120 Fishery Bulletin 120(2) 
hence do not signal selection or demo- 
graphic expansion. 
The final length of the mtDNA CO1 
fragment after alignment of 22 sequences 
of Typhlachirus and representatives of 
Soleidae was 586 base pairs. All haplo- 
types in Typhlachirus were weakly dis- 
tinguished (0.5% on average) and formed 
a single clade with high support indices 
(100%; Fig. 6). Typlachirus was obviously 
most closely related to species of the genus 
Brachirus and monotypic genus Dexillus, 
as predicted from their morphological 
similarity (Chapleau, 1989; Desoutter 
and Chapleau, 1997; Desoutter et al., 
2001b). The distance between Typhlachi- 
rus and the closest species, B. harmandi, 
was 14.4%. 
¢ T. elongatus 
= T. caecus Discussion 
-4.8 4 T. lipophthalmus 
PC1 
Although in Eschmeyer’s Catalog of 
Fishes 3 species of blind sole are recog- 
nized as valid (Fricke et al., 2021), our 
new data indicate possible monotypy of 
the genus Typhlachirus. According to the 
latest review of blind soles (Evseenko 
and Bolshakov, 2018), based on data 
from the literature on all known speci- 
mens of T: lipophthalmus, T. caecus, and 
T. elongatus, as well as on the study of 3 
Figure 5 
Plot of the principal component (PC) analysis of morphological characters 
and body proportions of Mekong blind sole (Typhlachirus elongatus) (circles), 
T. lipophthalmus (triangles), and T: caecus (squares). Each symbol represents 
a specimen captured in the Mekong River delta of Vietnam in 2018 or a 
specimen described in the literature (Chabanaud, 1948; Hardenberg, 1931b; 
Pellegrin and Chevey, 1940; Tan and Grinang, 2020). 
only 2 haplotypes shared by more than 1 individual: Hap3 
(MW646926) was found in 3 fish, and Hap5 (MW646928) 
was found in 6 fish. Results of the analysis of genetic diver- 
sity in the sample indicate a moderate degree of mtDNA 
COI variation: haplotype diversity is 0.802, nucleotide 
diversity is 0.0029, and the number of polymorphic sites 
is 9 (or 1.4% polymorphic sites). Estimates of Fu and Li’s 
(1993) F and Tajima’s (1989) D were not significant and 
specimens of T: elongatus (Evseenko and Bolshakov, 2018), 
the species of the genus Typhlachirus differ mainly in 
meristic characters (Table 4). Typhlachirus lipophthalmus 
differs from T: caecus in the number of rays in the dorsal 
fin and from T. elongatus in the number of rays in the anal 
fin. Typhlachirus caecus differs from T. elongatus in the 
number of vertebrae and rays in the anal fin (Table 4). It 
should be noted that the revision by Chabanaud (1948) 
Table 3 
Loadings of the first and second principal components (PC) from principal 
component analysis of morphological characters of Typhlachirus lipophthal- 
mus, T. caecus, and the Mekong blind sole (T. elongatus) from the sample that 
combines data for specimens collected in 2018 in the Mekong River delta and 
data from descriptions in the literature (Chabanaud, 1948; Hardenberg, 1931a; 
Pellegrin and Chevey, 1940; Tan and Grinang, 2020). Features include number 
of rays in dorsal fin (D), number of rays in anal fin (A), number of rays in caudal 
fin (C), body depth at pectoral fin base (BD), head length (HL), and number of 
pored scales on the horizontal branch of the ocular side lateral line not including 
scales on caudal fin (LL). 
D A Cc BD HL LL 
Component 
PC1 -0.0642 -0.0544  -0.0046 -0.0002 
PC2 0.7963 0.5990 0.0118 0.0013 
—0.0007 0.9964 
—0.0016 0.0840 
