Latitudinal Difference in the Species Richness of Photosymbiotic Ascidians Along the East Coast of Taiwan.

Some didemnid ascidians harbor cyanobacterial symbionts, and this is the only obligate photosymbiosis system known in chordates. These photosymbiotic ascidians are found only in tropical and subtropical waters, probably because the photosymbionts are vulnerable to low temperatures. We surveyed the photosymbiotic ascidian fauna along the east coast of Taiwan. The present and previous reports recorded 13 species in Taiwan, and the species richness and composition is different in five areas along the east coast. Along the middle-east, southeast, and south coasts, five or more species have been recorded, whereas only one species has been found along the northeast coast, and no species have been recorded on the north coast. This gap in the species richness is probably related to the Kuroshio Current, which travels from south to north along the east coast of Taiwan but changes to an easterly direction off the northeast coast. Increases in water temperature due to global warming could cause northward expansion of the distribution ranges of these ascidians in the future. Hence, the photosymbiotic ascidian fauna could be an environmental indicator in subtropical, shallow water, and the present study provides a basic dataset that illustrates the current status of photosymbiotic ascidians in Taiwan.