Ompok argestes, Sudasinghe, Hiranya & Meegaskumbura, Madhava, 2016

Sudasinghe, Hiranya & Meegaskumbura, Madhava, 2016, Ompok argestes, a new species of silurid catfish endemic to Sri Lanka (Teleostei: Siluridae), Zootaxa 4158 (2), pp. 261-271 : 262-267

Ompok argestes , sp. nov.

Ompok bimaculatus (not Bloch, 1794): Haig, 1952: 103 (in part); Deraniyagala, 1952: 51 (in part); Senanayake, 1980: 195 (in part); Pethiyagoda, 1991: 154 (in part).

Ompok ceylonensis (not Günther, 1864): Kottelat & Lim, 1995: 240.

Holotype: 2016.04.01.NH, 147 mm SL; Sri Lanka, Bentara River basin, Mathugama. H. Sudasinghe. Apr 2015.

Paratypes: 2016.05.01.NH 110 mm SL; same data as holotype. 2016.05.02.NH, 92.5 mm SL, Sri Lanka, Kelani basin, Karawanella. H. Sudasinghe, Jan 2015 . DZ 3199, 3200; same data as holotype. DZ 1380, 76.4 mm SL; Sri Lanka, Kalu basin, Dombagaskanda. H. Sudasinghe, Mar 2014 . DZ 1395, 3049, 87.3–94.7 mm SL, Sri Lanka, Deduru Oya basin, tributary Kolamunu Oya. H. Sudasinghe, Jan 2015 .

Diagnosis. Ompok argestes is distinguished from its Indian and Sri Lankan congeners by the combination of the following characters: body color pattern mottled; predorsal profile uniformly convex; maxillary barbels reach or extend slightly beyond the base of the dorsal fin; eye diameter 14.2–17.1% head length; body depth at anus 19.8–22.3% standard length (SL); head width 14.3–16.8% SL; caudal peduncle depth 5.6–6.5% SL; and vertebrae 48–52.

Description. For general appearance, see Figure 1 ; morphometric data are provided in Table 1 . Body laterally compressed; body depth greatest at dorsal-fin origin; head as broad as body, slightly depressed. Dorsal profile of head evenly convex ( Fig. 2 C).

Anterior profile of snout rounded. Anterior nostril tubular, placed anteromedial to base of maxillary barbel. Posterior nostril bordered by fleshy dorsal and ventral membranes, placed posteromedial to base of maxillary barbel.

Mouth terminal; lower jaw longer than the upper; gape almost horizontal, large; rictus beneath anterior orbital margin, separated from it by about a half-eye diameter. Rictal lobes well developed, subtended by deep submandibular groove. Thin supralabial fold extending from below orbit to base of maxillary barbel.

Jaw teeth depressible, villiform. Premaxillary teeth in narrow, gently-curved bands. Dentary teeth similar, bands extending from symphysis almost to mouth corners. First of row of dentary teeth just visible with mouth closed. Palatal teeth in two gently-curved arc-shaped patches on either side of midline.

O. argestes O. bimaculatus O. ceylonensis Pectoral-spine length 9.5 8.0–14.1 10.1 ± 1.8 8.6–11.8 10.9 ± 1.1 7.5–12.3 10.2 ± 1.4 Length of upper caudal-fin lobe 18.8 15.4–19.1 17.9 ± 1.3 18–21.3 19.7 ± 2.4 13.5–21.1 18.2 ± 2.3 Length of lower caudal-fin lobe 18.1 14.5–19.0 17.7 ± 1.5 16.7–19.1 17.9 ± 1.7 13.1–20.7 17.7 ± 2.3 Head length 22.5 22.5–24.1 23.5 ± 0.7 19.5–21 20.2 ± 0.6 19.8–25.2 22.5 ± 1.7 Head depth 13.5 13.3–16.4 14.4 ± 1.1 10.8–13.5 12.3 ± 1 10.8–14.3 12.7 ± 1.1 Head width 15.3 14.3–16.8 15.3 ± 0.8 11.5–13.7 12.5 ± 0.7 12.3–16.5 14.0 ± 1 Percent of head length

Snout length 41.0 34.3–41.5 39.9 ± 2.4 38.3–42.6 40.7 ± 1.5 37.4–44.2 41.1 ± 1.6 Eye diameter 14.2 14.2–17.1 15.9 ± 1.0 15.5–19.3 17.5 ± 1.4 11.3–17.3 14.7 ± 1.5 Postorbital head length 54.6 46.9–55.9 51.3 ± 3.3 44.9–53.7 49.8 ± 2.6 45.0–54.3 50.8 ± 2.2 Interorbital width 54.7 44.6–54.7 51.4 ± 3.4 51.2–57.1 54.6 ± 2.1 46.8–57.8 51.5 ± 3.5 Internarial width 27.3 27.3–32.9 30.0 ± 1.9 29.1–32.4 30.9 ± 1.3 27.9–32.3 30.3 ± 1.5 Internarial length 8.7 8.7–14.0 11.1 ± 2.1 9.9–11.8 11 ± 0.7 7.4–12.5 10.4 ± 1.4 Maxillary-barbel length 136 121–166 142 ± 16.9 198–254 228 ± 20 108–166 135 ± 15.5 Mandibular-barbel length 24.0 11.5–28.6 21.5 ± 5.4 34.4–41.9 38.3 ± 2.9 16.1–33.7 23.5 ± 4.8 Two pairs of barbels, slightly flattened along entire length. Maxillary barbel reaching base of dorsal fin or slightly beyond. Mandibular barbel reaching posterior border of eye or slightly beyond.

Eyes small, subcutaneous (without free orbital margin), located slightly anterior to midway of head, above posterior region of suparalabial fold. Dorsal orbital margin visible dorsally; ventral-orbital margin only just visible ventrally.

Gill membranes separate, overlapping, free from isthmus. Gular fold well developed, V-shaped. Gill rakers long, thin; 3+9 (1).

Dorsal fin small, with i,3 (8) rays. Pectoral fin with distal margin broadly convex, tip rounded, extending to base of second anal-fin ray. Proximal two-thirds of first unbranched ray of pectoral fin co-ossified into a slender spine with shallow oblique striae on dorsal and ventral surfaces; pectoral spine smooth (5) or with 13 (1) or 17 (1) posterior serrations. Pectoral fin with 12 (2), 13 (5), 14 (1) branched rays. Depressed pelvic fin reaching base of second anal-fin ray, its distal margin convex, with i,6 (8) rays. Distal margin of anal fin straight, with 56 (1), 57 (2), 59 (1), 60 (2), 62 (1) or 63 (1) rays, separate from caudal fin. Integument over anal fin thickened for approximately two-thirds of ray length on anterior region of fin, gradually reducing to about half of ray length posteriorly. Caudal peduncle slender. Caudal fin deeply forked, lobes elongated, with rounded tips; upper lobe slightly longer than lower; principal rays i,7,7,i (2), or i,7,8,i (5). Vertebrae 10+38=48(1), 9+40=49(1), 9+42=51(1) or 10+42=52(1).

Statistical analysis. Ompok argestes , O. bimaculatus and O. ceylonensis separate distinctly from each other in morphological space ( Fig. 2A , Table 2 ). Principal component analysis shows the three species to be well separated: the PC(1) axis, which explains 92% of the variance, is a size axis (all of the variables load heavily with high positive loadings on this axis); the PC(2) axis represents 4.35% of the variance, with maxillary barbel length, mandibular barbel length, pectoral spine length, head width and pre-pelvic length, contributing most heavily. The results of the Mann-Whitney U test are provided in Table 3 . Pre-dorsal length, pre-pelvic length, caudal-peduncle length, body depth at anus, head depth, head width, and eye diameter showed significant differences between O. argestes and O. ceylonensis (P ≤ 0.05 and U <30). Pre-dorsal length, post-dorsal length, pre-anal length, pre-pelvic length, dorsal-fin base length, anal-fin base length, head length, head width, eye diameter, inter-orbital width, maxillary-barbel length and mandibular-barbel length showed significant differences between O. bimaculatus and O. ceylonensis (P ≤ 0.05 and U <30).

Coloration. In 70% alcohol: body and head mottled greyish brown, fading to cream ventrally. Humeral region with a hazy black blotch, a faint line of melanophores proceeding along lateral line up to caudal-fin base. A grayish-black region beneath operculum, anterior to humeral spot. Maxillary barbel greyish brown throughout; mandibular barbel cream.

In life: body and head mottled greyish brown, fading to cream ventrally. Humeral region with a hazy black blotch, not prominent. Dorsal, pectoral and caudal fins with scattered melanophores; anal fin slightly mottled greyish brown; pelvic fin hyaline. Maxillary barbel dorsolaterally greyish brown, ventrally cream; mandibular barbel cream.

Habitat and distribution. Ompok argestes was collected from among submerged roots and marginal vegetation in slow-flowing, shaded regions of rivers and streams with a substratum of mud or leaf debris. It is an uncommon species, evidently restricted the lowlands of southwestern Sri Lanka ( Fig. 2 B).

Etymology. The specific epithet argestes is a Homeric (Greek) term for ‘southwest’, a reference to the region of Sri Lanka in which the species occurs.

Figures

FIGURE 1. A, Ompok argestes, color in life (DZ 3049), Sri Lanka, Deduru Oya basin; B, O. argestes, coloration in 70 % alcohol (DZ 3201), Sri Lanka, Bentara basin.

FIGURE 2. A, Factor scores of PC 1 vs. PC 2 of the principal components analysis of measurements of Ompok argestes (black, n = 7), O. bimaculatus (red, n = 8) and O. ceylonensis (green, n = 8) based on 24 external measurements; B, Distribution of the species of Ompok in Sri Lanka: ● Ompok argestes (location of holotype in red); ● O. ceylonensis; The shape of predorsal profile in C – E, shape of the predorsal profile in C, Ompok argestes, D, O. ceylonensis and E, O. bimaculatus. Scale bar 5 mm.

Tables

TABLE 1. Morphometric data for Ompok argestes (n = 8; 2016.04.01. NH, 2016.05.01. NH, 2016.05.02. NH, DZ 1380, 1395, 3049, 3199, 3200), O. bimaculatus (n = 8; NBFGRKU. 2016.04.10.01 – 08) and O. ceylonensis (n = 16; DZ 1388, 1389, 1391 - 1394, 1423 - 1424, 3000 - 3005, 3211, 3212). Holotype values are included in range, mean and standard deviation.

	holotype	range	mean ± s.d.	range	mean ± s.d.	range	mean ± s.d.
Standard length (mm)	147	76.4–147	104 ± 21.6	145–255	171 ± 37	97.2–289	197 ± 52.4
Percent of standard length
Predorsal length	34.3	34–36.8	35.6 ± 1.1	29.1–31.5	30.3 ± 0.9	29.6–37.1	33.4 ± 1.9
Postdorsal length	68.5	66.5–69.2	68.0 ± 0.9	68.1–71.9	69.7 ± 1.3	66.0–71.1	68.4 ± 1.6
Preanal length	35.8	35.8–39.6	37.8 ± 1.4	34.5–36.4	35.4 ± 0.7	34.3–44.1	39.9 ± 3.3
Prepelvic length	32.2	32.2–34.7	33.6 ± 1.0	30.6–32.9	31.5 ± 0.8	31.0–40.0	35.8 ± 2.5
Caudal peduncle length	3.4	2.9–3.5	3.3 ± 0.3	3.2–4.6	3.9 ± 0.6	3.4–4.9	4.0 ± 0.5
Caudal peduncle depth	6.5	5.6–6.5	6.1 ± 0.4	5.7–7.2	6.6 ± 0.5	5.6–7.3	6.5 ± 0.5
Body depth at anus	21.1	19.8–22.3	21.2 ± 0.9	20.8–23.5	21.9 ± 0.9	19.6–26.4	23.6 ± 2.2
Dorsal-fin height	12.3	11.5–14.5	13.0 ± 1.1	10.4–13.9	12.3 ± 1.2	8.9–14.3	11.7 ± 1.5
Dorsal-fin base length	2.0	1.9–2.6	2.3 ± 0.3	1.1–1.5	1.3 ± 0.3	1.3–3.2	2.1 ± 0.6
Anal-fin base length	62.4	59.1–62.4	61.5 ± 1.2	62.4–68.7	64.9 ± 2.4	51.7–65.0	59.7 ± 3.9
Pelvic-fin height Pelvic-fin base length Pectoral-fin height	6.7 2.9 15.8	5.4–7.3 2.5–4.1 15.2–17.4	6.7 ± 0.7 3.3 ± 0.6 16.4 ± 0.9	5.8–7.5 2.4–3.4 15–18	6.5 ± 0.6 3 ± 0.4 17.1 ± 1	5.4–7.1 2.2–3.6 12.5–19.4	6.6 ± 0.5 3.1 ± 0.5 16.6 ± 2.1

TABLE 2. Component loadings of principal component analysis of the morphometric measurements of Ompok argestes, O. bimaculatus and O. ceylonensis.

Measurement	Component 1	Component 2
Standard length	0.994	-0.016
Predorsal length	0.986	-0.115
Postdorsal length	0.992	0.003
Preanal length	0.973	-0.178
Prepelvic length	0.980	-0.179
Caudal peduncle length	0.968	-0.064
Caudal peduncle depth	0.985	0.092
Body depth at anus	0.981	-0.085
Anal-fin base length	0.983	0.083
Pelvic-fin height	0.977	0.049
Pelvic-fin base length	0.901	0.031
Pectoral-fin height	0.970	0.142
Pectoral-spine length	0.935	0.200
Head length	0.987	-0.130
Head depth	0.981	-0.090
Head width	0.968	-0.191
Snout length	0.983	-0.147
Eye diameter	0.971	0.143
Post orbital head length	0.974	-0.161
Inter orbital width	0.990	-0.096
Inter narial width	0.985	-0.129
Inter narial length	0.959	-0.098
Maxillary-barbel length	0.765	0.619
Mandibular-barbel length	0.792	0.573
Percentage of total variance explained	92.004	4.351