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Mystus cavasius (Hamilton, 1822)
Bagrus cavasius Valenciennes, in Cuvier & Valenciennes, 1840: 409; Jacquemont, 1835-1844: Pl. XIV Fig. 2; Bleeker, 1854: 113.
Macrones cavasiusGünther , 1864: 76 (in part); Day, 1877: 447, Pl. C Fig. 1 (in part); Day, 1889: 155 (in part); Jenkins, 1910: 140.
Mystus cavasius Shaw & Shebbeare, 1937: 91, Fig. 90, Pl. 3 Fig. 3; Chauhan, 1947: 276; Chauhan & Ramakrishna, 1953: 411; Motwani et al., 1962: 21; Bhuiyan, 1964: 63; Singh, 1964: 89; Qureshi, 1965: 42, Fig. 106; Babu Rao & Chattopadhyay, 1969: 887, Pl. 2 Fig. 1; Rahman, 1974: 2, Fig. 1G; Pillai & Yazdani, 1977: 5; Jayaram, 1977: 29, Fig. 21A (in part); Jayaram & Singh, 1977: 262; Jayaram, 1981: 196, Fig. 92A (in part); Shrestha, 1981: 155, Fig. 73; Dutt et al., 1982: 27 (in part); Sharma & Dutt, 1983: 334 (in part); Husain & Tilak, 1984: 275; Sen, 1985: 136, Fig. 74; Edds, 1986a: 5; Edds, 1986b: 17; Sharma & Rajput, 1986: 566; Barman, 1988: 49; Datta Munshi & Srivastava, 1988: 235, Pl. XXIX Fig. 2; Rahman, 1989: 200; Roberts, 1989: 124 (in part); Talwar & Jhingran, 1991: 559, Fig. 184 (in part); Sen, 1992: 180, Fig. 58; Dutta et al., 1993: 25; Roberts, 1994: 248 (in part); Shrestha, 1994: 52, Fig. 79; Sen, 1995: 561, Pl. XXVI Fig. 2; Husain, 1997: 595; Jayaram, 1999: 235, Fig. 118C (in part); Menon, 1999: 200 (in part); Nath & Dey, 2000: 89, Fig. 78, Pl. 2-12; Barman, 2002: 263, Fig. 66; Jayaram & Anuradha Sanyal, 2003: 46, Fig. 5 (in part); Mishra et al., 2003: 26.
Mystus (Mystus) cavasius Jayaram, 1954: 532, Fig. 2 (in part); Motwani & David, 1957: 11; Majumdar, 1958: 368; Srivastava, 1968: 71, Fig. 45; Misra, 1976: 87, Fig. 18 (in part); Gupta, 1985: 17, Pl. IB.
Material examined. BMNH 18220.127.116.115, 113 mm SL; India (photograph examined) . BMNH 1918.104.22.168 (1), 121.8 mm SL ; BMNH 1922.214.171.124-128 (5), 57.8-71.5 mm SL; India : Bombay Presidency, Deolali district [=Maharashtra, Nasik district] , Darna River . BMNH 19126.96.36.199 (1), 108.9 mm SL; India : Bombay Presidency, Deolali district [=Maharashtra, Nasik district] , Unanda River . CAS 94220 (1), 80.7 mm SL; India : Orissa , Rushukulya River near Purushotampur, 15-20 km inland . CAS 50327 (32), 160.5- 67.4 mm SL; Nepal : Chitawan Valley, Dudara River, tributary to Rapti River . OSUS 15972 (2), 57.4-68.0 mm SL; Nepal : Sunsari , Sapt Kosi River, bought in market in Itahari . OSUS 17352 (1), 75.2 mm SL; Nepal : Nawalparasi , Narayani River at Taadi Ghat . OSUS 17434 (2), 75.5-77.9 mm SL; Nepal : Nawalparasi/Chitawan , Narayani River below Rapti River confluence . UMMZ 189647 (2), 84.6-88.0 mm SL; India : West Bengal , Santal Parganas, Kanloi River near Kotalpukur Railway Station . UMMZ 208686 (1), 63.7 mm SL; Bangladesh , Sylhet , Surma (Meghna) drainage, Shari River, 6.8 km S of Jaintapur on Sylhet-Shillong highway . UMMZ 208750 (10), 59.3-96.5 mm SL; Bangladesh : Sylhet , Surma (Meghna) drainage, Gowain River and Khal at Gowainghat . UMMZ 238800 (5), 130.1-152.4 mm SL; India : West Bengal , market in Calcutta . UMMZ 244745 (2), 97.9-103.9 mm SL; India : West Bengal , Mansai River, 1 km after Amtala on JalpaiguriCoochbehar road, 26°19'30"N84°14'4"E . UMMZ 244869 (2), 90.1-90.2 mm SL; India : West Bengal , Tista River at Tista Barrage, 26°45'1"N88°35'11"E . UMMZ 244939 (1), 100.3 mm SL; India : West Bengal Hooghly River at Kalna, 23°13'30"N88°22'39"E . USNM 205615 (2 paratypes of M. mukherjii ZBK ), 76.3-77.3 mm SL; India : Bihar, Ranchi district , Subarnarekha River, below waterfalls at Hundru .
Diagnosis. Mystus cavasius differs from other congeners with a long-based adipose fin (except M. seengtee and M. falcarius ZBK ) in having a combination of a black spot in front of the dorsal-spine base, a dark humeral mark, a body without distinct midlateral stripes, very long maxillary barbels reaching to caudal-fin base, dorsal spine short and feebly serrate, tall dorsal fin, and 13-22 gill rakers. Mystus cavasius differs from M. seengtee in having fewer gill rakers on the first gill arch (13-22 vs. 23-28; Table 1) and a more gently sloping predorsal profile (making an angle of 20-25° with the horizontal vs. 30-35°; Fig. 2), and from M. falcarius ZBK in having fewer rakers on the first gill arch (13-22 vs. 22-29; Table 1), a straight or gently concave (vs. markedly concave) dorsoposterior margin of the dorsal fin (Fig. 3), a faint (vs. very prominent) dark spot at the base of the dorsal spine and a ovoid (vs. crescentic) dark humeral mark.
Description. Biometric data are given in Table 2. Head depressed; dorsal profile evenly sloping (at angle of 20-25° to horizontal) and ventral profile almost straight. Bony elements of dorsal surface of head covered with thin skin; bones readily visible, especially on posterior half of neurocranium, and ornamented with numerous fine, radial grooves. Anterior cranial fontanelle extending from behind snout to line through posterior orbital margins, separated from posterior fontanelle by narrow epiphyseal bar. Posterior fontanelle extending to base of supraoccipital spine. Supraoccipital spine elongate, slender and with blunt tip; extending to anterior nuchal plate. Eye ovoid, horizontal axis longest; located entirely in dorsal half of head. Gill openings wide, extending from exposed surface of posttemporal to beyond isthmus at line through mouth corners. Gill membranes free from isthmus. First branchial arch with 13-22 long, slender gill rakers.
Mouth subterminal, fleshy upper lip extending anteriorly beyond upper jaw. Oral teeth small and villiform, 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 to point extending posteriorly well past level of premaxillary band; band width narrower than premaxillary band at midline, widening laterally and then tapering to a sharp point posterolaterally.
Barbels in four pairs. Maxillary barbel long and slender, extending to caudal-fin base. Nasal barbel slender, extending to vertical through base of pectoral spine. Inner mandibular-barbel origin close to midline; thicker and longer than nasal barbel and extending to base of posteriormost pectoral-fin ray. Outer mandibular barbel originating posterolateral of inner mandibular barbel, extending to vertical through middle of dorsal-fin base.
Body moderately compressed. Dorsal profile rising evenly but not steeply from tip of snout to origin of dorsal fin and sloping gently ventrally 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 and midlateral in position. Vertebrae 21+17=38 (2), 20+19=39 (1), 21+18=39 (5), 22+17=39 (2), 21+19=40 (9), 22+18=40 (10), 21+20=41 (3), 22+19=41 (4), 23+18=41 (1), 21+21=42 (1) or 22+21=43 (1).
Dorsal fin with spinelet, spine, and 6 (1) or 7 (38) rays. Origin of dorsal fin anterior to mid-body, about two-fifths of body. Dorsal fin margin straight or slightly concave, with first two fin rays longer than others. Dorsal fin spine moderately long, straight and slender, posterior edge with 3-4 indistinct serrations. Serrations fewer, lower and less distinct in smaller specimens. Anterior nuchal plate acutely triangular.
Pectoral fin with stout spine, sharply pointed at tip, and 6 (1), 7 (5), 8 (21), 9 (9) or 10 (3) rays. Anterior spine margin smooth; posterior spine margin with 12-22 serrations along entire length (serrations fewer in smaller specimens). Pectoral fin margin straight anteriorly, convex posteriorly. Postcleithral (humeral) process short and slender, with concave dorsal edge and extending to anterior tip of anterior nuchal plate.
Pelvic fin origin at vertical through posterior end of dorsal-fin base, with i,5 (39) rays and slightly convex margin; tip of adpressed fin not reaching anal fin origin. Anus and urogenital openings located at vertical through middle of adpressed pelvic fin. Males with a short genital papilla reaching to base of first anal-fin ray.
Adipose fin with very long base and deeply incised posterior portion, spanning almost all of postdorsal distance. Anal fin origin located at approximately middle third of adipose fin, fin with iv,6 (7), iv,7 (20), iv,8 (10) or iv,9 (2) rays and curved posterior margin.
Caudal peduncle moderately deep. Caudal fin deeply forked, with i,6,6,i (1), i,6,7,i (6), i,6,8,i (2), i,7,7,i (18) or i,7,8,i (12) principal rays; upper lobe slender and lanceolate, lower lobe pointed. Procurrent rays extending only slightly anterior to fin base.
Coloration. In 70% ethanol: dorsal surface of head and body uniform brownish gray in some individuals, silvery white in others. Dark spots in front of base of dorsal spine and on humeral region, faint in some specimens. Some individuals with distinct dark stripe along lateral line, consisting of densely aggregated melanophores and with pale stripe without melanophores immediately above. Ventral surfaces of head and body dirty white; adipose fin brownish gray. All fins hyaline, with melanophores on fin membranes on some individuals, usually more densely aggregated along margins with fin rays. Dorsal half of barbels gray dorsally, gradually turning to dirty white on ventral half and tips.
Distribution. Known from the Ganges, Brahmaputra, Mahanadi, Subarnarekhar and Godavari river drainages in northern India, Nepal and Bangladesh (Fig. 4). Records of M. cavasius from the Indus River drainage further to the west presumably refer to this species, but no material for verification was available to us.
Habitat and biology. Mystus cavasius is known from a wide variety of habitats, including both fast- and slow-flowing rivers and streams (Nath & Dey, 2000), where it reportedly feeds on invertebrates and, to a smaller extent, smaller fishes (Bhatt, 1971). In the Ganges, this species spawns in August and September (Bhatt, 1971).