118 
Fishery Bulletin 120(2) 
Figure 3 
Photograph showing the general view of a Mekong blind sole (Typhlachirus 
elongatus), 71 mm in standard body length, caught on May 2010 in the Mekong 
River delta of Vietnam. The arrow points to the right pectoral fin. The scale 
is 1 cm. 
varying degrees. The dorsal and anal fins join with the 
caudal fin. The pectoral fin on the right side of the body is 
present in all specimens. The pectoral fin on the left side is 
a transparent membrane; it has a wide base and 3-6 
widely spaced rays, half covered by the operculum. The 
lateral line is straight, extending to the caudal fin on both 
sides of the body. Both sides of the body are covered with 
ctenoid scales. The right side of the body is uniformly col- 
ored pinkish brown, and the left side is pinkish white. 
Morphology 
Meristic characters and proportional measurements are 
presented in Table 1. The body is right-sided, strongly com- 
pressed, broad in front, and tapered toward the tail. The 
caudal fin merges with the dorsal and anal fins; the caudal 
peduncle is absent. The greatest body depth is 37-47% SL. 
The head is large, and its length is 20-31% SL. The ante- 
rior profile of the head is rounded, and the snout protrudes 
slightly anterior to the mouth. The epicranial complex con- 
sists of 5(6) pterygiophores associated directly with the 
erisma, the 0(1) pterygiophore between the erisma and sec- 
ond neural spine, and 6(7) pterygiophores attached between 
the neural spines of the second and third vertebrae. The edge 
of the isthmus is far behind the vertical through the corner 
of the mouth. The mouth is terminal, and its anterior half is 
curved in the ventral direction; the length of the mouth is 
33-37% HL. A short dermal fringe surrounds the jaws. The 
teeth on the jaws are present only on the left side. The teeth 
on both jaws are long, thin, pointed, and slightly curved. 
The number of rows is from 1 to 4, and the number of rows 
increases from front to back. 
The left eye is absent (the eye is covered under skin and 
muscles, and the frontal bone and lateral ethmoid bone 
limit the eye orbit). Only the lower (right) eye, located just 
above the corner of the mouth, is visible; the eye is not 
pigmented. The anterior nostril on the ocular side of the 
body is tubular and located above the edge of the maxilla; 
from below it is covered with a valve. 
The posterior nostril on the ocular side 
of the body is rounded, located between 
the eye and the anterior nostril, and cov- 
ered from above by a valve. The anterior 
nostril on the blind side of the body is 
rounded, located above the middle of the 
maxilla, and surrounded by cilia, and it 
has a valve. The posterior nostril is tubu- 
lar and located above the corner of the 
mouth. 
Gill rakers are absent from the gill 
arches. The gill membranes are covered 
with skin. The upper end of the gill 
opens at the level of the eye. Scales are 
small, are ctenoid on both sides of the 
body, and have 5-10 cteni. On the ocu- 
lar side, the largest scales are located 
in the anteriodorsal part of the body, 
size of scales decreases in the caudal 
direction; scales on the blind side are 
approximately equal to the smallest scales on the ocular 
side of the body. 
The lateral lines on the blind side and ocular side are 
straight; the number of pored scales on the ocular side to 
the end of the scale cover is 84—108, and the number on the 
blind side is 120-159. The lateral line on both sides of the 
body continues to the caudal fin and runs between 6 and 7 
rays; it additionally contains from 23 to 33 perforated scales 
on the ocular side and from 30 to 50 scales on the blind side. 
The dorsal fin begins at the most protruding part of the 
rostrum; the anal fin begins somewhat in front of the level 
of the base of the right pectoral fin. The length of the rays 
in the dorsal and anal fins gradually increases in the caudal 
direction. The caudal fin is long and pointed. 
The pectoral fin on the ocular side of the body is present 
in all specimens; it contains from 1 to 3 unbranched rays 
(Fig. 4A). The fin size is comparable in size to the scales. 
The pectoral fin on the blind side is presented in the form 
of a transparent membrane with a broad base, half cov- 
ered by the operculum; it contains 3-6 rays (Fig. 4B). Both 
rays in the pelvic fin are branched and simple. The right 
pelvic fin is connected to the anal fin by a membrane; the 
left ventral fin is free. The anus is located in the anterior 
quarter of the body, between the anal and pelvic fins, and 
is displaced to the left side. The right side of the body is 
uniformly colored pinkish brown, each edge of the scales is 
bordered in black, and the left side is pinkish white. 
Biology 
Little is known about the biology of this species. Typhlachi- 
rus elongatus is a benthic fish and lives in muddy brackish 
waters on soft, silty substrates. In the Mekong River delta, 
the locations where specimens were captured are concen- 
trated within the estuarine ecotone formed by the 
6 branches of the river. In the Hau River, T: elongatus were 
more common in the river’s main course, and in the estua- 
rine areas and the coastal sea zone, they were practically 
