Cathorops (Cathorops) melanopus (Günther, 1864)

Figs. 19 and 20

Arius melanopus Günther, 1864: 172. Type locality: río Motagua, Guatemala. Syntypes: BMNH 1865.4.29.51-53; Regan, 1907: 126 (in part) [description, Guatemala].

Tachisurus melanopus; Eigenmann &Eigenmann,1888:146 [only name]; Eigenmann & Eigenmann, 1890: 88 [synonymy].

Cathorops melanopus; Taylor & Menezes, 1978 [only name]; Marceniuk & Ferraris, 2003:449 [distribution, Guatemala, río Motagua];Acero P., 2003: 837 [only name]; Kailola, 2004:132 [only name]; Marceniuk & Menezes, 2007: 45 [distribution, Guatemala, río Motagua]; Betancur-R. et al., 2007:349 [only name]; Betancur-R. & Willink, 2007 [key features and distribution, Guatemala, río Motagua].

Material examined. BMNH 1865.4.29. 51-53, 3 (1, 172.0 mm SL), Guatemala, río Motagua, syntype of Arius melanopus; AMNH 35241, 2, 115.0-143.0 mm SL, Guatemala, Izabal, río Motagua at Finca Hopi; UMMZ 197336, 3,126.0-188.0 mm SL, Guatemala, Izabal, río Motagua at Finca Hopi.

Diagnosis. Cathorops melanopus differs from all congeners by having 16-18 gill rakers on first arch (vs. 19-20 in C. belizensis, 14- 15 in C. fuerthii, 37-40 in C. hypophthalmus, 20-24 in C. mapale, and 19-22 in C. tuyra), 19-20 rays on anal fin (vs. 21-23 in C. agassizii, 21 in C. aguadulce, 21-24 in C. arenatus, 22-25 in C. fuerthii, 22-23 in C. hypophthalmus, 24-27 in C. manglarensis, 21-24 in C. mapale, 25-27 in C. multiradiatus, 22-25 in C. spixii, and 22-23 in C. steindachneri), snout length 5.3-5.5% SL (vs. 6.9- 8.9% SL in C. belizensis, 6.3-7.9% SL in C. fuerthii, 6.1-8.0% SL in C. higuchii, 8.6-9.6% SL in C. hypophthalmus, 6.0-8.6% SL in C. kailolae, 6.2-8.0% SL in C. manglarensis, 7.3-7.6% SL in C. steindachneri, 7.4-8.3% SL in C. taylori), and posterior margin of pectoral-fin spine with prominent serrations (vs. short and inconspicuous serrations in all other species, except in C. aguadulce, C. kailolae, C. multiradiatus, and C. tuyra) (Fig. 20).

Cathorops melanopus is further distinguished from C. aguadulce by possessing a shorter snout (5.3-5.5 vs. 9.3-11.6% SL), shorter distance from tip of snout to dorsal-fin origin (30.0- 32.6 vs. 39.0-40.7% SL), shorter distance from tip of snout to posterior margin of dorsomedian groove of neurocranium (17.9- 19.1 vs. 22.9-26.4% SL), and by lacking fleshy papillae intercalated with gill rakers on first two arches (vs. papillae present). Cathorops melanopus is additionally distinguished from C. belizensis in having a shorter head (22.3-23.0 vs. 25.8-31.9% SL), shorter distance between lateral cornu of lateral ethmoid and external limb of supracleithrum (19.3-19.6 vs. 22.2-26.4% SL), smaller distance between the anterior nostrils (3.6-4.4 vs. 5.1- 6.1% SL), shorter distance between posterior nostrils (4.4-5.1 vs. 5.9-7.6% SL), larger orbital diameter (4.5-5.0 vs. 3.6-4.4% SL), smaller interorbital distance (11.1-12.1 vs. 12.9-15.1% SL), narrower mouth(8.9-9.7 vs. 10.0-13.2% SL), narrower premaxilla(5.1- 5.8 vs. 6.3-11.9% SL), and intense black pigmentation on pelvic fin (vs. pale pigmentation on pelvic fin) (Fig. 19). Cathorops melanopus is further distinguished from C. higuchii in possessing a shorter distance from tip of snout to posterior margin of dorsomedian groove of neurocranium (17.9-19.1 vs. 20.9-26.3% SL), larger orbital diameter (4.5-5.0 vs. 3.3-4.4% SL), shorter distance between lateral cornu of lateral ethmoid and external limb ofsupracleithrum(19.3-19.6 vs. 20.4-25.9% SL), andintenseblack pigmentation on pelvic fin (vs. pale pigmentation on pelvic fin) (Fig. 19). Cathorops melanopus also differs from C. kailolae in possessing a shorter distance from tip of snout to dorsal-fin origin (30.0-32.6 vs. 33.1-38.0% SL), shorter distance from tip of snout to pelvic-fin origin (46.2-49.1 vs. 50.4-52.6% SL), shorter dorsal-fin spine (17.2-18.1 vs. 19.3-24.9% SL), higher caudal peduncle (8.7-9.0 vs. 6.6-8.0% SL), and by lacking fleshy papillae intercalated with gill rakers on first two arches (vs. papillae present). Cathorops melanopus is additionally distinguished from C. mapale species group in possessing a narrower cephalic shield at supracleithrum area (16.7-16.9 vs. 17.3-19.8% SL), shorter head (22.3-23.0 vs. 24.4-28.9% SL), narrower body (19.6- 20.1 vs. 20.5-22.3% SL), shorter distance from tip of snout to dorsal-fin origin (30.0-32.6 vs. 33.4-38.7% SL), shorter dorsal-fin spine (17.2-18.1 vs. 18.5-24.2% SL), and intense black pigmentation on pelvic fin (vs. pale pigmentation on pelvic fin) (Fig. 19).

Description. (Tables 1 and 6). Head short and depressed, profile slightly convex at level of frontals and supraoccipital. Body broader rather than deeper on pectoral girdle area. Cephalic shield rugose, relatively short and narrow on lateral ethmoid, frontal, supracleithrum, and epioccipital areas. Osseous bridge formed by lateral ethmoid and frontal long and slender, evident under skin. Dorsomedian groove of neurocranium relatively deep and large, its margins well marked, progressively narrower posteriorly. Supraoccipital process long and wide on posterior portion, profile straight. Nuchal plate crescent-shaped and moderate in size. Snout quite short, rounded on transverse section. Anterior and posterior nostrils close to one another. Eye lateral and relatively large. Interorbital distance and distance between nostrils and orbit short. Maxillary barbel surpassing base of pectoral-fin spine, external mental barbel surpassing margin of gill membrane, internal mental barbel reaching margin of gill membrane.

Mouth relatively small, lower jaw arched.Lips relatively thick. Vomerine tooth plates absent. One pair of small, narrow and elongated accessory tooth plates, quite distant from one another. Accessory tooth plates with small molariform teeth. Premaxilla quite narrow and long. Dentary with not so pronounced posterior projection, with many sharp teeth on anterior portion and few small molariform teeth on posterior portion.

Soft pectoral-fin rays 10. Pectoral-fin spine short; anterior margin with few granules on basal two thirds, distal third with short serrations; posterior margin straight on basal fourth, distal three quarters with long serrations. Soft dorsal-fin rays 7. Dorsalfin spine short, as long as pectoral-fin spine; anterior margin with granules on basal two thirds, distal third with short serrations; posterior margin serrated for almost its entire length. Pelvic fin high, with 6 rays.Anal fin high and long at base, with 19-20 (19) rays. Upper and lower lobes of caudal fin of moderate length, upper lobe longer than lower lobe. Caudal peduncle high.

Acicular gill rakers on first arch 16-18 (17), 5 or 6 (5) on upper limb, 11 or 12 (11) on lower limb. Spike shaped gill rakers on second arch 16-18, 4 to 6 on upper limb, 12 or 13 on lower limb. Mesial surfaces of all gill arches with developed gill rakers. Lateral and mesial surfaces of first and second gill arches without fleshy papillae intercalated with gill rakers.

Coloration in alcohol. Dorsal and lateral portions of head dark, ventrally light beige. Body with same dark color, progressively lighter towards lateral line and rather light beige under lateral line. Maxillary barbel dark, mental barbels light, fins dark especially towards edge.

Sexual dimorphism. Sex of specimens was not examined (see Material and Methods).

Distribution. Material examined is from the río Motagua in Guatemala, type locality of the species (Fig. 6). Probably present in the Honduran portion of the río Motagua, near its mouth. The species is restricted to freshwaters.

Vernacular names. Dark Sea Catfish, bagre prieto.

Remarks. Because C. melanopus is restricted to the freshwaters of río Motagua (in Guatemala and probably Honduras), the recognition of specimens from Mexico, Nicaragua, Costa Rica and Panama as C. melanopus by previous authors (e.g. Steindachner, 1876; Meek & Hildebrand, 1923; Miller, 1966; Gilbert & Kelso, 1971; Castro-Aguirre et al., 1999) is erroneous.