Kagoshima University Research Center for the Pacific Islands, Occasional Papers No.34, 137-140, 2001
Part 2, Section 1, Report 2. The Progress Report of the 1999 Survey of the Research Project“Social Homeostasis of Small Islands in an Island-zone”

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A COMMENT ON THE GEOLOGICAL FORMATION OF THE MISHIMAISLANDS (TAKESHIMA, IOUJIMA AND KUROSHIMA) AS INFERRED FROMTHEIR FRESHWATER CRUSTACEAN FAUNAS
Hiroshi SUZUKI

Abstract
Freshwater crustacean faunas of the Mishima Islands, which constitute Takeshima, Ioujima andKuroshima, were investigated. Three species, Caridina typus, Macrobrachium lar and Geothelphusa
dehaani were recorded. The geological formation of the Mishima Islands was discussed, with notes on
the biology and distribution patterns of their freshwater crustacean faunas. It is revealed that the three
Islands were connected to the Chinese mainland about 1.0 million years ago, wherein only Takeshima
had been submerged by sea-level rising that exterminated its G. dehaani population.
Key words: crustacean fauna, formation process, Geothelphusa dehaani, Mishima Island, sea-level rising

Introduction
The composition of crustacean fauna inhabiting the inland waters of islands was noted tocoincide with geological history, life history of organisms, and so on. In the Ryukyu Islands located between the Kyushu Island and Taiwan, S HOKITA (1979, 1996) discussed the formation of
the freshwater crustacean faunas, particularly the origin of the Japanese freshwater crabs, bycomparing the composition and geographical distribution of the crustacean fauna inhabiting theinland waters of the Islands. In addition, he speculated that the ancestral species of the freshwater
crustaceans, especially the landlocked and true freshwater species, already existed on the Ryukyu
Islands about 1.0 Ma (million years ago), in stead of later immigration via the last land bridge
linking the Ryukyus with the Chinese mainland.
Following S HOKITA (1996), this study compares the freshwater crustacean fauna of theMishima Islands (Takeshima, Ioujima and Kuroshima) and infers their geological formation,
which remains obscure in the light of geological studies.
The author expresses his sincere thanks to Captain K. Yoshinaga and the crew of the training vessel Nansei Maru, Kagoshima University.

Material and Methods
A biological survey was conducted in the inland waters of the Mishima Islands (Fig. 1) on
September 27-29, 1999. From the brackish water to the freshwater areas on each island, decapod
crustaceans were collected manually or with a scoop net. Sampling was done during daytime
only. Specimens were fixed with 10% formalin and preserved in 70% ethyl alcohol for future
study.

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Hiroshi SUZUKI

Fig. 1. Map showing the sample sites (solid circles) in Takeshima and Kuroshima, and their locations.

Results and Discussion
Among the three islands, Ioujima lacked a river system hence, no freshwater crustaceans
were found in the island. However, three species belonging to three families (Table 1) were collected from Takeshima and Kuroshima. Among them, Caridina typus (Fig. 2) andMacrobrachium lar (Fig. 3) are diadromous crustaceans, which inhabit both freshwater andseawater areas at certain stages of their life cycle. Both species are common in the area along
where the Kuroshio Current flows. The distribution of the two species may indicate that the crustacean faunas of Takeshima and Kuroshima are strongly effected by the Current, wherein it
served both as transport and barrier mechanisms for interisland migrations.

A Comment on the Geological Formation of the Mishima Islands (Takeshima, Ioujima and Kuroshima)as Inferred from Their Freshwater Crustacean Faunas

139

Table 1. List of Inland Water Crustacean Decapods Captured in Takeshima and Kuroshima
Family and Species

Takeshima

Kuroshima

Life stylepresent

present

Diadromous

Macrobrachium lar

present-----

Diadromous

Potamidae
Geothelphusa dehaani-----

present

Freshwater

Atyidae
Caridina typus
Palaemonidae

Fig. 2. Dorsal view of Caridina typus collected inTakeshima.

Fig. 3. Dorsal view of Macrobrachium larcollected in Takeshima.

Fig. 4. Dorsal view of Geothelphusa dehaani collected in Kuroshima.

Geothelphusa dehaani (Fig. 4) is a true freshwater crab, which was found in Kuroshima but
not in Takeshima. Since G. dehaani also inhabits the adjacent islands, such as Kuchierabujima
and Yakushima (SUZUKI & T SUDA, 1991; SUZUKI & S ATO, 1994), it suggests that this crab
might have occurred in Takeshima about 1.0 Ma when the island was connected to the Chinesemainland.
KIMURA (1996) noted that the Ryukyu Islands nearly connected to the Chinese mainland,through Taiwan as a land bridge, during two development periods approximately during 1.6

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Hiroshi SUZUKIFig. 5. Distant view of Takeshima, Ioujima, and Kuroshima. Left; Takeshima (left side) andIoujima (right side), Right; Kuroshima.

to1.0 Ma and 0.2 to 0.02 Ma. He mentioned that there was a deep channel at the Kerama andTokara gaps. Based on the depths around the Mishima Islands, there is no possibility thatTakeshima and Kuroshima were connected to the Kyushu Island about 0.2 to 0.02 Ma and, inthat period, G. dehaani immigrated into the two islands. The maximum altitudes of Takeshima,Kuroshima (Fig. 5), Kuchierabujima, and Yakushima are 194 m, 622 m, 657 m and 1935 m, respectively. Takeshima and Kuroshima were connected to the Chinese mainland about 1.0 Ma,
but thereafter only Takeshima was submerged by sea-level rising and was never connected withKyushu.
Thus, it is inferred that the population of G. dehaani in Takeshima was possibly exterminated by submergence since 1.0 Ma, whereas that of Kuroshima has been isolated from other island populations for a long time, probably more than one million years. The G. dehaanipopulation in Kuroshima is being studied with regards to speciation.

References
SHOKITA, S. 1979. Bull. Coll. Sci., Univ. the Ryukyus，28： 193-278 ．
------ 1996. J. Geogr.，105： 343-353 ．
K IMURA, M. 1996，J. Geogr.，105 ：259-285 ．
SUZUKI, H. & TSUDA, E. 1991. Proc. Biol. Soc. Wash., 107: 318-324.
鈴木廣志・佐藤正典 1994．かごしま自然ガイド 淡水産のエビとカニ．137ページ．西日本 新 聞 社刊 ， 福 岡 ．［ SUZUKI, H. & SATO, M. 1994. Kagoshima Nature Guide,Freshwater Shrimps, Crayfish, and Crabs. 137 pp. Nishinihon Shinbunsha, Fukuoka.］