![]() ![]() Dorsal-saddle count was taken posterior to the opercle. Counts of abdominal vertebrae include the Weberian complex ( n = 5 vertebrae), with the first caudal vertebra being the first one with its hemal spine posterior to the anteriormost anal-fin pterygiophore, and counts of caudal vertebrae including the urostyle complex. Counts of vertebrae were made on micro-CT (computed tomography) scans. The last branched ray of the dorsal and anal fins, sharing a pterygiophore with the penultimate ray, is given as ½. Total ray counts are given for paired fins branched ray counts are given for unpaired fins. All measurements were taken to the nearest 0.1 mm using digital calipers and from the left side when possible. Measurements and counts follow Armbruster (2012) except for caudal-peduncle length, which was measured from the rear of the anal-fin base to the middle of the caudal-fin base, and prepectoral, prepelvic, and preanal lengths, which were taken from the anterior tip of the snout to the origin of each fin. ![]() Tissue samples for DNA analysis were taken from one or more specimens at most sites by excising fins and placing them in 95% ethanol. ![]() Specimens were collected in Cambodia, Malaysia, and Thailand using nets of various sizes, euthanized in MS-222, fixed in 10% formalin, and transferred to 70% ethanol for storage. In the process, we discovered an undescribed species of Nemacheilus, described herein, in large tributaries of the Mekong River in the Khorat Plateau ecoregion ( Abell et al., 2008) of Thailand. Analyses of morphological and molecular data from across the hypothesized distributions of these two nominal species were conducted to test their monophyly as well as examine their relationships to other species of Nemacheilus for which molecular data were available. pallidus based on the descriptions in Kottelat (1990) even though some specimens were from, or collected close to, the type localities of both species. Recently collected specimens from several drainages in Cambodia, Malaysia, and Thailand could not confidently be assigned to either N. troglocataractus another 13 species occur in Indonesia, and five occur in India ( Fricke et al., 2019). TWELVE species of Nemacheilus currently are recognized in mainland southeast Asia: N. Nemacheilus masyae occurs throughout mainland southeast Asia, including in the Chao Phraya, Mae Klong, Mekong, and coastal drainages of the Malay Peninsula. The new species is known from the Songkhram and Mun river drainages in Thailand and appears to be restricted to the Khorat Plateau ecoregion of the Mekong River basin. masyae, which reaches a much larger size-to 66.2 mm SL-and otherwise is easily distinguished from the new species. Molecular phylogenetic analyses suggest that the new species is most closely related to N. ![]() The new species, described herein, is small-with a maximum-known standard length of 28.6 mm-and has a distinctive color pattern of dusky black bars along the side of the body that cross over the back and join the bars on the other side. masyae, and an undescribed species of Nemacheilus occurs in large tributaries of the Mekong River in Thailand. Analyses of morphological and molecular data from recently collected specimens of Nemacheilus from Cambodia, Malaysia, and Thailand indicate that N. ![]()
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