Rhyacoschistura, Kottelat, 2019

Kottelat, Maurice, 2019, Rhyacoschistura larreci, a new genus and species of loach from Laos and redescription of R. suber (Teleostei: Nemacheilidae), Zootaxa 4612 (2), pp. 151-170 : 152-159

Rhyacoschistura , new genus

Type species. Rhyacoschistura larreci , new species.

Etymology. Rhyacoschistura is derived from the Greek word ῥύαξ, -ΚΟς (rhyax, meaning a rushing stream, a torrent) and the genus name Schistura (itself based on σΧΙστός [schistos; divided] and Οὐρά [oura; tail]). In reference to the torrent and hill stream habitat of the species placed in the genus. Gender feminine.

Diagnosis. Rhyacoschistura is distinguished from the other genera of Nemacheilidae by the combination of: the modified male pectoral fin (especially the first branched ray wider than the following rays, without a membrane between the branches except near the tip, covered by small pointed tubercles at maturity; more details below); the capsule of the anterior chamber of the air bladder in two halves, connected by a manubrium; the lower lip with a wide median interruption, the two halves forming an acute angle, not in contact medially, wide and fleshy medially, partly free from the jaw and connected to the isthmus by a frenum; the suborbital flap is present or absent; the body depth is about equal from behind head to caudal-fin base; depth of caudal peduncle 1.1–1.3 times in its length; caudal fin emarginate; and scales very distinct, covering whole body, including predorsal and prepectoral areas.

Pectoral fin of males slanted upwards and expanded laterally; first branched ray rigid, about 4 times wider than following rays; anterior branch not further branched, posterior branch branched again, without membrane between them, except near tip ( Fig. 1 ). Branched ray 2: anterior branch unbranched, posterior branch branched again. Following rays branched once or twice; space between branches of branched ray 2 very narrow (or lacking a membrane) and space increasingly wider in posterior rays. Dorsal surface of first branched ray and part of membrane just along posterior branch covered by small pointed tubercles. Unculiferous pads (sensu Conway et al. 2012) present in R. suber , along posterior margin of all branched rays; along anterior and posterior margins of first branched ray, posterior one covered by small tubercles, smaller than those on ray.

Air bladder capsule in two halves, separated and connected by a manubrium; no posterior chamber of air blad- der visible in abdominal cavity.

Mouth arched, about 2–2.5 times wider than long ( Fig. 2 ). Upper lip thin, with median notch, and thin and shallow wrinkles. Lower lip with wide median interruption, sometimes leaving tip of jaw exposed; two halves forming an acute angle (when mouth closed and in properly fixed specimens), not in contact medially. Along median interruption, anterior edge of lower lip extending backwards until about medial extremity of postlabial groove; space between median interruption and postlabial groove narrow. Each half of lower lip fleshy medially, about 1.5–2 times thicker than at corner of mouth, not forming a ‘cushion’ (see Kottelat 2018). Medial portion with 0–3 narrow sulci, lateral portion with thin shallow wrinkles.

Colour pattern very variable, also intraspecifically. Smallest known individuals (up to about 25–30 mm SL) plain grey to yellowish, then a dusky midlateral stripe appears, which develops into a series of faint short bars, then into a pattern of irregular bars. Pattern at caudal-fin base: a black bar, arched, extending on base of rays, almost complete, except along upper and lower extremities.

Caudal fin emarginate, with 8+8 or 8+7 or 7+8 branched rays. Dorsal fin with 7½ branched rays; margin slightly convex; origin above or slightly in front of pelvic-fin origin.

Male of R. larreci with suborbital flap (modified lateral ethmoid), extremity of free part usually globulous, with a swollen cap of unculiferous tissue representing about half of flap, and with small tubercles along posterior edge ( Fig. 3a ). Suborbital flap missing in R. suber .

Included species. Rhyacoschistura larreci and R. suber ( Kottelat, 2000) .

Remarks. Among nemacheilid genera from northeast India and continental Southeast Asia, Rhyacoschistura shares the sexually dimorphic male rigid, thickened, curled, first branched pectoral-fin ray with only a narrow or no space between the branches with Mustura, Pteronemacheilus , Malihkaia , Physoschistura , Petruichthys and (based on photographs) apparently Protonemacheilus . Although in many other genera of Nemacheilidae the first branched pectoral-fin ray of males is also significantly thicker than the other rays and than in females, the type of modification is different (see below).

Rhyacoschistura is distinguished from these genera in having larger and distinctly conical and pointed tubercles on the first branched pectoral-fin ray and smaller, less sharp tubercles along the posterior margin, or on unculiferous pad behind the ray (vs. numerous small, granulose or conical tubercles on ray and fins, in several species very densely set).

In males of Mustura, the posterior branch of branched rays 1 and 2 is not further branched (vs. branched in Rhyacoschistura ); the dorsal fin margin is straight to concave (vs. slightly convex); the caudal fin is forked (vs. emarginate); the pelvic-fin origin is under the base of branched dorsal-fin rays 1–3 (vs. below or slightly in front of dorsal-fin origin); the colour pattern is variable, usually including a midlateral row of very irregular, vertically elongated blackish blotches, more or less connected to a middorsal row of irregularly shaped saddles; the black pattern at the caudal-fin base is made of two spots: a black spot over upper procurrent rays and upper two principal rays; and a vertically elongated patch of densely-set dark brown pigments along the base of the principal rays of the lower lobe (vs. a black bar, arched, extending on to the base of rays, almost complete, except along upper and lower extremities).

In males of Pteronemacheilus , the membrane is present between the two branches of the first branched ray (although narrow) (vs. branches are adjacent, without membrane between them in Rhyacoschistura ). Branched ray 2 has an elevated flange of skin on its dorsal side, on the proximal ⅓–½. Branched rays 3 and 4 are connected by thick tissues forming a hard elongate swelling (rod) between them on their dorsal surface; the rays have a slight curve downwards at about the distal extent of the rod, creating a shallow concavity in the ventral surface of the fin (vs. rays 3–4 and adjacent membranes not modified). Males have neither a suborbital flap nor a suborbital groove (vs. present in one species). The caudal fin is deeply emarginate (vs. emarginate).

In males of Physoschistura the two branches of the first branched ray are close together, without a membrane between them, except near the very tip. Branched ray 2 is thinner and the branches are close but with a narrow membrane between them. The unculiferous pads along branched rays are very narrow and the small conical tubercles are present only on a narrow band along the posterior edge of branched ray 1. Besides, the two halves of the air bladder capsule are joined medially (vs. connected by a manubrium) and the posterior chamber is free, well developed and in direct contact with the capsule (vs. absent).

In males of Petruichthys the first branched pectoral-fin ray is not significantly thickened but the space between the branches is very narrow; there are patches of small conical tubercles on the dorsal surface of branched ray 1 and its anterior and posterior flanges, and on the posterior flange of ray 2. Males have a suborbital flap. Besides, in Petruichthys the nares are separated from each other, the anterior one at the extremity of a short tube (vs. nostrils adjacent, anterior naris on anterior face of a valve-like flap); the supratemporal canal of the cephalic lateral line system has two pores on each side of the head (vs. a median pore and one on each side); the postlabial groove extends along the whole posterior edge of the lower lip, and the frenum is in the groove, under the lip (vs. postlabial groove not complete and frenum visible).

In males of Malihkaia , the pectoral fin is strongly falcate, the unbranched and first branched rays are rigid, arched and curved upwards; the first branched ray is about 5 times wider than the other branched rays, flattened dorso-ventrally, reaching beyond pelvic-fin base; there are no membranes between the branches and the sub-branches ( Kottelat 2017a). The second branched ray is slender, branched only at tip and the membrane between the branches is very narrow. Besides, Malihkaia has a deeply furrowed lower lip (vs. smooth, with thin, shallow furrows or wrinkles in Rhyacoschistura ) and the suborbital flap is not globulous and does not have small tubercles along its posterior margin (vs. globulous and with small tubercles along its posterior margin).

The morphology of the male suborbital flap (globulous, with tubercles at the extremity) is shared by Rhyacos- chistura, Mustura, Physoschistura and Petruichthys , and has not yet been reported in other nemacheilid genera in South and Southeast Asia.

See Kottelat (2018) for a more detailed discussion of the modified pectoral-fin rays and for diagnoses of the remaining genera of Nemacheilidae present in northeastern India and mainland Southeast Asia.

Figures

FIGURE 1. Rhyacoschistura larreci, left pectoral fin, dorsal view, of: a, male, MHNG 2727.011, holotype, 57.1 mm SL; and b, female, CMK 28034, 68.8 mm SL.

FIGURE 2. Rhyacoschistura larreci, CMK 28034, 68.8 mm SL; mouth.

FIGURE 3. Rhyacoschistura larreci, head of: a, male, with suborbital flap (arrow), MHNG 2727.011, holotype, 57.1 mm SL; and b, female, CMK 28034, 68.8 mm SL.