Tải bản đầy đủ - 0trang
Chapter 57. Distribution of modern ostracods in the shelf seas off China
806 Q.-H. ZHAOAND P.-x.WANO
l-Bathymetry. sampling stations and sediment types in the shelf seas off China (for graphical reasons
some of the stations are omitted).
Rich ostracod faunas were found in all the shelf seas under consideration. A total of 116 species
belonging to 68 genera were identified in the Bohai Gulf and the Yellow Sea, 126 species representing 83 genera in the East China Sea and 190 species from 90 genera in the northern part of the
South China Sea.
The distribution of ostracod abundance (number of valves per gram of sediment) and diversity
(number of species per sample) is shown in Text-figs. 2,3 and Table 1. It can be seen that both the
abundance and diversity increase generally from the north to the south, from the coast to the shelf
edge. The lowest values (<10 species per sample, < 1 valve per gram) occur in the estuarine and
coastal zones with water depths of less than 20m and in the central part of the Huanghai Sea with
2-Distribution of ostracod abundance in the surface sediments of the shelf seas off China (number of
valves per gram of dry sediment).
TEXT-FIG.3- -Distribution of number of species of ostracods in the surface sediments of the shelf seas off China.
808 Q.-H. ZHAOAND P.-X. WANO
TABLEA AVERAGE OSTRACOD
OF THE SHELF
Water Depth Range
Huanghai Sea and Bohai Gulf
East China Sea
North Part of South China Sea
Huanghai Sea and Bohai Gulf
East China Sea
North Part of South China Sea
average number of valves per gram of sediment
average number of species per sample
water depths of over 50 m. The low concentration and diversity of ostracods can be attributed to
high sedimentation rate and to low water temperature and salinity, since the central part of the
Huanghai Sea is occupied by the Huanghai Cold Water Mass and the estuarine and coastal zones
are dominated by diluted water of variable temperature. The middle and outer shelves of the East
4-Occurrence of some cold-water and warm-water species in the shelf seas off China:
Cold-water species: 1, Howeina camptocytheroidea; 2, Acanthocythereismutsuensis; 3 , Sarsicytheridea bradii; 4, Finmarchinella huanghaiensis; 5, Cytheromorpha acupunctata; 6, Loxoconcha hattorii; 7, Cytherop
Warm-water species : 8, Cytherelloidea senkakuensis; 9, Paoenborchella iocosa P. malaiensis; 10, Cytherelloidea yingliensis; 1 1, Uroleberis faveolata; 12, Neonesidea haikangensis.
R a d i m e l l a spp.
M k r i t h e orientalis
Neoqtheretta s n e l l i i
P a i j e h r d e l l a i-a
+ P. malaiensis
B i c D m u q t h e r e euplectella
U r o l e k e r i s fowolata
Abrorythereis q u a n q d o ~ n s i s
Acanthoqthereis ? n i i t s d
A u r i l a qnba
Cushmanidea s b j j n i c a
Widmamella bradyi + W. b r a d y f o d s
B y t h w e r a t i ~cassidoidea
Hirsutocythere ? hanaii
k k q t h e r e retiaiiata
Anphileberis g i b k r a
Sinocytheridsa latiovata + S. longa
Bradleya j aponica
Munseyella p q i l l a
KabayaShiiM m a i e m i s
Spinileberis furuyaersis + S. pulcfira
distribution of common species of Ostracoda in the shelf seas off China.
810 Q.-H. ZHAOAND P.-X. WANO
China Sea and the northern part of the South China Sea, on the contrary, are controlled by the
Kuroshio Current and hence are distinguished by high temperature and salinity, apart from the low
sedimentation rate of terrigenous material. As a result, the ostracod concentration and diversity are
high in these areas, with more than 10 valves per gram and more than 20 species per sample.
As revealed by distribution patterns of individual species in the sea areas studied (see Wang and
Zhao, 1985; Wang, Min and Gao, 1980; Wang et al., in press; Zhao, Wang and Zhang, 1985),
temperature, salinity and water depth are major environmental factors controlling the areal distribution of Ostracoda.
The China Sea extends across three climatic zones : tropical, subtropical and temperate zone.
However, the bottom-water isotherms are not parallel to the latitudes and their pattern is complicated by the influence of the water masses and currents. For example, warm-water species typical
of the South China Sea - Bicornucythere euplectella (Brady), Cytherelloidea yingliensis Guan,
Neonesidea haikangensis (Guan), Uroleberisfoveolata (Brady), Neocytheretta snellii (Kingma), Loxoconcha sinensis Brady, Xestoleberis variegata Brady, Paijenborchella malaensis Kingma, P. iocosa
Kingma etc., also occur in the outer shelf area of the East China Sea and are believed to be brought
northwards by the warm water of the Kuroshio Current (Text-fig. 4). On the other hand, cold-water
species like Finmarchinella huanghaiensisZhao, Cytheropteron sawanensis Hanai, Loxoconcha hattorii Ishizaki, Cytheropteron acupunctata (Brady) and Howeina camptocytheridea Hanai are mainly
bound to the Huanghai Cold Water Mass, but are distributed by the coastal currents in the northwestern part of the East China Sea as well (Text-fig. 4). The latitudinal occurrence of common species is summarised in Text-fig. 5 . In respect to temperature adaptation, three types of species may
Aurila q n b a
&Salinity ranges of common species of Ostracoda in the coastal areas of the China Sea.
0common; - few; --- rare.
SpMleberis funyaensis + S. pulchra
T a d l a opima
kptocythere m t r i c l i v n s a
Sinocytheridea latiovata + S. lmga
partocythere l i t t o r a l i s
Lamconcha h a t t o r i i
+ M. pupilla
?iicfimwnellabradyi + W. b r a d y f o d s
rathocythereis ? niitsuwi
P t e v
irsutocythere ? hanaii
"df"y" j e arsicythetldea bradii
lmccythere o b s a
haythere M c u l a t a
m t d a hanaii
adleya a l b a t m s i a
7-Depth ranges of common ostracod species in the shelf seas off China.
812 Q.-H. ZHAOAND P.-X. WANO
be distinguished in the shelf seas off China: 1. warm-water species inhabiting the South China Sea
and/or the East China Sea, with a few occurrences in the southern part of the Huanghai Sea; 2.
coldwater species distributed mainly in the Huanghai Sea and the Bohai Gulf; 3. eurythermal
species found in coastal waters all along the shoreline of the Chinese mainland.
As for the water salinity, there is a general tendency for it to increase southwards and seawards
in the sea areas studied. In the open seas, where the salinity variation is negligible, all ostracod species
are stenohaline in nature and the water temperature is the main factor controlling their distribution,
whereas in coastal and estuarine areas with significant salinity variations in space and time, the
ostracod fauna is dominated by euryhaline and hypohaline forms. According to our ecological
survey of living ostracods along the Chinese coast (Zhao, 1984, 1985), the salinity ranges of common eury and hypo-haline species are shown in Text-fig. 6. Worth noting are some euryhaline
genera such as Sinocytheridea and Albileberis which are dominant and widespread in the Chinese
coastal waters, but have as yet never been reported elsewhere. The world-wide brackish-water form
Cyprideis torosa (Jones) is conspicuously absent along the Chinese coast and is probably replaced
by the endemic species Sinocytheridea latiovata Hou et Chen.
Depth ranges of common ostracod species in the shelf seas off China are given in Text-fig. 7.
According to depth occurrence it is possible to distinguish supralittoral, littoral, inner shelf and
other forms. However, it has been shown that the water depth in itself is not a purely independent
factor in determining the faunal distribution, because many environmental factors such as hydrostatic pressure, density, temperature, salinity, pH, oxygen, diaphaneity, nutrients, etc., change with
it. If the same environmental conditions occur at different depths in different areas, the depth range
of ostracod species may also be changed. Examples are the warm-water species Neonesidea haikangensis, Uroleberis foveolata and Loxoconcha sinensis which spread all over the shelf in the South
China Sea ranging from the inner to outer part, but are limited to the outer shelf in the East China
Sea where the Kuroshio current is running through. The difference in their depth occurrences can
obviously be ascribed to the temperature.
In our previous works, several ostracod assemblages have been dstinguished for each sea off
China: three assemblages for the Bohai Gulf (Wang and Bian, 1985), five for the Huanghai Sea
(Wang, Min and Gao, 1980), nine for the East China Sea (Wang and Zhao, 1985), and five for the
South China Sea (Zhao, Wang and Zhang, 1985). When all these data from individual seas are compared with each other and are summarised, five major ostracod assemblages may be recognised for
the entire shelf of the China Sea (Text-fig. 8).
Assemblage I (Pl. 1, Figs. 1-9) is dominated by Sinocytheridea fatiovata, S . fonga Hou et Chen,
and Neomonoceratina crispata Hu and is widely distributed in coastal and estuarine areas less than
20 m deep all along the Chinese coastline. Some species (e.g.,Albileberis sinensis Hou, Leptocythere
ventriclivosa Chen and Loxoconcha ocellata Ho) are common in all the areas studied except the
South China Sea where they are absent presumably due to its relatively high temperature. Moreover, the species composition also varies with the difference in salinity. Thus, Tanella opima Chen,
Propontocypris euryhalina Zhao, Spinileberis furuyaensis Ishizaki and Kato and S. pulchra Chen
are more frequent in oligo-and mesohaline waters, while in poly and euhaline waters some elements
from Assemblage I1 ( Wichmannella bradyformis, Bicornucythere bisanensis, Sinocythere sinensis
and S.? reticulata among others) are common. In fact, this is an eurythermal and euryhaline assemblage.
Assemblage I1 (Pl. 1, Figs. 10-25) is composed of eurythermal, but stenohaline, shallow-water
Modern Osrracoda in Sherf Seas offChina 813
8-Distribution of five major ostracod assemblages in the shelf seas off China.
species and occurs on the inner shelf at water depths of about 20-50 m throughout all the seas under
consideration. Wichmannella bradyformis (Ishizaki), Bicornucythere bisanensis (Okubo), Mwtseyella pupilla Chen, M . japonica (Hanai), Cytheropteron miurense Hanai are dominant and Aurila
cymba (Brady), Cushmanidea spp., Wichmannella bradyi (Ishizaki), Stigmatocythere spinosa (Hu),
Sinocythere sinensis Hou, S . ? reticulata Chen, Nipponocythere bicarinata (Brady), Neomonoceratina crispata and Sinocytheridea latiovata are common. Again, there are variations in species composition related to water temperature. For example, there are warm-water forms like Loxoconcha
sinensis, Hemikrithe orientalis Bold and Copytus posterosulcus Wang in the South China Sea,
and cold-water forms like Finmarchinella huanghaiensis, Cytheropteron sawanensis and Loxoconcha
hattorii in the Huanghai Sea. Generally, this is an eurythermal but stenohaline assemblage.
Assembage I11(PI. 1,Figs. 26-32; P1.2, Figs. 1-3) is distinguished by the cold-water character of
its dominant and common species with a relatively low species diversity. The assemblage occurs in
the central and deeper part of the Huanghai Sea (beyond the 50 m isobath) occupied by the Huanghai Cold Water Mass and, hence, can be called the Cold Water Mass Assemblage. Two subassemblages may be distinguished: the Northern Subassemblage is characterised and dominated by the
typical cold-water species Howeina camptocytheroides, Sarsicytheridea bradii (Norman) and and
Acanthocythereis mutsuensis Ishizaki, with F. huanghaiensis, C. sawanensis and others as common
forms; while in the Southern Subassemblage more eurythermal, cold-water species like Amphileberis gibbera Guan, Buntonia hanaii Yajima, Kobayashiina donghaiensis Zhao and Krithe sawanensis Hanai are dominant and Ambocythere reticulata Jiang and Wu, Nipponocythere obesa (Hu),
Cluthia ishizaki Zhao and others are common. The boundary between the two subassemblagesis
at approximately latitude 36"N.
Assemblage IV (Pl. 2, Figs. 4-25) is distributed beyond the 40-50 m isobath in the middle and
outer shelves of the East China Sea and the northern part of the South China Sea, being closely
related to the open-sea water and the Kuroshio Current with its branches. The assemblage is
characterised by a great number of warm-water species and a high diversity and abundance; hence
Modern Ostracoda in Shelf' Seas off China 815
it may be termed a warm-water assemblage. The dominant and characteristic species include Argilloecia hanaii Ishizaki, Cathetocytheretta apta Guan, Cytherelloidea senkakuensis Nohara, C .
yingliensis, Loxoconcha sinensis, Neocytheretta snellii (Kingma), Neonesidea haikangensis, Uroleberis foveolata and Xestoleberis variegata among others. The assemblage may be further subdivided
into at least two subassemblages: the East China Sea Subassemblage with more frequent Bradleya
japonica Benson, Hirsutocythere? hanaii, Cytheropteron uchioi Hanai, C . subuchioi Zhao and Semicytherura minaminipponica Ishizaki, and the South China Sea assemblage with Cytherelloidea
leroyi Keij, Bradleya albatrossia Benson, Cytherella posterotuberculata Kingma, Cytheropteron
sinensis Zhao and Loxoconcha pterogona Zhao.
Assemblage V (PI. 2, Figs. 26-32) is found in the coral reef area in the southern part of
Hainan Island and the Xisha Islands. All the major elements such as Radimella virgata Hu, R.
macroloba Hu, R . parviloba Hu, Loxoconcha tumulosa (Hu), L . lilljeborchi Brady, Treibelina rectangulata Hu, Paranesidea s p . and Keijia novilunaris Zhao are tropical in nature. Therefore, this
is a tropical coral-reef assemblage.
To sum up, the distribution of ostracod assemblages in the area is principally determined by the
water temperature and salinity, which in turn are controlled by the distribution pattern of water
masses and currents, apart from the latitude. Both Assemblage I and Assemblage 11 are basically
consistent in species composition from north to south, implying the similarity of coastal waters
along the entire shoreline of the Chinese mainland. The two assemblages differ from each other in
their salinity adaptation. All the other three assemblages are related to temperature conditions:
Assemblage 111 represents the Huanghai Cold Water Mass, Assemblage IV corresponds to the
warm-water Kuroshio system, and Assemblage V to the tropical surface water in the southern part
of the South China Sea.
Despite the numerous contributions to ostracod studies in East Asia, a systematic zoogeographical study is still wanting. In a discussion on ostracod zoogeography of the Pacific, Benson (1964)
divided the western coast of the Pacific into two realms: the Japonic Realm and the Indo-West
Pacific Realm, with a boundary running through the south of the East China Sea. This is, probably,
the only ostracod biogeographical zonation of East Asia now available in the literature. Since
then, a considerable amount of information has been accumulated on ostracod taxonomy and areal
PLATEI-Assemblage I : Fig. 1. Sinocytheridea latiovata Hou et Chen. Left valve, ~ 4 5 Fig.
longa Hou et Chen. Right valve, ~ 4 5 .Fig. 3. Neomonoceratina crispafa Hu. Left valve, x 50. Fig. 4.
Albileberis sinensis Hou. Right valve, x 55. Fig. 5. Leptocythere ventriclivosa Chen. Right valve, x 55. Fig. 6 .
Loxoconcha ocellata Ho. Left valve, x 50. Fig. 7. Tanella opima Chen. Right valve, x 55. Fig. 8. Propontocypris
euryhalina Zhao. Left view of carapace, x 55. Fig. 9. Spinileberis ficruyazsis lshizaki et Kato. Left valve, X 50.
Assemblage 11: Fig. 10. Bicornucythere bisanensis (Okubo). Right valve, X 40. Fig. 11. Wichmannella bradyformis
(Ishizaki). Right valve, ~ 4 5 Fig.
12. Wichmannellu bradyi (Ishizaki). Left valve, ~ 4 0 Fig.
13. Munseyella japonica (Hanai). Right valve, x 80. Fig. 14. Munseyella pupilla Chen. Right valve, x 80. Fig. 15. Cytheropteron
miurense Hanai. Right valve, ~ 6 0 Fig.
16. Aurila cymba (Brady). Right valve, x 50. Fig. 17. Stigmatocythere
spinosa (Hu). Left valve, x 60. Fig. 18. Sinocythere sinensis Hou. Left valve, x 60. Fig. 19. Nipponocythere bicarinata (Brady). Left valve, x 65. Fig. 20. Alocopocythere profusa Guan. Right valve, ~ 4 0 Fig.
. 21. Acanthocythereis niitsumai (Ishizaki). Right valve, x 45. Fig. 22. Loxoconcha hattorii lshizaki. Right valve, X 55. Fig.
23-Cytheromorpha acupunctata (Brady). Left valve, x 6 0 ; Fig. 24-Copytus posterosulcus P. Wang. Left valve,
x 55. Fig. 25. Hemikrithe orientalis Bold. Left valve, X 55.
Assemblage I I I : Fig. 26. Howeina camptocytheroidea Hanai. Right valve, X 50. Fig. 27. Sarsicytheridea bradii
(Norman). Right valve, x 35. Fig. 28. Acanthocythereis mutsuensis lshizaki. Left valve, X 30. Fig. 29. Finmarchinella huanghaiensis Zhao. Left valve, x 60. Fig. 30. Cytheropterorz sawanensis Hanai. Right valve, X 80. Fig.
31. Amphileberis gibbera Guan. Right valve, X 50. Fig. 32. Buntonia hanaii Yajima. Right valve, X55.