Laguna (2018) 25: 39-54

ISSN 2185-2995

Recent ostracode assemblages from Shushi Bay,
Tsushima Island, southwestern Japan and their ecologicaland zoogeographical characteristics
Toshiaki Irizuki1, Yuki Fujihara2, Hokuto Iwatani3, Shigenori Kawano4

Abstract: One hundred and ten ostracode species were identified in 23 surface sediment samples

collected from Shushi Bay, Tsushima, Nagasaki Prefecture, Southwest Japan. Four ostracodebioassociations and five biofacies were identified with R-mode and Q-mode cluster analyses.
The correlation coefficients between environmental factors and the selected species in eachbioassociation revealed the ecological characteristics of the species. This information is useful forreconstructing Holocene environments based on ostracode assemblages. The ostracode assemblages
reported here are nearly identical to those found on the Japanese mainland. No ostracode species
characterizing the Chinese or Korean ostracode faunas were found in this study.
Key words: Recent, Ostracoda, Shushi Bay, Tsushima Island, Japandemarcation for terrestrial animals (e.g., Tokuda, 1969;

Introduction

Sato, 1969). Given that benthic Ostracoda are very small

bivalved crustaceans (usually <1 mm, with a calcified

The Tsushima Strait, also known as the Korea Strait,

carapace) and do not have a planktonic larval life stage,is situated between one of the mainland islands of Japan

it is likely that this strait is a barrier for the dispersal of(Kyushu Island) and Korea (Korean Peninsula). It is

ostracodes into shallow seas or is a migration route of

approximately 200-km wide, and the average water depth

Ostracoda (Ishizaki, 1990a, b; Irizuki et al., 2005, 2009;is approximately 90 to 100 m (Fig. 1). The Sea of Japan

Tanaka, 2008; Tanaka et al., 2019). Many groups haveand the East China Sea are connected by this strait.

studied ostracodes along and off the coasts of Kyushu

Tsushima Island is located in the central part of the strait

Island, the Korean Peninsula, and China (e.g., Cai, 1988;

and has many small inlets and bays along its ria coast.

Wang et al., 1988; Zhao and Wang, 1988; Abe and Choe,

The Tsushima Strait is thought to be a zoological line of1

Institute of Environmental Systems Science, Academic Assembly, Shimane University, 1060 Nishikawatsu-cho, Matsue 690-8504, Japan.

2Interdisciplinary Graduate School of Science and Engineering, Shimane University, 1060 Nishikawatsu-cho, Matsue 690-8504, Japan.

3

School of Biological Sciences, The University of Hong Kong, Kadoorie Biological Sciences Building, Pokfulam Road, Hong Kong SAR, China.

4

Tochigi Prefectural Museum, 2-2 Mutsumi-cho, Utsunomiya 320-0865, Japan.

受付日：2018 年 9 月 4 日，受理日：2018 年 10 月 19 日，WEB 掲載日：2018 年 12 月 31 日

39Toshiaki Irizuki・Yuki Fujihara・Hokuto Iwatani・Shigenori Kawano
129°10′E
45°N

Hokkaido

Yellow
Yellow
Sea
Sea

129°30′E

Tsushima Island
34°40′N

Bohai
Sea
Sea ofofJapan
Sea
Japan

129°20′E

40°N

a
haim
us rtrentsThsimusnuTKoreanre
Cur C
Peninsula
Tsushima
HonshuIslandOyashio
Current

Shushi Bay(Fig. 2a)34°30′N

100m
35°N

Tsushima
StraitKyushu100m

34°20′N

rent
Cur
hio
s
o
r
Ku
30°N

East
East China
China
Sea
Sea

Pacific
Pacific Ocean
Ocean

34°10′N

100m

25°N
125°E

130°E

135°E

140°E

0

10

20km

145°E

Fig. 1 Locality of the study area.

1988; Iwasaki, 1992; Bodergat et al., 2002; Irizuki et

The distance intervals between sample sites were

only one study of modern ostracode assemblages

layer and the color and smell of sediments were recorded.

approximately 300 m. The thickness of the oxygenatedal., 2006; Tanaka, 2008; Tanaka et al., 2019). However,Samples for ostracode analysis were subsequently

from a short sediment core in Asou Bay along thecollected from the uppermost 1 cm of surface sediments

western coast of Tsushima Island has been conductedusing a small spoon. Residual samples were washed on(Kawano et al., 2013). Thus, here we report modern

a 2-mm opening sieve on the ship, and materials andostracode assemblages from a bay along the eastern

macrobenthic animals remaining on the sieve were

coast of Tsushima Island to define their ecological anddescribed. The temperature, salinity, and dissolved

zoogeographical characteristics.

oxygen in bottom waters were measured using the
Quanta probe (Hydrolab).

Localities and Methods
Tsushima Island is located between Korea and Japan

Laboratory Procedure(Fig. 1), and Shushi Bay is located in the northeasternpart of the island (Fig. 1). It is a Y-shaped, narrow,

A 10 to 20 g wet sample was weighed and dried inelongated bay with the long axis of approximately

an oven at 70°C for 3 days. It was then weighed and theapproximately 1 km. The water depth increases toward

were weighed, and their dry weights were calculated

4 km (Fig. 2a). The width of the bay at its mouth iswater content was calculated. The remaining wet samples

the mouth of the bay, where it is approximately 40 m.

based on the water content. These wet samples wereSeveral small rivers and streams flow into the bay;

washed through a 250-mesh sieve (63-μm opening),

relatively larger rivers are present at Hamakusu, Omasu,

dried in an oven at 60°C, and weighed again. The mud

and Shushi (Fig. 2a).

content of the samples was calculated. Under a binocular

A total of 23 surface-sediment samples (SS1 to SS23;

stereomicroscope, ostracode specimens were obtained

Fig. 2a) were collected with an Ekman-Burge grabfrom the coarser sediment remaining after dry-sieving

sampler from a small ship from July 13–15, 2006.

with a 115-mesh sieve (125-μm opening). If more than40

Recent ostracode assemblages from Shushi Bay, Tsushima Island,
southwestern Japan and their ecological and zoogeographical characteristics

34˚38’129˚
26’

Hamakusu

28’

27’

1

a

129˚
26’

Omasu

4

3

21

70–80

5

50–70

12

7

14
16

17

21

16

8

9

19

8

20

18

Shushi Bay

19
22

N

22

-5 m
-10 m
-20 m

-5 m
-10 m
-20 m

Akasaki

10

0.5

17

15

23

0

7

14

18

N

30–50
<30

6

6

15

Shushi

80–90
4

5

12

0

1.0 km34˚36’
34˚38’

Diversity (H′)

0.5

1.0 kmc

Abundance (N/g)

3≤
2
3

4

3

2.0–2.5

4

100–20011

50–1005

1.5–2.0

25–50

6

13

6

13

16

10–25
<10

7

7

34˚37’

d200≤

2.5–3.0

11
5

b

90≤

2

11

13

34˚37’

Mud content (%)

1

2
3

28’

27’

17
8

8

15

18

18

9

9
19
N

0

0.5

N

22

20
-5 m
-10 m
-20 m

20

22

-5 m
-10 m
-20 m

10

0

1.0 km

0.5

1.0 km34˚36’

Fig. 2 Maps showing spatial distribution of sample sites (a), mud content (b), diversity (c), and abundance (numberof valves/1-g sample) (d).

200 ostracode specimens were found in a sample, aResults

sample splitter was used to divide the sample. Each
carapace was counted as two valves. Scanning electron

Environment on the study dates

taken using the low-vacuum mode of a JEOL JCM-5000

ranged from 17.71°C at site SS8 to 21.82°C at site SS23

Water temperature: The temperature of bottom waters

micrographs of selected ostracode specimens were(Table 1). Although bottom-water temperature depends

Neoscope and a JEOL JSM T-220A at the Department ofprimarily on weather and measurement time on the

Earth Science, Interdisciplinary Faculty of Science andinvestigation days, it was highly negatively correlated

Engineering, Shimane University.

with water depth (r = −0.94, Table 1).

Salinity: The salinity of bottom waters ranged from

32.42 at site SS23 to 34.30 at sites SS7 and SS8 (Table 1).
It was highly positively correlated with water depth (r =41

Toshiaki Irizuki・Yuki Fujihara・Hokuto Iwatani・Shigenori Kawano

Table 1 List of sample and site information. f.s. and m.s in Sediment mean fine sand and medium sand, respectively.

No.

Latitude

Water
Bottomdepth (m) temp.(°C)

Bottomsalinity

Mud
Thickness
BottomMacrobenthos,
Sediment content of oxic layer
DO(mg/l)Materials
(%)
(mm )
5.74
mud
84.7
8
granules, plants,
annerids, few
Theora
6.14
mud
84.9
15
annerids, plants,
few shells,
Chlamys
6.20
sandy
71.4
10
shells (coquina),mudgranule, plants,
bryozoans, sea
urchins
6.00
mud
98.6
10
few shells, Theora,
gastropods

Longitude

Date

Time

SS1

34° 37.796′ 129° 26.850′

7/13/06

13:35

5.2

21.24

32.75

SS2

34° 37.644′ 129° 26.922′

7/13/06

13:55

12.0

20.92

32.95

SS3

34° 37.536′ 129° 26.699′

7/13/06

14:10

10.6

20.99

32.88

SS4

34° 37.475′ 129° 26.886′

7/13/06

14:23

16.4

20.31

33.28

SS5

34° 37.437′ 129° 27.136′

7/13/06

14:50

23.0

19.03

33.94

6.37

mud

97.5

10

annerids, Ophiura,
plants, sea
urchins, few shells

SS6

34° 37.202′ 129° 27.210′

7/13/06

15:10

27.4

18.51

34.13

6.56

mud

96.7

15

SS7

34° 36.992′ 129° 27.372′

7/13/06

15:35

31.3

17.83

34.30

6.73

mud

91.8

15

SS8

34° 36.901′ 129° 27.504′

7/13/06

15:50

28.9

17.71

34.30

6.72

mud

89.4

10+

SS9

34° 36.667′ 129° 27.587′

7/13/06

16:08

26.5

18.59

34.06

6.39

50.9

10

SS10 34° 36.229′ 129° 27.263′

7/13/06

12:17

15.7

20.59

33.08

6.46

muddy
m.s.
muddy
f.s.

51.6

5

SS11 34° 37.553′ 129° 27.110′
SS12 34° 37.392′ 129° 26.261′

7/13/06
7/13/06

14:40
9:45

13.4
11.0

20.69
20.76

33.08
33.02

6.00
5.96

m.s.
mud

27.0
92.5

20
10

SS13 34° 37.196′ 129° 26.209′

7/13/06

9:20

11.3

20.72

33.01

6.01

muddy f.m.s.

56.2

2

SS14 34° 36.995′ 129° 26.331′

7/13/06

9:05

18.7

19.43

33.74

6.06

mud

98.4

5

shells, Theora,
Raetellops, plants
annerids, Theora,
Raetellops
shells, Mytilus,
Raetellops, sea
urchins
gravels, shells,
Modiolus
granules, shells(coquina),
Spondylus,
Barbatia, Cardita,
Acestashells (coquina)
plants, few shells,
Theora, Acesta
gravels, shells(coquina),
Chlamys, Dosinia,
barnacles
plants, annerids,
Raetellops, Paphia

SS15 34° 36.835′ 129° 26.381′

7/13/06

8:55

10.1

20.90

32.88

6.51

mud

79.7

15

SS16 34° 36.955′ 129° 26.523′

7/13/06

10:30

18.8

19.70

33.69

5.96

mud

88.7

15

SS17 34° 36.950′ 129° 26.775′

7/13/06

10:50

27.4

19.35

33.81

5.94

mud

97.0

10

SS18 34° 36.839′ 129° 27.059′

7/13/06

11:07

26.4

19.40

33.74

6.28

sandy
mud

84.2

5

SS19 34° 36.620′ 129° 27.155′

7/13/06

11:25

29.0

18.99

33.94

6.22

mud

95.6

5-10

SS20 34° 36.456′ 129° 27.056′

7/13/06

11:55

17.3

20.35

33.28

6.15

muddy
m.s.

37.6

10

SS21 34° 37.023′ 129° 26.083′
SS22 34° 36.428′ 129° 27.302′

7/13/06
7/13/06

10:10
16:30

14.4
26.4

20.38
17.88

33.21
34.24

5.60
6.30

mud
mud

86.7
91.5

2
10-

SS23 34° 36.654′ 129° 26.271′

7/13/06

16:55

2.5

21.82

32.42

6.04

muddy
f.s.

37.0

5-10

42

Theora, Ophiura,
annerids
plants, shells, sea
urchins, few
annerids
annerids, Theora,
sea urchins, plants
granules, shells(coquina),
Chlamys, Acesta,
Protothaca,
Mytilus,
Dentalium,oysters, corals,
plants, bryozoans,barnacles,
annerids,
Raetellops,
Theora, plants
gravels, shells(coquina),
Barbatia, plants
plants, few shells
Raetellops, plants,
shells, Ophiura,
annerids
gravels, plants,
few shells

Recent ostracode assemblages from Shushi Bay, Tsushima Island,
southwestern Japan and their ecological and zoogeographical characteristics

4

Diversity

3
2

4

21

1

17

2
15
1214

16

10
1113
3
5 6

9
18
20
7
1922

8

0.4
0.2
0.1

Equitability

1
0

0.8
0.6

0

10

20

30
40
Species Number

50

60

Fig. 3 The relationships among species number, diversity, andequitability in each sample.

Species number (S) and Shannon-Wiener’s function0.96) and negatively correlated with water temperature (r= −0.99).

(Shannon index, H ′) were used as ecological indices forwaters ranged from 5.60 mg/l at site SS21 to 6.73 mg/l atand Gibson (1969) function was used as an index of

species richness and diversity, respectively. The Buzas

Dissolved oxygen: Dissolved oxygen (DO) in bottom

equitability (E). Results are shown in Figs. 2c and 3 andsite SS7 (Table 1). It was highly positively correlated with

Table 2. Species number ranged from 12 in sample SS4

water depth and salinity (r = 0.56 and 0.55, respectively)

to 51 in sample SS8; the Shannon index ranged from 1.94

and negatively correlated with water temperature (r =in sample SS1 to 3.34 in sample SS9; and equitability−0.6). An oxygenated layer was found in all study sites.

ranged from 0.32 in sample SS22 to 0.73 in sample SS10.

Its thickness ranged from 2 to 20 mm (Table 1). Thus,

Abundance (number of valves/g of dry sediment) ranged

the bottom of Shushi Bay was in an oxic condition at thefrom 0.3 in sample SS23 to 522 in sample SS13 (Table 2time of our study.

and Fig. 2d).

Sediments: Samples collected from the innermost partof several inlets in the bay (SS10, SS11, SS20, and SS23)
were composed of muddy fine-to-medium sand with a

Ostracode bioassociation

many shell fragments, except sample SS23 collected

samples (Table 2). Twelve species were represented

river flowing into the bay (Table 1, Fig. 2b). Sample

cluster analysis of these species was conducted to

We identified more than 107 ostracode species in 23

mud content of 27.0% to 51.6%; the samples containedby total number of specimens more than 100. R-mode

from the mouth of the Shushi River, which is the largest

clarify ostracode bioassociations, which mean ostracodeSS9 collected from the bay mouth was also composedgroups consisting of coexisting species. Horn’s overlapof muddy medium sand, with a mud content of 50.9%.

index (Horn, 1966) was used as a similarity index,

Other samples also consisted of muddy sediments. Most

and clustering was performed via the unweighted pairsamples with a high mud content (> 90%) containedgroup method with arithmetic mean (UPGMA). The

shells of Raetellops pulchella and Theora fragilis (Table

PAST program (Paleontological Statistics) was used for1), which are representative molluscan species living inanalysis; it is a free, easy-to-use data analysis package foroxygen-poor bottoms in enclosed bays.

paleontological data (Hammer et al. 2001). The analysis

Ostracode assemblage structure

revealed four ostracode bioassociations (BC, LN, KA,
and NX; Fig. 4).

Severa l ecologica l i ndices were ca lculated toBioassociation BC: This bioassociation consisted

characterize ostracode compositions for 22 samplesof four species: Bicornucythere bisanensis (Okubo),

containing more than 100 specimens except sample SS23.

43

Aglaiocypris nipponica
Ambocythere japonica
Ambtonia obai
Amphileberis nipponica
Argilloecia hanaii
Argilloecia lunata
Aurila corniculata
Aurila cymba
Aurila hataii
Aurila munechikai
Aurila spinifera
Bicornucythere bisanensis
Bythoceratina cassidoidea
Bythoceratina hanaii
Bythoceratina sp.
Callistocythere alata
Callistocythere angulata
Callistocythere asiatica
Callistocythere hayamensis
Callistocythere hosonosuensis
Coquimba ishizakii
Coquimba poga
Cornucoquimba cf. ikeyai
Cornucoquimba cf. moniwensis
Cornucoquimba tosaensis
Cythere sp.
Cytherois cf. bingoensis
Cytherois nakanoumiensis
Cytherois uranouchiensis
Cytherois sp.
Cytheromorpha acupunctata
Cytheropteron donghaiense
Cytheropteron miurense
Darwinula sp.
Dolerocypria mukaishimensis
Dolerocypria? sp.
Hanaiborchella triangularis
Hemicytherura cuneata
Hemicytherura cf. cuneata
Hemicytherura kajiyamai
Hemicytherura tricarinata
Hemicytherura sp.
Krithe japonica
Lineocypris? sp.
Loxoconcha bispinosa
Loxoconcha epeterseni
Loxoconcha harimensis
Loxoconcha hattorii
Loxoconcha japonica
Loxoconcha kitanipponica
Loxoconcha cf. kosugii
Loxoconcha uranouchiensis
Loxoconcha zamia
Loxocorniculum mutsuense
Macrocypris decora
Microcythere sp.
Munseyella japonica
Neonesidea oligodentata
Neonesidea sp.
Neopellucistoma? sp.
Nipponocythere bicarinata
Paracathaycythere costaereticulata
Paracypris sp.

44
5
24

1

3
3
1

2

6

2
1
1
3
1

6
9

4
2

30

7

29

1
1

4
2

17

3

1

1

1

8
2

5
43

SS3

1

9

2
6

4

4

19

3

8

SS2

67

SS1

35

1

1

43

19

20

2

45

13

SS4

Table 2 List of ostracodes from Shushi Bay.

17

2
1

3

8

21

64

1

11
14
1

1

3

1

33

2

1

1
18
22

SS5

1
3

8

1

4

3

1

4

67

1

15
2

1

2

1

3
1

11
23
1
3

SS6
1

1

3
1

1
2
22

1
2

1
4
1
1
12

5
1

4
3

1
8
2

3

31

1

1
1
8

1
1

32
1

1
1

1
36

4

4
1

1

1
1

1
1

1

15

6

3

5

12
1

1

1
1
2

3
8

1

20

10

1
17

2
11
11

13
5

3

2
3
11

2

1

6

11
4

1

2

1

9

1
10

8
3

1

7
7

1

1

1
10

25
4

2
6
15
15

3
1

7

14
9

12

11

3
15

12
1

1
2

1
1

1

8

1

33

1

2

13

46

1

2

23
19

3
21

2

1

3

2

3

3
1
19
5

3

2
1

3
2

4

30

2

2
1

1

28

35

1

11

1

1

2
2

39

2
1

21
1

2

13

1

2
8

5

1

1

2

2
16

2

21

5

12
1

1

1
3

2

3

16

10

4

3

1
4
9

11

2
6

12
2

13

4

2

3
5

80

3
2
30

2

8
23
1

8
21

28

5

4

16

11
11

2
3

1

3
2

1

12
1

4
5
3
4

7

10

1

7
6

5
1

1
87

1

11

2

1
1

2
1
3

13

3
28

1
5

1
34

13
5
1

1
4
21

3
3

1

1
1

2

16
3
1

5

1
7

3

5

7

1
2

24

5

25

1

13

48

3

4
23

10

2

4
3

8
1

82

16
1

3

1

3

2

1

1

41

4

1

4

SS10 SS11 SS12 SS13 SS14 SS15 SS16 SS17 SS18 SS19 SS20 SS21 SS22 SS23

2

SS9

5

7

25

SS8

1
2

4

1

18
1
9

SS7

Table 2 Irizuki et al.

Toshiaki Irizuki・Yuki Fujihara・Hokuto Iwatani・Shigenori Kawano

Loxoconcha epeterseni
Loxoconcha harimensis
Loxoconcha hattorii
Loxoconcha japonica
Loxoconcha kitanipponica
Loxoconcha cf. kosugii
Loxoconcha uranouchiensis
Loxoconcha zamia
Loxocorniculum mutsuense
Table 2 (Continued)
Macrocypris decora
Microcythere sp.
Munseyella japonica
Neonesidea oligodentata
Neonesidea sp.
Neopellucistoma? sp.
Nipponocythere bicarinata
Paracathaycythere costaereticulata
Paracypris sp.
Paracytheridea neolongicaudata
Paracytheroma sp.
Paradoxostoma spp.
Parakrithella pseudadonta
Pistocythereis bradyformis
Pistocythereis bradyi
Platymicrocythere sp.
Propontocypris attenuata
Propontocypris japonica
Propontocypris sp. 1
Propontocypris sp. 2
Pseudoaurila japonica
Pseudocythere sp. 1
Pseudocythere sp. 2
Pseudocythere sp. 3
Pseudopsammocythere tokyoensis
Robustaurila salebrosa
Sanyuania sp.
Schizocythere kishinouyei
Sclerochilus mukaishimensis
Semicytherura kazahana
Semicytherura miurensis
Semicytherura mukaishimensis
Semicytherura sasameyuki
Semicytherura wakamurasaki
Semicytherura sp.
Spinileberis quadriaculeata
Thalassocypria inujimensis
Thalassocypria? sp.
Trachyleberis niitsumai
Trachyleberis aff. verrucifera
Trachyleberis sp. 1
Trachyleberis sp. 2
Trachyleberis? sp.
Xestoleberis hanaii
Xestoleberis opalescenta
Xestoleberis sagamiensis
Xestoleberis setouchiensis
Xestoleberis sp.
Xestoleberis? sp.
Xiphichilus sp.
Cyprididae gen. et sp. indet. 1
Cyprididae gen. et sp. indet. 2Gen. et sp. indet.
Number of valves
Abundance (N/g)
Species Number
Diversity (H')
Equitability

45
4
2
1

36
1
4
1

119
10
27
2.71
0.56

2

11
2
2
1

142
10.9
17
1.94
0.41

1

7

1

2

5
8

4

3
1

1

24

3

6

263
160
34
2.76
0.46

1

1

1

1

4
2

30

203
31.3
12
2.02
0.63

1

256
80.6
32
2.66
0.45

6

6

1

1
3

1
4

2

3

2
2

17

2
1

3

4
1

5

18

35

1

1

8

1

1
5
23

1

5

1

2

2
1
1
3
1

6
9

4
2

188
88.2
35
2.59
0.38

1

5
1

3

1

1

1

2
3

6

1
1

2
4

1
3

8

1

4

3

1

176
267
43
2.92
0.43

1

1
4

5

1

2

1

2
1

3

1
9
2

213
296
51
3.3
0.53

2

1
9
1
2
5

3

6

1
1

5

2

29
4

1

3
1

1
2
22

1
2

1
4
1
1
12

5
1

4
3

1
8
2

3

169
106
44
3.34
0.64

7
1
1
2

4
2
7
2

1

3
1

6
2

1
4
1

6
7

1
1

15

6

3

5

12
1

189
156
35
3.25
0.73

2

6

13
2

9

3

2

6
4

1

1

3

1
2

10

1
17

2
11
11

5

212
189
32
3.06
0.66

1

8

18
1
1

9

7

1
3

3

1

2
2

1

1

1
10

25
4

2
6
15
15

1
162
24
26
2.35
0.4

1

1

7

2

17

1

2

2

3

1

12
1

1
2

1
1

1

186
522
33
2.97
0.59

5

5
5
8
1

3

6

1
1

22

5
1
1

2

23
19

3
21

2

1

201
37.7
27
2.4
0.41

2

1

3

1

1

1

9

1
1

30

2

2
1

1

165
4.23
24
2.55
0.53

1
2

29

18

26
1

3

1

1
2

2

13

1

2
8

1
152
6.26
29
2.92
0.64

1

21

2

14

1

1

2
2

4

12
1

1

1
3

2

3

193
25.1
22
2.09
0.37

1

3

3

3

2

5

13

4

2

5

194
152
41
3.22
0.61

2

2
1

5

5

6

1

1

1
2

2

2

2
2

1

13
1

8
21

28

5

4

240
90.9
40
2.65
0.35

1

3
1

13

2
2

2

1
1

2

1

1

2

1

16

7

10

1

7
6

1

211
287
42
3.14
0.55

1

25
1

6

2
2

7

4

1
4

2
3

4

3

2
5

1

1
5

1
34

13
5
1

1
4
21

3

154
24.7
15
2.09
0.54

2

13

1

1

8

7

1
2

248
184
41
2.57
0.32

1
1

2
2

9

1

1

2

1

2

1

1

16
2

4
23

10

2

4
3

1

14
0.31
5

3

2

Recent ostracode assemblages from Shushi Bay, Tsushima Island,
southwestern Japan and their ecological and zoogeographical characteristics

Toshiaki Irizuki・Yuki Fujihara・Hokuto Iwatani・Shigenori Kawano

Ostracode biofacies

N. oligodentata
NX
X. hanaii
A. nipponica
KA
C. donghaiense
K. japonica
L. bispinosa
LN
N. bicarinata
A. obai
B. bisanensis
C. angulata
BC
C. acupunctata
S. quadriaculeata

To identify ostracode biofacies, we conducted Q-mode

cluster analysis for 22 samples containing greater than

100 specimens and for 59 taxa including more than 3
specimens in either sample. The clustering method was

the same as for the R-mode cluster analysis. The analysis
revealed five ostracode biofacies (IM, SB, IS, MM, andMS) (Fig. 7).

Biofacies IM (inner mud biofacies): This biofacies

0.4 0.5 0.6 0.7 0.8 0.9 1.0
Horn’s overlap index

was composed of eight samples (SS1, SS3, SS4, SS5,
SS12, SS14, SS16, and SS21). It was divided into two

Fig. 4 Dendrogram showing the result of R-mode cluster

sub-biofacies. Sub-biofacies IM1 (SS1, SS3, SS12,

analysis of main species. BC, LN, KA, and NX mean

and SS21), distributed in the innermost part of the

ostracode bioassociations.

bay at water depths of 5.2 m to 14.4 m (Fig. 7), wascomposed of mud to sandy mud with a mud content of
71.4% to 92.5% (average 83.8%). The dominant species
were those from bioassociation BC (B. bisanensis, C.

Cytheromorpha acupunctata (Brady), Spinileberis
quadriaculeata (Brady), and Callistocythere angulata

acupunctata, S. quadriaculeata, and C. angulata).

inlets in Shushi Bay (Fig. 5) and have been observed insubordinate species. Species diversity and equitability

Species from bioassociation LN (e.g., L. bispinosa) wereOkubo. They were abundant in the innermost part ofranged from 1.94 to 2.76 (av. 2.28) and 0.40 to 0.54 (av.

the inner to middle part of enclosed Japanese bays (e.g.,0.45), respectively (Fig. 3, Table 2). Density was low to

Okubo, 1979; Ikeya and Shiozaki, 1993; Irizuki et al.,moderate (11–160, av. 55). Sub-biofacies IM2 (SS4, SS5,

2011).

SS14, and SS16), distributed in the inner part of the bay at

Bioassociation LN: This bioassociation consisted

water depths of 16.4 m to 23.0 m, was composed of mud,of three species: Loxoconcha bispinosa Kajiyama,

with a mud content of 88.7% to 98.6% (av. 95.8%). The

Nipponocythere bicarinata (Brady), and Ambtonia obai

dominant species were B. bisanensis and K. japonica.

(Ishizaki). They were abundant in the inner to middle

Subordinate species were L. bispinosa, N. bicarinata,part of Shushi Bay (Fig. 6) and have been observed inand A. obai. All species were members of bioassociation

the middle part of enclosed Japanese bays (e.g., Ishizaki,

LN. Species diversity and equitability ranged from 2.02

1968; Yasuhara and Irizuki, 2001; Yasuhara et al., 2007;

to 2.92 (av. 2.50) and 0.41 to 0.64 (av. 0.53), respectively.

Irizuki et al., 2008, 2009, 2011, 2015, 2018).

Density was low to moderate (6 to 81, av. 39).

Bioassociation KA: This bioassociation consisted of

Biofacies SB (seaweed beds biofacies): This biofacies

three species: Krithe japonica Ishizaki, Amphileberis
nipponica (Yajima), and Cytheropteron donghaiense

was composed of two samples (SS2 and SS15) with mud

Bay near the bay mouth (Fig. 6) and have been observed

were collected from the inner part of the bay at watercontents of 79.7% and 84.9% (av. 82.3%); the samples(Zhao). They were dominant in deeper parts of Shushi

depths of 10.1 m and 12.0 m (Fig. 7). X. hanaii andin muddy middle to outer bays at water depths of more

S. quadriaculeata were the dominant species in thisthan 20 m (e.g., Yasuhara and Irizuki, 2001; Yasuhara etbiofacies. X. hanaii lives on and around seaweeds and seaal., 2007; Irizuki et al., 2008, 2009, 2011, 2015, 2018).

grasses (Okubo, 1984; Kamiya, 1988). S. quadriaculeata

Bioassociation NX: This bioassociation was composed

lives in mud to sandy mud in the inner part of enclosedof two species: Neonesidea oligodentata (Kajiyama) andJapanese bays (Ikeya and Shiozaki, 1993). Species

Xestoleberis hanaii Ishizaki. They were found in mostdiversity and equitability were moderate (H′= 2.55 andsites in the bay (Fig. 5) and are phytal species (living on

2.71, av. 2.63; E = 0.53 and 0.56, av. 0.54) (Fig. 3, Table 2).

seaweeds and/or sea grasses in shallow coasts and rocky

Density was low (4 and 10, av. 7).

shores) (e.g., Okubo, 1975, 1984; Kamiya, 1988; Irizuki et

Biofacies IS (inner sand biofacies): This biofacies wasal., 2008).

46

Fig. 5. Irizuki et al.
Recent ostracode assemblages from Shushi Bay, Tsushima Island,
southwestern Japan and their ecological and zoogeographical characteristics
129˚ 26’
34˚38’

Bioassociation BC
129˚ 26’

28’

27’

28’

27’

1

3

4

3

B. bisanensis

5

4

BC

5

C. acupunctata

14

34˚37’

N

N

0

0.5

0

1.0 km

0.5

1.0 km34˚36’
34˚38’
1

3

3

S. quadriaculeata

C. angulata

34˚37’

N

N

0

0.5

20

0

1.0 km34˚36’

0.5

1.0 kmBioassociation NX

34˚38’

N. oligodentata

X. hanaii
13

13
7

34˚37’

8

8
18

9

9

N

0

N

20

0.5

1.0 km

0–10

10–25

0

20

0.5

1.0 km34˚36’

0

25–50

50–100100–200

200–400

400≤

Isobath

-5 m
-10 m
-20 m

Fig. 5 Spatial distribution of abundance (number of valves/10-g sample) of ostracode species inbioassociations BC and NX. Scale bars = 0.1 mm.

47

Fig. 6. Irizuki et al.

Toshiaki Irizuki・Yuki Fujihara・Hokuto Iwatani・Shigenori Kawano

Bioassociation LN

129˚26’34˚38’

27’

3

4

5

Bioassociation KA

129˚26’28’

27’

K. japonica

5

L. bispinosa

28’

6
7

34˚37’

17
8
18

19

0

0.5

22

N

22

N

0

1.0 km

0.5

1.0 km34˚36’
34˚38’

3

4

A. nipponica

5

N. bicarinata

5

6
13
7

34˚37’

8

19
N

0

20

0.5

22

N

22

0

1.0 km

0.5

1.0 km34˚36’
34˚38’

3

A. obai5

C. donghaiense

734˚37’

8
18

0

22

N

N

0.5

1.0 km

0–10

10–25

0

0.5

1.0 km34˚36’

0

25–50

50–100100–200

200–400

400≤

Isobath

-5 m
-10 m
-20 m

Fig. 6 Spatial distribution of abundance (number of valves/10-g sample) of ostracode species inbioassociations LN and KA. Scale bars = 0.1 mm.

48

Recent ostracode assemblages from Shushi Bay, Tsushima Island,
southwestern Japan and their ecological and zoogeographical characteristics

SS15
SS2
SS3
SS21
SS12
SS1
SS4
SS14
SS5
SS16
SS10
SS11
SS20
SS13
SS6
SS17
SS19
SS22
SS7
SS8
SS9
SS18

a

0.3

0.4

0.5 0.6 0.7 0.8
Horn’s overlap index

0.9

SB

34˚38’

129˚
26’

28’

27’

Biofacies(sub-biofacies)

IM1

IM1
IM2
SB
IS
MM
MS

IM2

IS

b

34˚37’

MM
N
-5 m
-10 m
-20 m

MS

1.0

0

0.5

1.0 km34˚36’

Fig. 7 Dendrogram showing the result of Q-mode cluster analysis (a) and spatial distribution of ostracode biofacies andsub-biofacies (b) (IM1, IM2, SB, IS, MM, and MS).

composed of four samples (SS10, SS11, SS13, and SS20)

Biofacies MS (middle sandy mud biofacies): This

with a mud content of 27.0% to 56.2% (av. 43.1%); thebiofacies consisted of two samples (SS9 and SS18) with

inlets in the bay at water depths of 11.3 m to 17.3 m (Fig.

samples were collected from the middle to outer partsamples were collected from the inner part of small

a mud content of 50.9% and 84.2% (av. 67.6%). The

7). N. oligodentata, Aurila spp., and Loxoconcha spp.

of the bay at water depths of 26.4 m and 26.5 m (Fig.

were dominant in this biofacies. They live on and around

7). N. oligodentata, Loxoconcha spp., Aurila spinifera,

seaweeds around rocky tidal shores, sea grass beds,

and Cytheropteron spp. were common in this biofacies.

and sandy bottoms under the influences of open waters

Species diversity and equitability were highest (H′= 3.22(e.g., Okubo, 1975; Kamiya, 1988). Species diversity

and 3.34, av. 3.28; E = 0.61 and 0.64, av. 0.63) (Fig. 3,

and equitability were high (H′= 2.97–3.25, av. 3.10; E =Table 2). Density was high (106 and 152, av. 129).

0.5–0.73, av. 0.63) (Fig. 3, Table 2). Density was highest
compared to the other biofacies (156–522, av. 325).

Discussion

Biofacies MM (middle mud biofacies): This biofacies

was composed of six samples (SS6–SS8, SS17, SS19,

Relationships between ostracodes and environmental

93.7%); the samples were collected from the middle

Because ostracode specimens in this study were mostly

factors

and SS22) with a mud content of 89.4% to 97.0% (av.
to outer part of the bay at water depths of 26.4 m toempty valves with minor amounts of carapace and few

donghaiense were dominant in this biofacies. They werestudy site was uncertain, they were not components ofappendages and because the sedimentation rate at each31.3 m (Fig. 7). K. japonica, A. nipponica, and C.

census assemblages. However, the correlation coefficients

members of bioassociation KA. Phytal species such as
N. oligodentata and X. hanaii, which were members of

between environmental factors measured in this study

bioassociation NX, were also present. Species diversity(water depth, salinity, dissolved oxygen, and mudcontent) and the percentages and density of species in

was moderate to high (H′= 2.09–3.30, av. 2.69), andeach ostracode bioassociation are discussed here (Table

equitability was low to moderate (E = 0.32–0.53, av. 0.40)(Fig. 3, Table 2). Density was moderate to high (25–296,

3). Species in bioassociation BC showed a negative

av. 158).

correlation with salinity, dissolved oxygen, and water49

Toshiaki Irizuki・Yuki Fujihara・Hokuto Iwatani・Shigenori Kawano

Table 3 List of the correlation matrix.
Percentage
Bicornucythere bisanensis
Cytheromorpha acupunctata
Spinileberis quadriaculeata
Callistocythere angulata
Loxoconcha bispinosa
Nipponocythere bicarinata
Ambtonia obai
Krithe japonica
Amphileberis nipponica
Cytheropteron donghaiense
Neonesidea oligodentata
Xestoleberis hanaii

mud content
0.30
0.22
-0.06
-0.01
0.31
0.41
0.49
0.57
0.48
0.38
-0.58
0.05

diversity
-0.72
-0.52
-0.23
-0.35
-0.46
-0.48
-0.32
-0.30
-0.02
0.10
0.72
0.04

salinity
-0.48
-0.58
-0.66
-0.69
-0.18
-0.03
0.24
0.65
0.79
0.73
0.16
-0.29

DO
-0.63
-0.47
-0.25
-0.37
-0.42
-0.30
-0.16
0.18
0.47
0.54
0.35
0.04

Table 3. Irizuki
et al.
water depth bioassociation
-0.59
BC
-0.62
BC
-0.69
BC
-0.74
BC
-0.24
LN
-0.10
LN
0.15
LN
0.68
KA
0.78
KA
0.77
KA
0.25
NX
-0.30
NX

Abundance
Bicornucythere bisanensis
Cytheromorpha acupunctata
Spinileberis quadriaculeata
Callistocythere angulata
Loxoconcha bispinosa
Nipponocythere bicarinata
Ambtonia obai
Krithe japonica
Amphileberis nipponica
Cytheropteron donghaiense
Neonesidea oligodentata
Xestoleberis hanaii

mud content
0.13
-0.19
-0.55
-0.51
-0.14
-0.06
0.35
0.41
0.33
0.24
-0.52
-0.45

diversity
-0.29
0.02
0.24
0.25
0.01
0.00
-0.12
-0.01
0.21
0.30
0.58
0.65

salinity
-0.34
-0.46
-0.51
-0.52
-0.28
-0.01
0.22
0.71
0.73
0.62
0.04
0.24

DO
-0.25
-0.09
-0.11
-0.09
-0.04
0.05
0.14
0.47
0.62
0.64
0.22
0.40

water depth bioassociation
-0.39
BC
-0.43
BC
-0.46
BC
-0.46
BC
-0.28
LN
-0.05
LN
0.19
LN
0.68
KA
0.66
KA
0.60
KA
0.08
NX
0.25
NX

depth, suggesting that they live in the enclosed inner partthe optimal zones of bioassociations BC, LN, and KA

KA showed a positive correlation with all environmental

relationships between anthropogenic pollution andof the bay. On the other hand, species in bioassociationdiffered. Irizuki et al. (2011, 2015, 2018) discussed thefactors, suggesting that they live in deeper mud bottoms

embayment species. In particular, B. bisanensis andof the bay under some influences of oxic open waters.

K. japonica showed clear relationships between TOC

and percentage of these species (Fig. 8) indicated that

correlation with TOC content, the latter a negative

In particular, the relationship between water depth

content and their abundance; the former showed positive

species in bioassociation BC were dominant in the bay

correlation. Thus, the optimal zone is also related to theat water depths of less than 20 m. In contrast, species inamounts and sources of organic matter, which are bates

bioassociation KA were dominant at water depths greaterfor ostracodes.

than 20 m. Moreover, B. bisanensis in bioassociation

Species in bioassociation NX did not show any

BC was the most dominant species in sandy mud to mud

significant correlations with environmental factors.

and C. donghaiense in bioassociation KA were the mostsediment than X. hanaii (Fig. 8). Both species live on

bottoms at water depths less than 20 m; K. japonica

However, N. oligodentata was more abundant in coarser

dominant species in mud bottoms at water depths ofor around seaweed and sea grass beds in an intertidal

20–30 m and at greater that 25 m, respectively. Thus,

zone (e.g., Kamiya, 1988); thus, their valves are easily

they are useful as an indicator of water depth. Species

transported from their habitats after their death. X. hanaii

in bioassociation LN did not show any high correlations

is relatively small in size (adult size: 0.6–0.7 mm long),with environmental factors, but the majority of themwhile N. oligodentata is larger (adult size: 1.0–1.2 mm

were found in mud bottoms at water depths of 15–20

long). Thus, we inferred that small juvenile specimens

m; this was a transition zone from the dominance ofof X. hanaii had been transported easily by low-energy

bioassociation BC to that of bioassociation KA. Thus,

currents, and they were therefore more abundant than50

Fig. 8. Irizuki et al.

Recent ostracode assemblages from Shushi Bay, Tsushima Island,
southwestern Japan and their ecological and zoogeographical characteristics

Bioassociation BC

*

(%)

(%)

B. bisanensis

*

30
20

C. acupunctata

20

40

16

*

12

* *

*

8

10

4

0

0

(%)

(%)

S. quadriaculeata

C. angulata

12

12
8

8

4

4
0

0

5

10

15

20

Water depth (m)

(%)

*

16

25

30

10

15

5

10

20

15

20

Water depth (m)2530Bioassociation LN

(%)

0

35

*

0

5

10

15

20

Water depth (m)

25

30

*

30

12

35

*

K. japonica

40

16

30

Bioassociation KA(%)

L. bispinosa

25

X. hanaii

10

0

20

Water depth (m)

*

30

*

20

35

*
* *

*

20

8

10

4

0

0

(%)

(%)

N. bicarinata

16

A. nipponica

16

12

12

8

8

4

4
0

0

(%)

(%)

A. obai10*

C. donghaiense

20

*

16

8

12

6
4

8

2

4

0

5

(%)

N. oligodentata

400Bioassociation NX

12
8

0

35

0

5

10

15

20

Water depth (m)25

Mud content

30

35

0

20–70%

05

70–90%

10

15

20

Water depth (m)

25

30

35

90–100%Fig. 8 Panels showing the relationships between water depth and percentages of main species.
* shows most dominant species in each sample.

51

Toshiaki Irizuki・Yuki Fujihara・Hokuto Iwatani・Shigenori Kawano

N. oligodentata in fine sediments.

Iwasaki, 1992; The Association for the Environmental

and water depth. Water depth also reflects a difference inal., 2019 in press). Wide areas of low-salinity estuaries

Conservation of the Seto Inland Sea, 2009; Tanaka et

Ostracode biofacies are primarily related to substrate

and tidal flats were not developed along Shushi Bay;

several environmental factors, such as salinity, organic

thus, samples could not be collected from those areas

matter content, water temperature, and dissolved oxygen.

in this study. Other representative species living along

The boundary between biofacies IM and MM, both of

the turbid and tidal flats in Chinese and Korean coasts

which consisted of sediments with high mud content (>(e.g., Zhao and Wang, 1988; Zhao and Whatley, 1988),90%), was situated at a water depth of 20–25 m; this is

Sinocytheridea impressa and Neomonoceratina spp.,

because the optimal zone of each dominant species, B.

bisanensis in biofacies IM and K. japonica in biofacies

where the tidal range is significant and many rivers

mentioned above. The fact that biofacies SB contained

is attributed to the fact that a similar environment isflow, were also not found in Shushi Bay. However, thisMM, was situated at a water depth of 20 m in the bay

not developed in Shushi Bay now because the Tsushima

the intertidal phytal species (bioassociation NX) X.

Island is surrounded by relatively deep seas. Wide tidal

hanaii indicates that seaweed and sea grass beds, orflats and turbid conditions might have been developed

rocky shores with seaweeds, are developed in shallower

along Shushi Bay during the low sea-level glacial periods

areas near sites SS2 and SS15. Biofacies IS and MS,

and species in Chinese and Korean coasts might haveboth of which consisted of coarser sediments, contained

migrated through the Tsushima Strait. However, thesemostly high diversity (> 3.0) and high equitability (> 0.6)

species are thought not to have been able to live in Shushi

assemblages composed of species from several different

Bay due to the subsequent transgression.

habitats. Biofacies IS was situated in sandy coasts,
where wave action dominated other areas, suggestingthat ostracodes are always influenced by the sorting andConclusions

winnowing effects of waves. Whereas, biofacies MS was
situated in deeper bay channels, suggesting that shallower

1. At least 110 ostracode species were identified in 23

there.

Tsushima, Nagasaki Prefecture, Southwest Japan.

surface sediment samples collected from Shushi Bay,sandy and phytal specimens were transported and mixed

2. Four ostracode bioassociations and five biofacies wereComparisons with recent Ostracoda in other enclosed

revealed on the basis of R-mode and Q-mode cluster

bays in Japan

analyses, respectively.

3. The correlation between environmental factors

Ostracode assemblages in Shushi Bay were basicallymeasured in this study and ostracode bioassociations

identical to those in enclosed bays of the Kyushu andclarified ecological characteristics of main ostracode

Honshu Islands (e.g., Ishizaki, 1968; Irizuki et al., 2006,

species. This information is useful to reconstruct2009). Only one species belonging to Bicornucythere,

Holocene environments based on ostracode assemblages.

B. bisanensis, was found in the present study; this

4. Ostracode assemblages were basically similar to thosespecies is abundantly distributed in inner to middle

from the Japanese mainland. No continental ostracodeenclosed bays at water depths of 5–10 m (Ikeya andelements were found in this study.

Shiozaki, 1993) from northern Japan (e.g., Ishizaki,1971) to southwestern Japan (e.g., Iwasaki, 1992). Alongthe eastern coasts of the Eurasian continent in eastern

Acknowledgments

Asia, several morphotypes of Bicornucythere have beenreported (Abe, 1988; Abe and Choe, 1988; Iwasaki, 1992;

We express deep gratitude to the Tsushima Coast Guard

Yasuhara and Irizuki, 2001; Irizuki et al., 2018). Two of

Office and the Kamitsushima-cho Koto Fishermen's

Keijella bisanensis form M of Abe, 1988) or wide tidal

in Shushi Bay. We are indebted to two anonymousthem live in relatively low salinity (B. sp. M equivalent to

Cooperative Association to collect sediment samples

flat areas (B. sp. P equivalent to Keijella bisanensis form

reviewers and a chief editor for revising our manuscript.

P of Abe, 1988) in the Honshu and Kyushu Islands (e.g.,

The authors would like to thank Enago (www.enago.jp)
52

Recent ostracode assemblages from Shushi Bay, Tsushima Island,
southwestern Japan and their ecological and zoogeographical characteristics

for the English language review. This study was partly

Island, Kagoshima Prefecture, southwestern Japan.

(C) from the Japan Society for the Promotion of Sciences

Irizuki, T., Seto, K. and Nomura, R. (2008) The impact

supported by the Grant-in-Aid for Scientific Research

Laguna 13: 13–28.

(Nos. 17540442 and 16K05589 to Toshiaki Irizuki).

of fish farming and bank construction on Ostracoda

in Uranouchi Bay on the Pacific coast of southwest
Japan- Faunal changes between 1954 and 2002/2005 -.

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