterior most 16-29 conspicuousı remainder indistinct (fixation artefactı pores hidden in midlateral groove resulting from dehydratation by ethanol). In anterior half of bodyı a small auxiliary pore or papillae above and below some lateral line tubes (usually by pair). Cephalic lateral line system with 6 supraorbitalı 4 + 11-12 infraorbitalı 8-9 preoperculo-mandibular and 3 supratemporal pores.

Anterior nare pierced in front side of a pointed triangular flap-like tube. Posterior nare adjacent to anterior one. Mouth archedı narrowı gape about 2.9-3.1 times in maximum head widthı occupying only median half of head width and lateral extremity of lips by far not reaching lateral outline of head in ventral view; gape about two times wider than long ( Fig. 3 a). Lips thick. Upper lip with a rounded median notch in which processus dentiformis is visibleı with numerous fine and shallow furrowsı edge finely crenulated. Processus dentiformis presentı well-developed. Lower lip with conspicuous V-shaped median interruption; broad median part with two or three sulciı narrower lateral partı shortı with a few fine and shallow furrows. Tip of lower jaw exposed. A shallow median concavity in lower jaw in which processus dentiformis is fitting. Inner rostral barbel not reaching corner of mouth; outer one reaching slightly beyond corner of mouth or vertical of anterior margin of eye. Maxillary barbel reaching below vertical of posterior half of eye. Intestine thin (about 0.3 mm diameter in 35.4 mm SL specimen)ı straightı without bend behind stomach ( Fig. 4 ); stomach comparatively smallı about 3 mm longı body of stomach wideı swollen. Air bladder without posterior chamber in body cavity.

Sexual dimorphism. None observed. No specimen with suborbital flapı groove or slit. No modified pectoral raysı no tuberclesı no patches of unculi. Ripe females deeper bodied.

Coloration. After three weeks in formalin. Head and body background colour pale yellowish brownı throat and belly whitish to pale yellowish; except otherwise statedı all markings dark brown to blackish. Head without distinct pattern. Body with 14-17 bars (4-6 predorsalı 3-4 subdorsalı 6-9 postdorsal)ı reaching downwards to level of pectoral finı or to ventral midline on caudal peduncleı continuous over back with contralateral bars under dorsal fin and on caudal peduncleı of regular width and shapeı wider than interspacesı anterior bars slightly broader than posterior ones. On predorsal areaı bars irregularly shaped and setı dissociated in blotches and short stripes and usually not meeting contralaterals.

A conspicuous black blotch on caudal-fin base. Black inner pigments forming a vertically elongated blotch occupying median 1 / 3 of caudal-fin baseı not reaching dorsal and ventral midlines; overimposedı a dark brown bar extending ventrally beyond base of lower unbranched principal rayı dorsally to base of branched ray 5 (from top). A smaller blotch from above base of upper unbranched principal rayı reaching or not dorsal midline. Space between bar on caudal-fin base and preceeding bar paler than rest of body background coloration. A wide inner axial stripe made of chevron-shaped elements present but faintly markedı not visible in front of dorsal fin.

Dorsal fin hyalineı with a small black blotch at base of simple and first branched raysı followed by two or three blackish blotches on or near base of following raysı space between blotches hyaline; on median third of rays (near branching in branched rays)ı black pigments along edge of rays and between segments. Caudal fin hyalineı on proximal two thirds of principal raysı black pigments along edge of rays and between segments. Anal fin hyalineı with black blotch on base of simple rays; on median third of rays (near branching in branched rays)ı black pigments along edge of rays and between segments. Pelvic and pectoral fins hyaline; on median third of rays (near branching in branched rays)ı black pigments along edge of rays and between segments. In smallest specimen (21.7 mm SL; Fig. 2 b)ı body greyishı bars more faintly markedı more irregularly shaped.

Notes on biology. A dissected female (35.4 mm SL) had one ripe ovary with 14 yellow ova about 1.2-1.4 mm diameter and about 31 whitish oocytes. Its stomach and intestine contained remains of unidentified insect larvae.

Distribution. Schistura systomos is presently known only from the type localityı a waterfall on Xe Setı a tributary of Xe Doneı near edge of Bolaven Plateau ( Fig. 5 ). It was collected between stones and blocksı in about 50-100 cm depth (dry season) below the waterfall.

a Etymology. From the classical Greek σύστοµος ( systomos )ı meaning with a narrow mouth. An adjectiveı indeclinable.

Remarks. Schistura systomos has the smallest mouth in the genus Schistura and possibly among loaches of the family Nemacheilidae . The mouth occupies only the median half of head width at its location ( Fig. 3 a)ı while in the other species it occupies most of head widthı with the lipsı when seen in ventral viewı extending to or almost to the lateral outline of the head ( Fig. 3 b). As mentioned earlierı the genus Schistura is very diverse andı as presently understoodı polyphyletic. A number of species group can be distinguished by morphological characters and by colour patternı most of which will likely be recognised as distinct genera. Two species of small size ( S. geisleri ı S. isostigma ) have the mouth somewhat narrower than other Schistura (not quantified) but they are distinguished by the pale yellow background and the colour pattern made of a midlateral row of dark brown to black blotches and a row of saddles along dorsal midline ( Kottelatı 1990ı 1998). They also have a suborbital flap in male that is lacking in S. systomos .

Similarlyı two species of Physoschistura ( P. brunneana ı P. pseudobrunneana ) also have the mouth somewhat narrower than species of Schistura (not quantified)ı slightly compressed and deep body and headı and the eyes partly or almost visible in ventral view; S. systomos is distinguished by the absence of a posterior chamber of the air bladder in the body cavity (vs. presence)ı the absence of a suborbital flap in male (vs. presence)ı the arched mouth (vs. U-shaped)ı and the lower lip with V-shaped median interruptionı the median part with two or three sulciı the lateral part shortı with a few shallow furrows (vs. with wide median interruption forming two lateral triangular pads with deep sulci). Physoschistura is another genus to which a number of species have been added in recent years (Chen et al. ı 2011; Lokeshwor & Vishwanathı 2012 a- b; Suvarnarakshaı 2013) that do not seem related to the type species ( P. brunneana ) and seem to represent one or more distinct genera. A number of species placed in Schistura are apparently also related to these ‘Physoschistura’ ı for example S. geisleri ı S. isostigma ı S. maepaiensis and S. shuangjiangensis . There is no character suggesting that S. systomos could be more closely related to any of these ‘Physoschistura’ rather than to ‘Schistura’. In factı S. systomos likely represents a genus of its own. It relationships will be addressed in a forthcoming review of the genera of Southeast Asian Nemacheilidae .

Compared to other nemacheilidsı the stomach of S. systomos appears proportionally smaller (not quantified) and the intestine is very thin and straight. In the great majority of nemacheilidsı the intestine has at least one conspicuous bendı a loop or a complex coiling. The stomach and intestine of the two dissected specimens contained remains of insect larvae that could not be identified.

Figures

Fig. 1. Schistura systomos ı MHNG 2767.056 ı holotypeı 35.0 mm SL; Laos: Champasak Province: Bolaven Plateau: Tad Set waterfall.

Fig. 2. Schistura systomos ı CMK 23401 ı paratypes; Laos: Champasak Province: Bolaven Plateau: Tad Set waterfall. aı 35.4 mm SL; bı 21.7 mm SL.

Fig. 3. Ventral view of mouth of Schistura species: aı narrow mouth in S. systomos ı MHNG 2767.056 ı holotypeı 35.0 mm SL; and bı wide mouth in other Schistura species (here S. clatrata ı CMK 23398 ı 81.5 mm SL; collected together with S. systomos).

Fig. 4. Schistura systomos ı CMK 23401 ı paratypeı 35.4 mm SL; digestive tract. Scale bar 1 mm.

Fig. 5. Laos: Champasak Province: Bolaven Plateau: Tad Set on Houay Set; type locality of Schistura systomos; 19 January 2013.

Tables

Table 1. Morphometric data of holotype and two largest paratypes of Schistura systomos. Hı holotype; Pı paratype.

	P	H	P
Standard length (mm)	34.7	35.0	35.4
Total length (mm)	42.0	42.3	42.5
In percent of standard length
Total length	121.1	120.7	119.9
Head length (dorsal)	21.3	22.1	19.8
Head length (lateral)	22.6	22.3	24.2
Predorsal length	54.5	54.9	55.1
Prepelvic length	50.8	51.1	51.6
Pre-anus length	65.7	67.4	66.4
Pre-anal length	74.6	76.2	76.3
Head depth	12.4	12.8	12.5
Body depth at dorsal-fin origin	15.1	19.1	17.1
Depth of caudal peduncle	11.2	11.1	11.2
Length of caudal peduncle	18.5	17.4	17.8
Head width	14.4	14.6	14.9
Body width at dorsal-fin origin	11.8	15.4	13.4
Snout length	9.6	9.6	10.4
Eye diameter	4.6	4.6	4.8
Interorbital width	8.3	9.8	8.2
Length of dorsal fin	18.2	18.4	15.8
Length of upper caudal-fin lobe Length of median caudal-fin rays	20.4 15.8	22.4 16.6	19.6 15.7
Length of lower caudal-fin lobe	20.2	21.1	19.8
Length of anal fin	18.0	17.8	16.6
Length of pelvic fin	16.6	17.3	15.2
Length of pectoral fin	18.5	17.9	17.5
In percent of dorsal head length
Snout length	45	43	53
Eye diameter	21	66	75
Interorbital width	39	44	41
Mouth width	23	21	24
In percent of lateral head length
Snout length	43	43	43
Eye diameter	20	21	20
Interorbital width	37	44	34
Mouth width	22	21	20