Exostoma tenuicaudata, Tamang, Lakpa, Sinha, Bikramjit & Gurumayum, Shantabala Devi, 2015

Tamang, Lakpa, Sinha, Bikramjit & Gurumayum, Shantabala Devi, 2015, Exostoma tenuicaudata, a new species of glyptosternine catfish (Siluriformes: Sisoridae) from the upper Brahmaputra drainage, northeastern India, Zootaxa 4048 (3), pp. 441-445 : 442-444

Exostoma tenuicaudata , sp. nov.

(Fig. 1)

Type material. Holotype: ZSI/ APFS /P-929, 108.0 mm SL; India: Arunachal Pradesh: Upper Siang district, a fast flowing mountain drainage, confluence directly to Siang River, about 3 km from Bomdo village on main road to Tuting, 28°44.04' N 94°51.97' E, 429 m asl; L. Tamang, 26 October 2011.

Paratypes. ZSI/ APFS /P-930, 4, 82.7–103.2 mm SL; same data as holotype, 26–28 Oct 2011.

Diagnosis. Exostoma tenuicaudata is distinguished from its congeners by the presence of an elliptical plicated adhesive pad medially between the anterior margin of the snout and the mid-upper lip margins, further connected posteriorly by a fleshy pad up to the mid-upper lip; and a more slender caudal peduncle (3.6–4.7% SL or 13.8–17.2% CPL). The following combination of characters further distinguishes this species from its congeners: snout length 60.2– 61.8% HL; interorbital width 24.4–26.4% HL; interdorsal distance13.0–13.5% SL; caudal peduncle depth 3.6–4.7% SL and 13.8–17.2% CPL; CPL 26.3–28.0% SL; prepectoral length 16.8–18.9% SL; predorsal length 34.9–39.0% SL; prepelvic length 42.4–44.6% SL; preanal length 68.1–70.1% SL; length of adipose-fin base 30.8–32.9% SL; length of dorsal-fin base 7.9–10.8% SL; length of anal-fin base 3.9–5.3% SL; caudal-fin length 14.3–19.4% SL, maxillary barbel 63.5–72.1% HL, nasal barbel 19.9–26.3% HL; eye diameter 10.0–16.0% HL; pectoral-fin rays 11–12; and adipose-fin not confluent with dorsal-fin base.

Description. Biometric data are given in Table 1 . Head and body depressed and moderately broad. Body moderately compressed anteriorly, degree of compression increasing from pelvic-fin origin to caudal-fin base. Dorsal profile rising gently from snout tip to dorsal-fin origin, with slight depression at occipital base, gently declining thereafter towards caudal-fin base. Ventral profile flat from snout to pelvic-fin origin, gently ascending towards caudal-fin base. Snout almost semicircular in dorsal aspect, with a medial incision.

Four pairs of barbels: nasal, maxillary, inner and outer mandibular. Maxillary barbel dorso-ventrally flattened, with broad base and pointed tip, tip extending to base of second pectoral-fin ray. Ventral surface of maxillary barbel with adhesive pad consisting of 25–30 elongate plicae, followed by series of small rounded plicae (faint in 1 individual), tip smooth. Ventral surface of snout between anterior maxillary barbel bases with numerous small rounded plicae. Snout broadly rounded with a minute medial incision (Fig. 2). An elliptical plicated adhesive pad situated between anterior snout margin and upper lip, further connected posteriorly to upper lip by a fleshy pad (Fig. 2). Outer mandibular barbel moderately flattened with pointed tip, not reaching pectoral-fin origin. Inner mandibular barbel minute, originating at labial flap, arising through notch between median and lateral lobes of lower lip.

Nares situated midway between snout tip and anterior orbital margin. Eye small, rounded, subcutaneous, situated dorsally midway between anterior margin of nares and posterior margin of operculum. Humeral process poorly developed. A series of small tubercles running longitudinally on dorsum of head, dense on opercular region (prominent in adult males, weak in adult females).

Mouth inferior, transverse, anteriorly reflected. Upper lip continuing into maxillary barbel. Lower lip post-labial groove continuous. Upper and lower jaw each divided (lower jaw broadly, upper narrowly) medially by a fleshy medial frenum. Teeth in patches, embedded in skin, sparsely distributed, varying in size, mobile, oar-shaped, distally flattened, directed somewhat posteriorly. Upper lip fleshy, thick, bi-crescentic. Lower jaw surrounded by well-developed continuous post labial fold consisting of one median lobe with larger lobes laterally, median lobe without a notch in the middle, posterior margin of labial fold smooth, entire. Upper half of lateral lobe with plicate adhesive pad (curving slightly inward, basally broad, anteriorly narrow, resembling a human fist with extended forefinger), extending anteriorly from each corner of mouth (Fig. 2), with 25–30 irregular to elongate plicae. Gill opening narrow, extending downward from level immediately dorsad to base of posteriormost pectoral-fin ray to level of base of first pectoral-fin ray.

Dorsal fin with i,6*(4) or i,5 (1) rays, devoid of spine. Adipose-fin base long, posterior end not contiguous with upper caudal-fin procurrent rays, its length slightly greater than three times length of dorsal-fin base. Caudal fin weakly forked, broad, with i,7,8,i *(1); i,7,7,i (2); or i,7,6,i (2) rays. Anal fin with ii, 4 rays, its base shorter than other fins.

Pectoral fin moderately enlarged, devoid of spine, with i,11*(4) or i,12 (1) rays, tip extending to or slightly beyond vertical through dorsal-fin origin; a series of rugose tubercles along each pectoral ray (more prominent in adult male than female); first ray element lanceolate, fleshy, thick, flattened, with 63–72 membranous plicae on ventral surface; plicae elongate, slightly arched, symmetrically and obliquely arranged. Pelvic fin somewhat rhomboidal, enlarged but smaller than pectoral fin, with i,5 rays; origin at vertical through base of penultimate or antepenultimate dorsal-fin ray; first ray element fleshy, thick, flattened, with 60–70 membranous plicae on ventral surface; plicae elongate, slightly arched, symmetrically and obliquely arranged; plicae difficult to discern when outer membranous plicae are removed; posterior tip of pelvic fin reaching almost to anterior margin of anus in adult (4), slightly beyond in young specimen (1).

Holotype Paratypes Range Mean±SD Mouth width (rictus) 45.7 38.7 38.3 37.1 35.7 35.7–45.7 39.1±3.9 Males with a large genital papilla, approximately conical, located immediately posterior to anus; females with a smaller, rounded genital papilla, located immediately posterior to anus in a U-shaped depression (Fig. 3a,b).

Interorbital and internarial distances almost equal. Length of first pectoral fin element equal to width of body at pectoral-fin origin. No thoracic adhesive apparatus. Lateral line complete, almost straight, mid-lateral in position.

Color in preservative. Dorsal and lateral surfaces of head and body brownish gray, ventral region between level of pectoral- and anal-fin origin cream to light milky white, venter light yellow. Dorsal region of body with three creamy to light-beige patches: first, a crescentic patch on occipital region; second on dorsal-fin origin; third, an ovoid patch on posterior end of dorsal-fin base. Lateral side of body with one elliptical beige patch on each side of body, situated just below and slightly posterior to middle of adipose-fin base, and two light beige-brown stripes running along entire flank on each side of body above and below lateral line.

Dorsal surfaces of paired fins light brown, ventral surfaces dusky except for light-yellowish proximal half. Postlabial flap around mouth pale yellow. Plicae on lateral lobe, maxillary barbel, first element of paired fins, and upper and lower lip surfaces light milky white. Dorsal-fin base dark brown, rays light brownish gray, interradial membranes and distal margin almost hyaline. Adipose fin light grayish brown with creamy distal margin. Anal-fin base and subdistal margin light grayish brown, tip hyaline. Caudal fin dark to light grayish brown except for two pale-yellow ovoid blotches on upper and lower lobes, upper one considerably larger than lower, extending almost to posterior fin margin, lower one situated nearer to fin base than to its posterior margin. Blotches approximately form an ‘L-shaped’ pattern when collectively viewed.

Distribution and habitat. Exostoma tenuicaudata was obtained in a steeply sloping mountain drainage with fastflowing water, debouching into the Siang River (Brahmaputra River basin), about 3 km from Bomdo village on the main road to Tuting, Upper Siang district, Arunachal Pradesh, northeastern India (Fig. 4).The drainage mostly consists of large, dark-brown boulders and with pebbles, cobbles, concrete fragments of various colors and minute gravels in certain places. The banks consist of mosses, shrubs, small trees, with larger trees further uphill. The gravel bed was more or less clean, with only a little algal slime over the substratum. The water was clear and cool. Other species collected at the type locality include Neolissochilus hexagonolepis , Pseudecheneis sulcata , Garra magnidiscus and Schistura sp.

Etymology. The Latin word tenui meaning (slender) and caudata (tail), in allusion to the slender caudal peduncle of this species. An adjective.

Tables

TABLE 1. Biometric data for Exostoma tenuicaudata (n = 5).

Standard length	108.0	103.2	90.3	93.3	82.8	82.8–108.0	95.5±10.1
in % of SL
Head length	20.5	20.2	21.9	21.4	22.8	20.2–22.8	21.3±1.0
Body depth at anus	11.3	9.6	11.1	10.6	10.9	9.6–11.3	10.7±0.6
Body depth at dorsal-fin origin	14.2	11.1	12.8	12.4	14.1	11.1–14.2	12.9±1.3
Body width at dorsal-fin origin	16.7	14.3	16.2	15.2	16.9	14.3–16.9	15.9±1.1
Head height at occiput	9.9	9.3	9.9	9.5	10.3	9.3–10.3	9.8±0.4
Head width	18.1	18.1	18.7	18.7	21.1	18.1–21.1	18.9±1.2
Rictus	8.3	7.6	8.3	7.5	8.7	7.5–8.7	8.1±0.5
Caudal-peduncle length	28.0	26.3	26.6	26.8	27.4	26.3–28.0	27.0±0.7
Caudal-peduncle depth	4.3	3.6	4.5	3.7	4.7	3.6–4.7	4.2±0.5
Pre-dorsal length	35.2	34.9	38.0	38.0	39.0	34.9–39.0	37.0±1.8
Pre-pectoral length	16.9	16.8	18.0	17.6	18.9	16.8–18.9	17.6±0.8
Pre-pelvic length	43.8	44.6	42.4	43.8	43.8	42.4–44.6	43.7±0.8
Pre-anal length	68.5	68.1	68.4	68.5	70.1	68.1–70.1	68.7±0.8
Pre-anus length	61.7	59.9	60.0	62.4	63.3	59.9–63.3	61.5±1.5
Dorsal to adipose distance	13.4	13.4	13.5	13.5	13.0	13.0–13.5	13.4±0.2
Post-adipose distance	12.9	10.9	9.2	8.4	9.9	8.4–12.9	10.2±1.7
Length of dorsal-fin base	10.8	7.9	9.8	9.5	10.0	7.9–10.8	9.6±1.1
Height of dorsal fin	15.5	15.9	15.1	15.9	18.1	15.1–18.1	16.1±1.2
Length of adipose-fin base	32.8	31.5	31.4	32.9	30.8	30.8–32.9	31.9±1.0
Pectoral-fin length	20.4	20.5	19.8	21.4	23.2	19.8–23.2	21.1±1.3
Length of first pectoral-fin element	19.7	19.0	19.1	19.8	21.8	19.0–21.8	19.9±1.1
Pelvic-fin length	17.8	17.4	17.5	18.2	19.6	17.4–19.6	18.1±0.9
Length of anal-fin base	4.6	3.9	4.8	5.2	5.3	3.9–5.3	4.7±0.5
Length of anal fin	10.6	10.0	10.2	10.9	13.0	10.0–13.0	10.9±1.2
Upper caudal-fin lobe length	14.2	15.4	14.1	13.5	18.2	13.5–18.2	15.1±1.9
Lower caudal-fin lobe length	16.4	14.3	14.7	16.4	19.4	14.3–19.4	16.2±2.0
Distance between pelvic and anal fin origins	30.6	28.4	26.2	27.6	26.2	26.2–30.6	27.8±1.8
Pelvic-anus distance	22.1	20.6	19.0	20.8	20.1	19.0–22.1	20.5±1.1
Vent-anal-fin origin distance	5.3	5.3	5.2	5.7	3.4	3.4–5.7	5.0±0.9
Adipose-fin height	3.3	1.1	2.2	2.6	2.6	1.1–3.3	2.4±0.8
In % of HL
Head width	88.0	88.8	85.2	87.5	92.8	85.2–92.8	88.5±2.8
Snout length	61.8	60.2	61.0	61.2	61.2	60.2–61.8	61.1±0.6
Eye diameter	10.5	10.0	16.0	11.2	12.4	10.0–16.0	12.0±2.4
Interorbital width	25.5	25.9	24.4	26.2	26.4	24.4–26.4	25.7±0.8
Rictus	40.1	36.3	37.7	36.4	37.1	36.3–40.1	37.5±1.6
Maxillary barbel length	69.2	69.0	68.3	63.5	72.1	63.5–72.1	68.4±3.1
Nasal barbel length	23.5	23.6	22.1	19.9	26.3	19.9–26.3	23.1±2.3
Inner mandibular barbel length	7.8	7.6	11.3	8.1	6.5	6.5–11.3	8.3±1.8
Outer mandibular barbel length	22.5	24.7	21.8	21.2	29.0	21.2–29.0	23.8±3.2
%CPL
Caudal-peduncle height	15.2	13.8	16.9	13.8	17.2	13.8–17.2	15.4±1.6