Misawa et al.: Population structure of Okamejei kenojei 
25 
OEC: n =31 (29) 
O YS: n=10 (7) 
+ SJ:77=74 (72) 
AEK: n=1 (8) 
□ OS: n=24 (27)' 
8 NP: 77=54 (69) 
Tsugaru Strait 
Tsushima 
Current 
Sea of Japan 
ihoshi 
China 
Honshu 
Kuroshio 
Current 
Yellow Sea 
Tsushima 
Strait 
'Osaka Bay 
'Seto Inland Sea 
East China Sea 
'Kagoshima 
Pacific Ocean 
120' E 130'E 140'E 
Figure 1 
Sampling area and number of examined specimens of Okamejei 
kenojei. Each symbol represents a sampling site. n=number of speci¬ 
mens used in mtCR analysis and numbers in parentheses=number 
of specimens used in morphological comparisons. EC=East China 
Sea; YS=Yellow Sea; SJ=Sea of Japan; EK=East coast of Kyushu Is.; 
OS=Osaka Bay; NP=Pacific coast of northern Japan. 
their lifetime, although some may produce up to 600 
over 4 years (Ishihara et al., 2009). The egg capsules 
(43-59 mm length) have tendrils on each corner, which 
anchor the capsules to the seafloor (Ishiyama, 1958). 
Skates generally have a low dispersal ability (Var- 
gas-Caro et al., 2017). In fact, previous tagging studies 
have suggested that some skates have relatively small 
home ranges (e.g., Walker et al., 1997), and population 
genetics have also revealed evidence of a local popu¬ 
lation structure of skates (e.g., Chevolot et al., 2006). 
Similarly, O. kenojei was formerly recognized as two 
allopatric species, Raja porosa, known from the East 
China Sea, Yellow Sea, Sea of Japan, and the Pacific 
coast of southern Japan, and R. fusca, known from the 
Pacific coast of northern Japan (Ishiyama, 1967). Ac¬ 
cording to Ishiyama (1967), the nominal forms could 
be distinguished by differences in exteimal characters, 
such as snout length, interorbital width, and cranium 
shape. Later, Boeseman (1979) suggested that R. po¬ 
rosa was a junior synonym of R. kenojei (=Okamejei 
kenojei), and was followed by Ishihara (1987), who con¬ 
sidered both R. porosa and R. fusca to be junior syn¬ 
onyms of O. kenojei owing to similarities in the distri¬ 
butional pattern of ventral sensory pores and similari¬ 
ties in clasper structure. Nevertheless, Ishihara (1987) 
recognized several morphological variants among local 
populations, in particular in reference to individuals 
from the Pacific coast of northern Japan, which were 
previously recognized as R. fusca, i.e., as 
the “northern form.” Ishihara (1987) sug¬ 
gested that the “northern form” differed 
from forms in other areas in having a 
bluntly angled and extremely translucent 
snout, numerous nuchal thorns, and black 
spots or reticulated patterns scattered over 
the entire dorsal surface of the disc. Such 
40' N morphological divergence among local pop¬ 
ulations has suggested the existence of a 
complex population structure within the 
species. 
Okamejei kenojei has considerable eco¬ 
nomic value in Korea and Japan (Ishihara, 
1990; Ishihara et al., 2009; Baeck et al., 
2011). Domingues et al (2018) suggested 
that management and conservation policies 
30” n f° r shark and ray fisheries should include 
information on genetic diversity. However, 
despite the potential population structur¬ 
ing and high economic value of O. keno¬ 
jei, there have been no studies of genetic 
structure at the population level for this 
species—information that is crucial for fish¬ 
eries management and conservation prac¬ 
tices. In some skate species, molecular ge¬ 
netic studies have revealed both population 
structure and demographic history, as well 
as the existence of cryptic species based 
on mitochondrial DNA (mtDNA) sequences 
(e.g., Valsecchi et al., 2005; Chevolot et al., 
2006; Griffiths et al., 2010; Griffiths et al., 
2011; Spies et al., 2011; Dudgeon et al., 2012; Frodella 
et al., 2016; Im et al., 2017; Vargas-Caro et al., 2017). 
The mtDNA control region (mtCR) has been frequently 
used to infer population structures within species be¬ 
cause of the high levels of nucleotide polymorphism 
evident in several skate species (Valsecchi et al., 2005). 
In this context, we examined the population structure 
of O. kenojei by analyzing mtCR sequences and mor¬ 
phological variations and present an outline of popu¬ 
lation boundaries and their connectivity for fisheries 
management. 
Materials and methods 
Samples 
For the mtCR analysis and morphological comparisons, 
a total of 293 individuals of Okamejei kenojei were col¬ 
lected from six regions along the Japanese Archipelago 
(regional populations abbreviated as EC (East China 
Sea); SJ (Sea of Japan); EK (East coast of Kyushu Is.); 
OS (Osaka Bay); NP (Pacific coast of northern Japan); 
and YS (Yellow Sea) (Fig. 1; Suppl. Table) (online only). Of 
these, 194 individuals (EC=31; YS=10; SJ=74; EK=1; 
OS=24; NP=54) were used for mtCR sequence analysis, 
212 individuals (EC=29; YS=7; SJ =72; EK=8; OS=27; 
NP=69) for morphological comparisons, and 113 indi- 
