Acrochordonichthys gyrinus, Chavalit Vidthayanon & Heok Hee Ng, 2003

Chavalit Vidthayanon & Heok Hee Ng, 2003, Acrochordonichthys gyrinus, a new species of akysid catfish (Teleostei: Siluriformes) from Thailand., Zootaxa 183, pp. 1-7 : 2-6

Acrochordonichthys gyrinus sp. nov. (Figs. 1, 2a & 3a)

Acrochordonichthys ischnosoma (non Bleeker) - Vidthayanon et al., 1998: 46.

Type material. Holotype: NIFI 2645 , 71.7 mm SL; Thailand: Phitsanulok Province, Yom River at Prompiram ; the Inland fishery Station of Phitsanulok Province and C. Vidthayanon, 29 Nov 1995.

Paratypes: NIFI 3133 , 2 ex., 69.3-69.6 mm SL; UMMZ 240152 , 1 ex., 69.6 mm SL; data as for holotype .

Diagnosis. Acrochordonichthys gyrinus can be distinguished from all congeners of the A. ischnosoma species group by a concave (vs. straight) posterior margin of the pectoral fin. Acrochordonichthys gyrinus further differs from A. ischnosoma by a smaller dorsal to adipose distance (7.4-8.6% SL vs. 9.0-10.1); from A. guttatus by a more slender body (body depth at anus 8.3-10.5% SL vs. 10.9-12.8) and a thicker humeral process (maximum width of humeral process 16.3% its length vs. 10.0-11.8); from A. mahakamensis and A. strigosus by a straight (vs. rounded) posterior margin of the adipose fin; from A. septentrionalis by a shallower head (9.8-11.4% SL vs. 12.8-13.6).

Description. Body moderately depressed. Dorsal profile rising evenly but not steeply from tip of snout to origin of dorsal fin, then sloping gently ventrally from there to end of caudal peduncle. Ventral profile flat to anal-fin base, the sloping gently dorsally from there to end of caudal peduncle. Anus and urogenital openings located at vertical through middle of adpressed pelvic fin. Head covered with small tubercles with poorly demarcated and indistinct margins, body with such tubercles arranged in 5-6 longitudinal rows on each side. Lateral line complete.

Head depressed and broad, acutely triangular when viewed laterally and with rounded snout margin when viewed from above. Gill openings narrow, extending from a point immediately ventral to base of pectoral spine to a point midway between base of pectoral spine and isthmus. Gill membranes attached to isthmus. Midline of cranium with fossa extending from base of supraoccipital to a point midway between anterior orbital margin and tip of snout; posterior half of fossa occupied by posterior fontanel, separated from slender anterior fontanel by wide epiphyseal bar. Supraoccipital spine elongate, with parallel sides and blunt posterior tip reaching interneural.

Barbels in four pairs. Maxillary barbel slender, extending two thirds of distance to base of pectoral spine. Nasal barbel short, extending to anterior margin of orbit. Inner mandibular-barbel origin close to midline; barbel thicker and longer than nasal barbel and extending to level of mouth rictus. Outer mandibular barbel originates posterolateral of inner mandibular barbel, extending almost to base of pectoral spine.

Eye ovoid, horizontal axis longest; located entirely in dorsal half of head. Orbit with free margin.

Mouth inferior, anterior fifth of premaxillary toothband exposed when mouth closed.. Oral teeth small and viliform, in irregular rows on all tooth-bearing surfaces. Premaxillary tooth band curved, continuous across midline, and of equal width throughout. Dentary tooth band much narrower than premaxillary tooth band at symphysis, tapering laterally. Dorsal fin located above anterior third of body; origin nearer tip of snout than caudal flexure; fin margin straight. Dorsal-fin spine long and slightly curved. Adipose fin with margin straight for entire length; posterior margin straight. Caudal fin emarginate. Procurrent rays symmetrical and extend only slightly anterior to fin base. Anal-fin base ventral to posterior half of adipose fin. Fin margin straight. Pelvic-fin origin at vertical behind posterior end of dorsal-fin base. Pelvic-fin margin slightly convex, tip of adpressed fin not reaching anal-fin origin. Pectoral fin with long, curved spine. Anterior spine margin smooth along entire length; posterior spine margin with six strong serrations along entire length. Pectoral-fin margin posteriorly concave.

Morphometric and meristic data as in Table 1.

Colour. Dorsal surface of head and body brown, with dark brown spots scattered randomly on dorsal surface of head. Abdomen, chest and ventral surface of head cream, without spots. Dorsal, pectoral, pelvic and anal fins cream with two dark brown bands, one at base of fins and another near edge. Caudal fin cream, with dark brown band near edge and dark brown patch at base of caudal peduncle. Barbels and pectoral spines cream, with dark brown spots on dorsal surfaces.

Distribution and habitat. Known only from the type locality, the Yom River at Phitsanulok, part of the upper Chao Phraya basin in Thailand.

Acrochordonichthys gyrinus inhabits the bottom of the river, which has a mixed sand/mud subtrate with a moderately strong current. Other fish species found syntopically include Crossocheilus reticulatus , Henicorhynchus lineatus , Botia helodes , B. modesta , Acantopsis spp., Akysis sp., Glyptothorax spp., Polynemus multifilis , Cynoglossus microlepis , and Cynoglossus sp.

Etymology. From the Latin gyrinus, meaning rounded or curved, in allusion to the concave posterior margin of the pectoral fin in this species.

Discussion

Ng & Ng (2001) divided Acrochordonichthys into two species-groups based on distinct differences in morphology. With its slender caudal peduncle (depth of caudal peduncle 4.7-5.3% SL), narrower head (head width 18.8-20.8% SL), and 39 vertebrae, A. gyrinus clearly belongs to the A. ischnosoma species-group as defined by Ng & Ng (2001). Besides A. gyrinus , there are five species of Acrochordonichthys belonging to the A. ischnosoma species group, viz. A. ischnosoma Bleeker , 1858, A. guttatus Ng & Ng , 2001, A. mahakamensis Ng & Ng , 2001, A. septentrionalis Ng & Ng , 2001, and A. strigosus Ng & Ng , 2001.

The shape of the posterior margin of the pectoral fin is a key character used to diagnose A. gyrinus from congeners in the A. ischnosoma species group. This is another character useful for diagnosing Acrochordonichthys species in addition to those discussed by Ng & Ng (2001), as there is no variation in the shape of the fin margin in all of the material we have examined.

The record of A. ischnosoma from the Chao Phraya River drainage by Vidthayanon et al. (1998) refers to A. gyrinus , and not A. septentrionalis as postulated by Ng & Ng (2001).