32 
Fishery Bulletin 11 7(1-2) 
to 
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o 
Y_ 
c 
Y_ 
o 
(U 
n 
E 
3 
Figure 6 
Plots of number of tail thorn rows of females within 6 re¬ 
gional populations of Okamejei kenojei. (A) EC, YS, and SJ, 
(B) EK, OS, and NP. 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. 
2016). Therefore, further study of the species based on 
nuclear DNA or other mtDNA markers, or both, would 
reveal more details in population structure and associ¬ 
ated morphological variations. 
In the haplotype network for ocellate spot skate, 
the OS haplotypes (Okl3 and Okl4) were distant from 
others, but these two haplotypes are closely related to 
each other in the network (Fig. 2). In addition, OS in¬ 
dividuals were characterized by a smaller eye, longer 
head and snout, and fewer nuchal thorns than those 
in other regional populations (Tables 3 and 4; Figs. 3 
and 4), clearly suggesting significant isolation of the 
OS population from the other populations. In fact, the 
OS individuals were collected only from Osaka Bay, 
situated at the innermost part of the Seto Inland Sea 
(Fig. 1), and O. kenojei were not collected from other 
marginal areas of Osaka Bay. This finding may suggest 
that this species is inhabiting patchy areas and exhib¬ 
iting discontinuity in around the Seto Inland Sea, and 
its distributional area is also isolated from other areas. 
Although one of two haplotypes of the ocellate spot 
skate in NP was shared with other regional popu¬ 
lations (Fig. 2, Okl), NP individuals clearly dif¬ 
fered in morphological characters from other pop¬ 
ulations, by having a broader disc, smaller eye, 
shorter head and snout, widely separated orbits, 
nostrils and gill openings, a greater number of 
nuchal thorns and numerous small black specks 
present on its dorsal surface (Figs. 3, 4, and 7). 
The size at maturity of males and females was 
also greater than that in other regional popula¬ 
tions (Figs. 5 and 6). Such morphological diver¬ 
gence of NP from the other regional populations 
agreed well with the findings of Ishiyama (1967) 
and Ishihara (1987). Although the former recog¬ 
nized NP individuals as a separate species, the 
sharing of a haplotype with other regional popu¬ 
lations (Okl) indicates the absence of reproduc¬ 
tive isolation between them. Although gene flow 
between NP and other regional populations has 
been limited, as suggested by a significant F ST , 
we treated the NP as the area of a local popula¬ 
tion within O. kenojei, as suggested by Ishihara 
(1987). Gene flow between it and the other popula¬ 
tions seems to be restricted by the Tsugaru Strait 
between the Sea of Japan and Pacific Ocean (Fig. 
1)—further evidence of a phylogeographical break 
across the Tsugaru Strait (Kai et al., 2014; Kai 
and Yamanaka 2 ). 
Size at maturity was clearly larger in NP indi¬ 
viduals than in other regional populations (Figs. 
5 and 6). Such differences between Sea of Japan, 
Sea of Okhotsk, and Pacific Ocean populations 
have been noted for several fish species. For ex¬ 
ample, Tamate (2012) showed that the body size 
of the anadromous masu salmon ( Oncorhynchus 
masou) returning to the coast from the Sea of Ok¬ 
hotsk was significantly smaller than that of the 
same species from the Pacific and Sea of Japan 
coasts. In contrast, Tohkairin et al. (2014) showed 
that the maximum body size of the marbled snailfish 
(Crystallichthys matsushirnae ) was much smaller in the 
Sea of Japan population than in the Pacific and Sea 
of Okhotsk population. Because the genetic divergence 
between NP and other populations was at the intraspe¬ 
cific level, some other evolutionary factors may have 
shaped the geographic size variations in O. kenojei. 
It should be noted that the NP haplotypes were sim¬ 
ilar to those of SJ, YC and EC, but distantly related 
to those of OS, suggesting that NP was colonized from 
SJ through the Tsugaru Strait, not from OS along the 
Pacific coast of Japan. Although no obvious geographic 
barriers are known between OS and NP, the absence of 
a major population of O. kenojei from the Pacific coast 
of central Japan suggests that the strong Kuroshio 
Current, originating from tropical waters, may have 
prevented dispersal of the species. In fact, the main 
2 Kai, Y., and T. Yamanaka. 2017. Tsugaru Straight hybrid 
zone between two Japanese marine sculpins (genus Cottius- 
culus). [Available at website, accessed May 2018] 
