In Asia, the distribution of horseshoe crabs has been recorded from the area which is surrounded by 4 lines drawn from the western part of Japan to West Bengal (India), from West Bengal to Java (Indonesia), from Java to Torres Strait, and from Torres Strait to the western part of Japan. However, no precise information about the distribution of each of the three Asian species, Tachypleus tridentatus, T. gigas and Carcinoscorpius rotundicauda, has been available. We made a preliminary field survey in 1979 to clarify the distribution of the Asian species. The following localities were found to yield horseshoe crabs in this survey, together with previous surveys made in 1975-1978 and with some reliable informations. T. tridentatus: Western part of Japan, Taiwan, southwestern part of the Philippines, and Kota Kinabalu of North Borneo (Malaysia). T. gigas: West Bengal (India), Phuket and Bangsaen (Thailand), Penang (Malaysia), Singapore, northeastern part of Sumatra and southern part of Kalimantan (Indonesia), and Kuching and Kota Kinabalu (Malaysia). C. rotundicauda: West Bengal (India), Phuket and Bangsaen (Thailand), Penang (Malaysia), Singapore, northeastern part of Sumatra and southern part of Kalimantan (Indonesia), Kuching and Kota Kinabalu (Malaysia), and southwestern part of the Philippines. From these results, it was confirmed that (1) all the three species are found sympatrically in the northwestern coast of Borneo (Kota Kinabalu), (2) two species, T. gigas and C. rotundicauda, are found along the southern coast of Borneo, the northern coast of Sumatra, the coasts of both sides of Malay Peninsula, and that of West Bengal of India, and (3) another combined distribution of two species (T. tridentatus and C. rotundicauda) was found along the southwestern coast of the Philippines and Palawan Islands. We think these evidences would be an important clue to explain the origin of these three Asian species of horseshoe crabs.
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