Plankton Benthos Res 6(1): 18–25, 2011
Plankton & Benthos
Research
© The Plankton Society of Japan
Zoogeography of planktonic brackish-water calanoid
copepods in western Japan with comparison with
neighboring Korean fauna
SAKIKO ORUI SAKAGUCHI1, HIROSHI UEDA1,*, SUSUMU OHTSUKA2, HO YOUNG SOH3 &
YANG HO YOON3
1
Usa Marine Biological Institute, Kochi University, Inoshiri 194, Usa, Tosa, Kochi 781–1164, Japan
Takehara Marine Science Station, Hiroshima University, Hiroshima 725–0024, Japan
3
Faculty of Marine Technology, Chonnam National University, Jeonnam 550–748, Korea
2
Received 30 July 2010; Accepted 26 October 2010
Abstract: Brackish-water calanoid copepods, defined as those predominantly inhabiting oligo- and mesohaline waters, were investigated at 45 rivers in western Japan during 2006–2009 to review their faunas and, for comparison,
4 rivers of Korea in 2006. Eight species (Acartia ohtsukai, A. sinjiensis, A. tsuensis, Pseudodiaptomus inopinus,
P. ishigakiensis, Sinocalanus sinensis, S. tenellus and Tortanus derjugini) were identified as brackish-water calanoid
copepods from Japan. Among them, P. inopinus was most often collected; it occurred in 38 rivers. Acartia tsuensis
and A. sinjiensis were also frequent species in Japan. Pseudodiaptomus ishigakiensis discovered from Kyushu
Island is new to the mainland of Japan. From the rivers of Korea six species were identified (A. ohtsukai, A. sp., P.
inopinus, P. sp., S. tenellus and T. dextrilobatus). Based on the present and previous studies, a total of 13 brackish-water
calanoid copepods have been recorded from Japan and Korea, of which eight species, including three continental relicts
(A. ohtsukai, S. sinensis and T. derjugini) in Kyushu Island, are common to both regions. Among them, only two species
(P. inopinus and S. tenellus) are distributed widely on both Japanese and Korean coasts of the Tsushima (Korea) Strait.
This indicates that the faunas of brackish-water calanoid copepods are very different between Japan and Korea despite
the short geographical distance (about 170 km) and that the Tsushima Strait has played the role of an effective barrier
for their dispersion.
Key words: brackish-water calanoid copepods, Japan, Korea, Tsushima (Korea) Strait, zoogeography
Introduction
Faunistic studies of brackish-water copepods are interesting from a zoogeographical viewpoint because conditions
of estuaries are different from those of marine and freshwater habitats in regard to that dispersion processes. Marine
copepods can most easily disperse to distant localities by
ocean currents and circulation, and hence their geographical isolation hardly occurs, at least at scales of several hundred kilometers. In contrast, freshwater copepods are generally confined to each enclosed freshwater body, such as
lakes and ponds, or rivers within the same basin. A possible
pathway for dispersal from one water body to another is
through occasional flooding and through underground
water connecting water bodies. However, no natural disper* Corresponding author: Hiroshi Ueda; E-mail, [email protected]
sal pathways, except for birds as vector, are considered to
exist between islands due to the barrier of sea water. Dispersion patterns of brackish-water copepods are regarded as
intermediate between marine and freshwater copepods.
Since brackish waters or estuaries are generally narrow
transitional zones between fresh- and marine waters, habitats of brackish-water copepods are very limited compared
with those of freshwater and marine copepods; in this
paper, “brackish-water copepods” are defined as those that
occur predominantly in oligohaline (salinity 0.5–5) or
mesohaline (5–18) waters and fall under Jeffries’ (1967)
category of “true estuarine” zooplankton. When flooding
occurs, however, they are flushed out into the sea together
with a brackish-water mass (Ueda et al. 2004) and some of
them are expected to reach neighboring estuaries by being
transported by tidal currents. By this manner, brackishwater copepods could expand their populations to far dis-
Brackish-water calanoid copepods in Japan
tant localities along the coast line. If flushed-out brackishwater masses are transported by ocean currents and copepods survive within the masses, they could disperse to distant islands across the sea as sometimes happens through
dispersion via ship ballast water. Although it has not yet
been reported definitely for brackish-water copepods, satellite observations have revealed that a bloom of harmful dinoflagellates on the southeast Korean coast was transported
to 500 km away to San-in district, western Honshu Island,
over about two weeks to one month by the Tsushima Warm
Current (Onitsuka et al. 2010).
It is generally considered that the Tsushima (Korea)
Strait between Kyushu Island and the Korean Peninsula has
persisted since at least the last transgression of about 7,000
years ago (Lim et al. 2006). Kyushu Island is now separated
from Korea by about 170 km, where Tsushima Island lies
at the mid point. The faunal difference in brackish-water
copepods between western Japan and southern Korea partly
results from geographical isolation since at least the last
transgression. Reversely, the similarity of their faunas suggests that the same species have survived in each region
without gene flow and/or that there has been gene flow between the regions. Thus the faunas of brackish-water copepods in Japan and Korea provide us an interesting theme to
know to what extent the Tsushima Strait has acted as a barrier to their dispersion.
One of the most urgent issues for brackish-water copepods is changes in their faunas due to global warming, as in
most biota. Human-mediated introduction also gives a great
impact on their faunas, because estuaries used as export
ports are extremely vulnerable to the invasion of alien zooplankton via ballast water. Intercontinental invasions of
East Asian copepods have been well documented from the
two large estuaries on the Pacific coast of the USA, i.e. San
Francisco estuary of California and the Columbia River estuary of Oregon (Ferrari & Orsi 1984, Orsi & Ohtsuka
1999, Bollens et al. 2002, Bouley & Kimmerer 2006,
Cordell et al. 2008). The alien species are presently the
dominant zooplankton and have changed the food web in
these estuaries (Cordell et al. 2007). These issues indicate
that a faunal study of brackish-water copepods is important
from the viewpoint not only of their zoogeography but also
for conservation of ecosystems in estuaries. We herein
review the distributions of planktonic brackish-water
calanoid copepods in western temperate Japan based on
samples collected from various river estuaries from 2006 to
2009, and compare them with those in South Korea by
using our samples and previous records from Korean estuaries to understand the faunal similarity and dissimilarity
between opposite sides of the Tsushima Strait.
Materials and Methods
Sampling trips in western Japan were conducted each in:
August 2006 for nine rivers on the east and south coasts of
Kyushu Island; October 2006 for four rivers on the west
19
coast of Kyushu; March 2007 for four rivers flowing into
Ariake Bay, western Kyushu Is.; November 2007 for 15
rivers on the east and north coasts of Kyushu Is. and nine
rivers on the north coast of the San-in district, western Honshu Is.; and August 2009 for 12 rivers all around Kyushu Is.
and four rivers on the north coast of the San-in district (Fig.
1, Table 1). In addition to these samples, copepod collections were also taken from three small rivers flowing to
Dokai Bay on the north coast of Kyushu Is. from January to
March 2008. In total, 83 samples were collected from 78
stations in the 45 rivers in Japan; hereafter rivers surveyed
are denoted by numbers denoted in Fig 1. A sampling trip
in Korea was made in October 2006 and a total of seven
samples from four rivers were made on the south and west
coasts. Sampling in each river was made generally at a site
within oligo- or mesohaline zones. If the number of brackish-water copepods in a sample was small and time was
available, the sampling site was changed to a more upstream or downstream site according to the salinity there.
Until 2008, copepods were generally taken by towing a 0.1mm mesh plankton net, except at rivers #24 and #26, where
a 0.3-mm mesh net used. For sampling in 2009, a 0.2-mm
mesh net with a lead attached to the mouth ring was used to
collect copepods successfully from the near-bottom zone,
because the common brackish-water copepod genus Pseudodiaptomus is known to be almost demersal during the
day (e.g. Fancett & Kimmerer 1985, Kouassi et al. 2001).
Methods of net towing varied according to conditions of
sampling sites. We generally threw and towed a net with a
rope from the shore or bank of a river, but if a bridge suitable for sampling was present, we lowered the net from the
bridge and towed it by walking on the bridge. If no suitable
shore or bridge was found, we carried a net into the river by
wearing waders and towed it by walking on the shallow
bed. At river #13, a vertical tow from near the bottom was
made in the center of the river using a rubber boat. Samples
from 2006 to 2008 were fixed in 2% buffered formalin and
those in 2009 were once sieved with a 0.2 mm mesh cloth
and fixed in 99.5% ethanol immediately after sampling. At
each sampling site, temperature and salinity in the surface
water were measured with a portable T-S meter (NS-3P,
Merbabu Corp. from August 2006 to March 2007 and CM21P, DKK-TOA Corp. in 2008 and 2009) or a CSTD (Compact-CTD, Alec Electronics Co. in November 2007). Those
in the bottom water were also measured for sampling in
2009 and for vertical sampling at river #13 in 2006; bottom
water for measurement was taken using a self-closing water
sampler (Ueda & Yoshimura 1992). Species occurrence was
based on adult and immature copepodid specimens that
could be identified at the species level. Percentage occurrence of the abundant species in the total copepod fauna
was estimated by enumerating 40 copepods, excluding
the number of benthic harpacticoids and symbiotic copepods such as Hemicyclops spp.
20
S. O. SAKAGUCHI et al.
Fig. 1. Map of temperate western Japan and southern Korea showing the locations of river mouths from which copepods were
collected. The river names are listed in Table 1.
Results
Temperature in the surface water ranged from 18.1–
33.1°C in the warmer seasons (August and October 2006,
August 2009) and from 8.5–14.1°C in the colder seasons
(March 2007 and January–March 2008) (Table 1). The temperatures at collection sites in October 2006 were almost
the same between western Kyushu Island (18.4–21.9°C)
and southern and western Korea (18.1–20.6°C). The temperatures in mid or bottom waters in August 2009 from
northern Kyushu Is. and northern San-in district were the
same as or slightly higher or lower than the surface temperature by usually 1.0°C, with the maximum difference of
2.1°C (higher in the bottom water) at a lower reach of the
river #24. Although the surface salinity varied greatly from
0.0 to 33.0, at most stations (64 of 88 stations in total) it
was within the range of that of oligo- or mesohaline waters.
More than a half of the stations in Kyushu Is. in 2006 were
in freshwater or oligohaline zones (salinity 5.0), whereas
those in other years and in Korea were mostly in meso- or
polyhaline zones. In half of the 16 rivers surveyed in 2009,
the mid- or bottom salinities were higher than the surface
salinities by 10.0.
A total of 11 brackish-water calanoid copepod species
were identified in 79 samples from 44 rivers of Japan and
Korea (Tables 1 and 2); these were Acartia ohtsukai Ueda
& Bucklin, A. sinjiensis Mori, A. tsuensis Ito, A. sp., Pseudodiaptomus inopinus Burckhardt, P. ishigakiensis Nishida,
P. sp., Sinocalanus sinensis (Poppe), S. tenellus (Kikuchi),
Tortanus dextrilobatus Chen & Zhang and T. derjugini
Smirnov (Table 1). Acartia sp. and Pseudodiaptomus sp.
from Korea are undescribed species predominantly inhabiting Korean estuarine waters (Soh HY, unpubl); P. sp. was
the same species as that studied by Eyun et al. (2007).
Copepods collected from five rivers in Japan (#9, #22, #23,
#37, #43) were only marine or freshwater forms or unidentified Pseudodiaptomus copepodids. Other predominating
non-brackish-water species, comprising 20% of total
copepods in each sample, were nine marine (A. erythraea
Giesbrecht, A. hudsonica Pinhey, Euterpina acutifrons
(Dana), O. brevicornis Giesbrecht, O. davisae Ferrari &
Orsi, O. oculata Farran, Parvocalanus crassirostris (Dahl),
Paracalanus parvus (Claus) s. l., Corycaeus sp.) and four
freshwater forms (Cyclops vicinus Ulyanin, Eodiaptomus
japonicus Burckhardt, Thermocyclops crassus Fisher, T.
taihokuensis Harada).
The most often-collected copepod was P. inopinus, which
occurred in 37 rivers (76% of the rivers surveyed) throughout Japan and Korea and was usually the predominant
species in these rivers. The second most often-collected
species in Japan was A. tsuensis, which occurred in 21
rivers. Sinocalanus tenellus and A. sinjiensis were also
common in Japan and occurred in 17 and 16 rivers, respectively. Although the salinity was measured only in the surface water until 2008, these four common species did not
show notable species-specific salinty ranges; the lowest was
Brackish-water calanoid copepods in Japan
21
Table 1. List of copepod species from river estuaries in Japan and Korea. Species in bold face are brackish-water calanoid copepods,
which are defined as those occurring predominantly in oligo- or mesohaline waters. The river numbers correspond to those in Fig. 1. Locations of the sampling stations (Sta.) are indicated by the distance (km) from the river mouse. Temperature and salinity in the mid- or bottom
layer are given in parentheses. Percentage occurrences of predominating species (20% of total copepods) in samples containing 40
copepods are given in parentheses; if species of immature copepodids (juveniles) was unsure due to co-occurrence of congeners, the values
for adults is presented in brackets. Less abundant species (20% occurrence) other than brackish-water calanoid copepods are omitted.
No.
River name
Sta.
Temp. (°C)
6–7 August 2006 (eastern and southern Kyushu Is.)
1
Kita-gawa
0.9
No data
2
Shiomi-gawa
2.8
29.8
2.9
29.3
3.2
29.7
3.5
26.6
3
Hitotsuse-gawa
1.9
28.8
4
Ôyodo-gawa
2.9
25.4
5
Honjô-gawa
1.6
27.3
6
Kimotsuki-gawa
0.6
24.8
7
Man-nose-gawa
2.5
28.4
8
Gotanda-gawa
1.3
No data
9
Sendai-gawa
1.8
27.8
Salinity
0.0
12.9
7.8
4.5
0.2
6.8
0.1
6.6
0.5
2.0
8.6
1.7
Species*1 (% occurrence)
Paracyclopina nana (98)
A. sinjiensis, P. inopinus (69), S. tenellus
A. sinjiensis, P. inopinus (90), S. tenellus
P. inopinus (94), S. tenellus
P. inopinus (100)
A. ohtsukai, A. tsuensis, P. inopinus, S. tenellus, O. davisae (48)
P. inopinus
A. tsuensis (49), P. inopinus (41), S. tenellus
P. inopinus
P. inopinus (99)
A. tsuensis (92), P. inopinus
Eodiaptomus japonicus (95)
20–21 October 2006 (western Kyushu Is.)
10 Kikuchi-gawa
1.0
21.9
7.6
11 Shira-kawa
1.0
19.2
8.1
3.0
18.4
0.1
3.3
18.4
0.8
12 Midori-kawa
0.0
21.7
5.5
2.0
21.2
1.0
13 Kuma-gawa
5.2
21.2
1.7
5.2*2 21.2 (24.0) 1.7 (27.9)
A. sinjiensis (70), P. inopinus, S. tenellus
A. sinjiensis
A. sinjiensis, P. inopinus, S. sinensis, O. davisae (76)
A. ohtsukai, A. sinjiensis, P. inopinus (24), S. sinensis, O. davisae (68)
P. inopinus (54), S. sinensis (35)
P. inopinus (95)
A. ohtsukai, A. sinjiensis [20], P. inopinus (42), S. tenellus
A. sinjiensis, A. tsuensis, P. inopinus (55), S. tenellus, (Acartia juveniles 25%)
17–19 March 2007 (western Kyushu Is.)
14 Rokkaku-gawa
6.1
15 Chikugo-gawa
6.1
16 Okinohata-gawa
2.9
17 Honmyo-gawa
7.6
S. sinensis (99), Tortanus derjugini
P. inopinus, S. sinensis (96)
S. sinensis (84), Tortanus derjugini
S. tenellus (100)
8.5
10.5
10.8
10.8
17.3
0.2
0.0
0.1
5–9 November 2007 (from eastern and northern Kyushu to northern San-in District, Honshu Is.)
18 Yasaka-gawa
2.4
17.7
18.5
A. tsuensis (84), P. inopinus
19 Aki-gawa
0.6
18.2
15.8
A. tsuensis (83), P. inopinus
20 Kunoura-gawa
0.7
18.0
16.2
A. tsuensis (40), P. inopinus (60)
21 Katsura-gawa
2.5
17.0
11.9
P. inopinus
22 Yakkan-gawa
1.1
17.3
11.5
(Pseudodiaptomus juveniles only)
23 Yamakuni-gawa
1.8
17.0
4.7
(Pseudodiaptomus juveniles only)
2.5
18.2
26.9
A. sinjiensis [72], A. tsuensis [24], P. inopinus
24 Yukinoura-gawa
4.4
16.8
5.1
P. ishigakiensis, S. tenellus (81)
4.4*3
16.8
5.1
A. sinjiensis, P. inopinus, P. ishigakiensis, S. tenellus (98)
25 Saza-gawa
4.1
17.5
8.8
A. tsuensis, P. inopinus, S. tenellus, O. davisae (57)
17.6
19.4
A. sinjiensis (39), P. inopinus (53), S. tenellus
26 Imari-gawa
2.6*3
3.1
17.6
16.2
A. sinjiensis, P. inopinus (33), S. tenellus, O. davisae (47)
27 Izumi-gawa
1.9
17.1
5.0
P. inopinus (93), S. tenellus
28 Muromi-gawa
1.5
20.5
14.3
A. tsuensis, P. inopinus (46), O. brevicornis (53)
29 Tatara-gawa
1.8
21.0
14.7
A. sinjiensis, A. tsuensis, P. inopinus (97), S. tenellus
30 Tsuri-kawa
0.7
20.1
15.4
A. sinjiensis, A. tsuensis, P. inopinus (68)
31 Shioiri-gawa
1.1
19.6
16.8
A. tsuensis (39), P. inopinus (32), S. tenellus
32 Onga-gawa
1.8
18.7
13.6
A. sinjiensis
33 Ayaragi-gawa
1.7
17.0
11.2
A. sinjiensis, A. tsuensis, P. inopinus (53), (Acartia juveniles 23%)
34 Awano-gawa
1.4
17.8
9.5
A. tsuensis (84)
35 Misumi-gawa
1.3
18.1
15.5
A. sinjiensis, P. inopinus (62)
36 Hashimoto-gawa
4.4
19.9
19.9
A. sinjiensis, A. tsuensis, P. inopinus (24), S. tenellus, O. davisae (47)
37 Tama-gawa
1.0
19.1
13.2
only one Paracalanus parvus s. l. adult in the whole sample
38 Takatsu-gawa
0.1
17.6
7.8
A. tsuensis (35 copepods, mostly Oithona juveniles, in the whole sample)
0.4
17.3
4.7
A. tsuensis (only 4 specimens in the whole sample)
39 Hamada-gawa
1.9
17.5
11.3
A. tsuensis, P. inopinus (82)
40 Gôno-kawa
1.8
16.8
7.1
A. tsuensis (only 2 specimens in the whole sample)
41 Sashimi-gawa
1.6
16.4
13.2
P. inopinus (32), S. tenellus (40)
22
S. O. SAKAGUCHI et al.
Table 1.
(Continued)
No.
River name
Sta.
Temp. (°C)
Species*1 (% occurrence)
Salinity
3 January, 20 February, 11 March 2008 (rivers flowing into Dokai Bay, northern Kyushu Is.)
43 Shinshinhori-kawa (Jan.)
0.1
12.0
33.0
O. davisae (66)
0.5
13.1
22.0
O. davisae (100)
43 Shinshinhori-kawa (Feb.)
0.3
9.8
20.2–22.0
S. tenellus, O. davisae (69)
1.0
9.6
25.6
S. tenellus, A. hudsonica (47), O. davisae (47)
44 Kinzan-gawa (Feb.)
3.2
10.2
24.3
P. inopinus (43), S. tenellus (45)
45 Wariko-gawa (Mar.)
0.7
14.1
18.9
A. hudsonica (30), O. davisae (58)
1.0
14.1
15.1
O. davisae (69)
18–22 August 2009 (from northern Kyushu to northern San-in District, Honshu Is.)
30 Tsuri-kawa
0.7
30.9 (30.9) 11.2 (21.7)
A. sinjiensis, A. tsuensis [46], P. inopinus (24)
26 Imari-gawa
3.1
27.6 (28.6) 6.2 (23.1)
A. sinjiensis (99), P. inopinus
24 Yukinoura-gawa
3.9
26.2 (28.3) 3.2 (21.8)
A. tsuensis (51), P. inopinus (48), S. tenellus
4.4
25.1 (24.5) 0.0 (0.1)
A. tsuensis, P. inopinus (99)
15 Rokkaku-gawa
2.8
31.4 (32.2) 3.9 (4.5)
P. inopinus (77), S. sinensis (23)
14 Chikugo-gawa
6.3
29.4 (29.1) 15.1 (15.4)
A. ohtsukai, P. inopinus (86)
10 Kikuchi-gawa
1.4
28.6 (28.4) 25.6 (25.4)
A. sinjiensis [42], A. ohtsukai, P. ishigakiensis, Paracalanus parvus s. l. (21)
1.8
29.0 (28.7) 20.0 (24.0)
A. sinjiensis, A. ohtsukai [59]
8
Gotanda-gawa
1.3
33.1 (33.5) 10.5 (12.2)
A. tsuensis (72), P. ishigakiensis (28)
7
Man-nose-gawa
2.5
28.8 (28.8) 1.3 (1.4)
P. inopinus (67), S. tenellus (33)
3
Hitotsuse-gawa
1.9
26.1 (26.9) 9.1 (28.7)
A. sinjiensis, A. tsuensis, P. inopinus, P. ishigakiensis,
Paracalanus parvus s. l. (42), Parvocalanus crassirostris (21)
3.5
26.3 (26.7) 5.5 (10.3)
A. sinjiensis, A. tsuensis, P. inopinus, S. tenellus, (Acartia juveniles 64%)
2
Shiomi-gawa
2.9
26.0 (27.1) 6.6 (27.2)
A. tsuensis, P. inopinus, P. ishigakiensis, (Pseudodiaptomus juveniles 85%)
18 Yasaka-gawa
2.4
31.2
20.6
A. tsuensis (71), P. inopinus (27)
2.9
32.9 (32.6) 12.8 (14.2)
A. sinjiensis, A. tsuensis [63], P. inopinus (22)
23 Yamakuni-gawa
2.2
31.1 (31.8) 11.4 (13.7)
A. sinjiensis (39), P. inopinus (61)
33 Ayaragi-gawa
1.6
25.5 (26.7) 2.6 (18.4)
A. sinjiensis, A. tsuensis [63], P. inopinus (20)
36 Hashimoto-gawa
4.2
25.4 (26.7) 0.4 (25.8)
A. tsuensis (50)
4.4
25.0 (25.8) 0.7 (21.2)
A. tsuensis (71), S. tenellus
39 Hamada-gawa
1.9
27.4 (27.8) 1.0 (23.7)
A. tsuensis (53), P. inopinus (42)
42 Ôhashi-gawa
3.1
28.1 (28.1) 8.0 (8.4)
A. sinjiensis (66), P. inopinus (30), S. tenellus
26–28 October 2006 (southern and western Korea)
46 Seomjin
16.8
18.9
6.5
47
Tamjin
19.2
19.4
13.5
48
Yeongsan
49
Mankyung
8.8
8.9
2.2
6.5
14.9
20.6
19.5
19.3
19.2
18.1
9.5
0.6
24.5
9.2
0.3
A. ohtsukai, A. sp. [21], P. inopinus, P. sp., S. tenellus,
Tortanus dextrilobatus, (Pseudodiaptomus juveniles 40%)
A. ohtsukai, A. sp., P. inopinus, P. sp., S. tenellus, Tortanus dextrilobatus,
O. davisae (61)
P. inopinus (89), Tortanus dextrilobatus
P. inopinus (84)
A. ohtsukai, A. sp., Paracalanus parvus s. l. (58)
A. ohtsukai, Paracalanus parvus s. l. (67)
(Cyclopidae juveniles 91%)
*1 Abbreviations: A., Acartia; O., Oithona; P., Pseudodiaptomus; S., Sinocalanus.
*2 Sample by vertical towing.
*3 0.3-mm mesh net used.
2.6 or less and the highest was 25.6 in river #43 for S. tenellus or 28.7 at the bottom of river #3 in 2009 for the other
three species. As for temperature, P. inopinus and S. tenellus occurred in both the warmer and colder seasons, in
which the temperature at the stations at which the copepods
occurred ranged from 10.2–32.9°C for P. inopinus and
9.8–29.8 oC for S. tenellus. The two Acartia species were
not collected in the colder season (March 2007 and January
to March 2008), and their temperature ranges were almost
equal to each other (25.0–33.1°C for A. tsuensis and
25.5–32.9°C for A. sinjiensis). It is notable that A. tsuensis
and/or A. sinjiensis occurred in a total of 50 samples from
27 rivers but that the two species co-occurred only in 11
samples from eight rivers. This suggests that their habitats
may be somewhat different from each other within an estuary; in fact, our unpublished observations have revealed
that A. tsuensis was distributed more upstream in river estuaries as compared to A. sinjiensis.
Acartia ohtsukai, S. sinensis, and T. derjugini from Japan
have been considered as continental relicts, which are restricted to the Ariake Bay estuaries (Hiromi & Ueda 1987,
Ohtsuka et al. 1995, Ueda & Bucklin 2006). In the present
study, S. sinensis and T. derjugini were collected only from
the rivers flowing into Ariake Bay. However, A. ohtsukai
was rarely found in the rivers outside the bay (#3 and #13)
as well. Pseudodiaptomus ishigakiensis, which has hitherto
Brackish-water calanoid copepods in Japan
Table 2. List of brackish-water copepods in temperate Japan
and Korea based on the present and previous studies. Species
without superscript numbers were collected in this study and the
others are cited from the references indicated by the numbers.
Japan
Calanoida
Acartia ohtsukai
A. sinjiensis
A. tsuensis
A. sp.
Eurytemora affinis
Pseudodiaptomus inopinus
P. ishigakiensis
P. poplesia
P. sp.
Sinocalanus sinensis
S. tenellus
Tortanus derjugini
T. dextrilobatus
Number of species
1)
4)
3)
9
Korea
1)
1,2)
2)
2)
2)
4)
12
Yoo et al. (1991), 2) Lee et al. (2007), 3) Mizuno & Miura (1984),
Soh et al. (2001)
been recorded only from the Nansei Islands, the islands between Kyushu Island and Taiwan (Nishida 1985, Oka et al.
1991), was newly collected from Kyushu Is. (rivers #2, #3,
#8, and #24) and river #24 was the northern limit of the
species.
Discussion
The present study and previous monographic studies on
the copepod faunas of Japan (Mizuno & Miura 1984, Chihara & Murano 1997) and Korea (Chang 2009) indicate
that brackish-water calanoid copepod species, which are defined as those predominantly inhabiting oligo- or meshohaline waters, belong to the genera Acartia, Eurytemora,
Pseudodiaptomus, Sinocalanus and Tortanus. Lee et al.
(2007) made a comprehensive study on copepod fauna in
various brackish waters throughout South Korea and described seven brackish-water calanoid copepods, viz. A.
ohtsukai (as A. pacifica; see Chang 2009), A. tsuensis, E.
affinis (Poppe), P. inopinus, P. poplesia (Shen), S. sinensis,
and S. tenellus, among which A. tsuensis and E. affinis were
new to the Korean fauna; E. pacifica Sato was also described but not dealt with in this study because it inhabits
basically neritic water. In addition, A. sinjiensis and T. derjugini were recorded from Korean estuaries by Yoo et al.
(1991) and Soh et al. (2001), respectively. Based on the present and these previous studies, 13 brackish-water calanoid
copepods are listed from Japan and Korea (Table 2). Eight
of them are common to both sides of the Tsushima Strait
and five inhabit only one side of the strait. Among the eight
common species, A. ohtsukai, S. sinensis and T. derjugini in
23
Japan have been regarded as continental relicts (Hiromi &
Ueda 1987, Ohtsuka et al. 1995, Ueda & Bucklin 2006),
which have been isolated from the continental populations
since the last transgression and remained restrictedly to the
turbid estuaries of Ariake Bay. The present study revealed a
few occurrences of A. ohtsukai in river #3 outside Ariake
Bay, but this species can still be regarded as a continental
relict because the high abundance of the species was observed only within the rivers of the innermost part of Ariake Bay (Ueda & Bucklin 2006). These continental relicts
were not found on the Japanese coast of the Tsushima
Strait.
Eurytemora affinis has been regarded as a circumboreal
euryhaline species distributed from cold-temperate to Arctic waters, but Lee & Frost (2002) suggested that it is a
species complex composed of genetically divergent populations. This ‘species’ is common to Japan and Korea based
on the literature. It is interesting that the Korean population
is recorded only from the east coast (Chang 2009). In
Japan, it has been known from Hokkaido Island and the
Japan Sea coast of the northern half of Honshu Island
(Mizuno & Miura 1984). These previous records and the
absence of E. affinis in the present samples suggest that the
‘species’ is not distributed on both coasts facing the
Tsushima Strait, i.e., the northern Kyushu Is. (and the Sanin district) and southern Korea, probably because the temperature is higher than that in its distribution range due to
warming by the Tsushima Warm Current. Accordingly it is
unlikely that the Japanese and Korean populations are exchanged with each other across the Tsushima Strait.
In Japanese estuaries, A. tsuensis is a very common and
sometimes the most abundant copepod species, whereas the
Korean population was recorded from only a few estuaries
in spite of intensive sampling throughout South Korea by
Lee et al. (2007). Considering that A. tsuensis is a warm
temperate-tropical species ranging from the Philippines
(Golez et al. 2002) to middle Japan (Ito 1956; Mie Prefecture), two hypotheses are possible to explain its limited
finding in Korean estuaries. One is that the species was
originally distributed in both areas but the Korean population propagates restrictedly in the southernmost part of
Korea because of limitation by temperature. The other hypothesis is that the species originally inhabited from tropical to temperate islands along the Kuroshio Current but recently expanded its distribution to Korea due to natural or
human-mediated dispersion. The species had never been
described from the Chinese continental coast (Shen & Song
1979), but Ohtsuka et al. (unpubl) recently found a few
specimens of A. tsuensis from an oligohaline zone of the
Oujiang River, Wenzhou, and Amoy. This finding from
China is a very similar case to that from Korea, seemingly
suggesting that recent dispersion is more likely because the
temperature cannot explain its rarity along the Chinese continental coast.
Another common Acartia species, A. sinjiensis, is distributed from Thailand (Ohno et al. 1996; identification was
24
S. O. SAKAGUCHI et al.
confirmed by Ueda H) to Poseta Bay near Vladivostok,
Russia and distantly in Australia (Ueda & Hiromi 1987),
indicating that the species is more eurythermal than A.
tsuensis. However, there have been no records from the
Chinese continental coast. The Korean population has been
recorded from Youngwang on the west coast (Yoo et al.
1991) and Yeosu on the south coast (Soh HY unpubl) but
was not found in Lee et al.’s (2007) collections. This suggests that the Korean population inhabits very limited estuaries compared with the Japanese population. Such a geographic pattern (common in Japan but rare in Korea) for A.
sinjiensis is the same as that for A. tsuensis, and therefore
the hypothesis that the Korean population was recently dispersed from Japan may also be applicable to A. sinjiensis.
Among the common species in Table 2, those which have
not been discussed yet are Pseudodiaptomus inopinus and
Sinocalanus tenellus. According to previous studies, these
species have latitudinally wide distribution ranges as follows: P. inopinus from Guangdong, China (Shen & Song
1979) to northern Hokkaido Island (Mizuno & Miura 1984)
and S. tenellus from Thailand (Shen & Song 1979) to
Sakhalin, Russia (Rylov 1932). Some of the previous
records of these species may need detailed taxonomical reexamination, because the existence of such an extremely
eurythermal species ranging from tropical or subtropical to
cold-temperate zones is unlikely. In fact, Sakaguchi & Ueda
(2010) recently described a new pseudodiaptomid species P.
nansei, which was formerly identified as P. inopinus (Oka et
al. 1991), from the Nansei Islands, southernmost Japan.
Even though obvious morphological differences may be not
observed, recent genetic studies have revealed divergence
between geographically-separated populations of wideranging coastal species. For example, Japanese and Korean
populations of the splash-pool harpacticoid copepod Tigriopus japonicus Mori, which is distributed widely throughout
Asian coasts, belong to different genetic clades from each
other (Ki et al. 2009). Lee & Frost (2002) indicated retarded evolution of morphological characters of the Eurytemora affinis species complex.
In conclusion, brackish-water calanoid copepods that frequently occurred on both sides of the Tsushima Strait belong to only two species, P. inopinus and S. tenellus; the
other brackish-water species are absent on one or both
sides, or are very limitedly found on the Korean side in
contrast to frequent occurrence on the Japanese side. Since
populations of brackish-water copepods in Japan and Korea
were not isolated from each other until formation of the
Tsushima Strait by linkage along the coast line, the present
differences between the two sides are considered to have resulted from geographical isolation after formation of the
strait. The two frequently-occurring common species probably had wide distribution ranges covering both Korea and
Japan before their geographical isolation. The great difference in brackish-water copepods between the two sides of
the Tsushima Strait indicate that the strait has played the
role of an effective barrier for their dispersion. Occasional
transportation from Korea to Japan by the Tsushima Warm
Current, which was recently observed for a bloom of a dinoflagellate (Onitsuka et al. 2010), is probably impossible
for brackish-water copepods.
Acknowledgments
We are grateful to Dr Chang CY of Taegu University,
Korea, for sending his illustration book, which was indispensable to the present study. This study was supported
by grants-in-aid for Scientific Research (#18208019,
#20380110) from the Japan Society for the Promotion of
Science (JSPS), and for the Bilateral Joint Scientific Program between Japan and Korea from JSPS and the Korea
Science and Engineering Foundation (2005–2007).
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