Akysis portellus sp. nov., a new species of catfish (Teleostei: Akysidae) from the Sittang River drainage, Myanmar

Acknowledgements: I am grateful to Patrick Yap for help in obtaining specimens of A. portellus, and the following for permission to examine material under their care: David Catania (CAS), Maurice Kottelat (CMK), Sven Kullander (NRM), Martien van Oijen (RMNH), Douglas Nelson (UMMZ), Susan Jewett (USNM), Isaäc Isbrücker (ZMA), Kelvin Lim (ZRC) and A.K. Karmakar (ZSI). Financial support by research grant R-154-000-318-112 of the National University of Singapore to Heok Hui Tan is acknowledged.


INTRODUCTION
Akysid catfishes are small to medium-sized hillstream catfishes generally found in fast-flowing streams and rivers in South and Southeast Asia. Members of the type genus, Akysis, are small species (typically not larger than 50mm SL) with tuberculate skin and a cryptic color pattern generally consisting of yellow patches or bands on a brown body. They have a distribution ranging from the Irrawaddy River drainage to the west, the Citarum River drainage to the east and south and the Lancanjiang (upper Mekong) drainage to the north. The genus had been divided ino two species groups (the A. variegatus and the A. pseudobagarius species groups) by Ng & Kottelat (1998), with members of the A. pseudobagarius group being reassigned to the genus Pseudobagarius by Ferraris (2007). There is considerable hidden diversity within the genus; more than half of the 20 valid species have been described in the last decade (Ng & Kottelat 1998, 2004Ng & Tan 1999;Arunkumar 2000;Ng & Freyhof 2003;Ng & Rainboth 2005;Ng 2006Ng , 2007Page et al. 2007).
Recently, specimens of Akysis collected from the Sittang River drainage in Myanmar were made available to me. This material was initially identified as Akysis longifilis, but close examination revealed enough differences to warrant its recognition as a distinct species. The description of this new species as Akysis portellus sp. nov. forms the basis of this study.

Description:
Biometric data in Table 1. Body moderately compressed. 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, then sloping gently dorsally from there to end of caudal peduncle. Anus and urogenital openings located at vertical through middle of adpressed pelvic fin. Skin tuberculate. Lateral line extending just posterior to base of last anal-fin ray. Vertebrae 16+16=32 (2), 17+15=32 (2), 16+17=33* (4) or 17+16=33 (2).
Head depressed and broad, with rounded snout margin when viewed from above. Anterior nostril tubular, base of nostril not in contact with base of nasal barbel. Gill openings narrow, extending from immediately ventral to posttemporal to onethird of distance from ventral midline of body to base of pectoral spine. Bony elements of dorsal surface of head covered with thick, tuberculate skin. Eye ovoid, horizontal axis longest; located entirely in dorsal half of head.
Barbels in four pairs. Maxillary barbel long and slender, extending to vertical through middle of dorsal-fin base. Nasal barbel slender, extending to dorsalmost limit of gill opening. Inner mandibular-barbel origin close to midline, extending to base of pectoral spine. Outer mandibular barbel originating posterolateral of inner mandibular barbel, extending beyond base of last pectoral-fin ray.

Coloration:
In ethanol: dorsal surface and sides of head medium grayish brown, with few darker brown spots randomly scattered throughout. Dorsal surface and sides of body dark grayish brown. Belly, chest and ventral surfaces of head and body light brown. Dorsal half of body with two elongate saddleshaped light brown spots: first on body at anterior three-quarters Image 1. Akysis portellus sp. nov., ZRC 51138, holotype, 32.7mm SL; dorsal, lateral and ventral views.

Etymology:
From the Latin portella, the diminutive form of porta, meaning door. The name is used as a noun and alludes to the relatively small mouth of this species.

Distribution:
Known from the type locality in the Sittang River drainage, southern Myanmar (Image 1).

DISCUSSION
Five other species of Akysis are recorded from Myanmar (Ng 2008): A. longifilis, A. pictus, A. prashadi, A. vespa and A. vespertinus with a sixth species, A. manipurensis, being known from the Chindwin River drainage (itself part of the Irrawaddy River drainage) in India (Vishwanath et al. 2007). Only A. longifilis is known to occur sympatrically with A. portellus sp. nov. in the Sittang River drainage (the type locality of A. portellus is about 90km downstream along the Sittang River from that of A. longifilis; Image 1). Although both species are superficially very similar, a side-by-side comparison reveals the differences outlined in the diagnosis. These differences in the gross morphology are not due to sexual dimorphism because the differences are consistent across members of both species. Although it cannot be excluded that the differences reflect different geographical variants of the same species, the highly distinct degree of difference in the shapes of the head and the mouth makes it unlikely that consistent morphological differences of this nature are merely due to geographical variation. Furthermore, the caudal peduncle of A. portellus appears to be more slender than that of A. longifilis, although this difference is not translated measurably (caudal peduncle depth 6.07.0% SL in A. portellus sp. nov. vs. 5.67.2 in A. longifilis). There also appear to be slight differences in color between the two species: the pelvic and anal fins of all A. portellus specimens examined appear to lack any markings, while those of A. longifilis usually have very few brown spots that form indistinct transverse bands through the middle of the fins. Finally, A. portellus sp. nov. has fewer vertebrae than A. longifilis (3233 vs. 3335), although the overlap in vertebral counts diminishes its value as a diagnostic character. Despite the diversity of the group, the occurrence of two species of Akysis in close sympatry is only known from the Endau River drainage, where A. microps and A. hendricksoni occur syntopically in the Kahang River (Ng & Tan 1999), and possibly in the Mekong River drainage (although known distribution of the sympatric species are separated by greater distances than is the case for A. longifilis and A. portellus sp. nov.; see Ng & Sabaj 2005, Image 2). Given the close overall similarity between A. longifilis and A. portellus, it is not surprising that an unpublished phylogenetic analysis using nuclear and mitochondrial markers revealed them to be sister taxa. The evoluionary implications of this are still being investigated and will be reported on at a later date.