Mai et al. (1992) described one specimen as the name Scaphidonichthys sp. from Phu Lap, Tan Phu, Dong Nai
province near the Dong Nai River at elevation (11°27’17.23” N, 107°29’4.40” E, 155 m). We consider this species
conspecific with O. dongnaiensis based on examination of its morphometrics (Mai et al. 1992) similar to our O.
dongnaiensis specimen (UNS00856) collected at the same locality.
Morphological data. Predorsal scales and eye diameter are considered reliable taxonomic characters, and to
our knowledge, the short eye diameter (4.2−5.6 times in HL) in the two new species is extreme among all
Onychostoma species from southeast Asia (Taki 1975, Kottelat 1998, Lothongkham & Musikasinthorn 2005).
Onychostoma krongnoensis is most similar to O. dongnaiensis, but differs most dramatically in colour in life.
Onychostoma krongnoensis is dark turquoise on the back and yellow to pinkish orange on lateroventral surface
(Fig. 2), and O. dongnaiensis is light turquoise on the back, and bluish silver-white on the lateroventral surface
(Fig. 5). Onychostoma krongnoensis also has a slightly deeper body (3.5−4.3 vs. 3.1−3.4 times in SL) and a slightly
shorter caudal-peduncle (3.8−4.2 vs. 4.1−4.5 times in SL). The number of epidermal tubercles on the snout also
may vary in the two species with more on O. krongnoensis, although seasonal and ontogenetic variation has not
been studied.
Molecular data. The newly collected specimens from the Krong No and Dong Nai rivers are embedded within
a clade containing all Mekong Onychostoma species (Fig. 6). This clade receives 100% bootstrap support.
Onychostoma meridionale was also embedded within this clade.