Invasive Alien Species Triops (Branchiopoda, Notostraca) in Japan and Its Ecological and Economic Impact

The generally-accepted tadpole shrimp (notostracan) classification of Longhurst does not include information on the world richest Triops collection in Budapest, Hungary. Besides, his “Triops granarius” (an ‘Asian’ species in the broadest sense), which is ranging from Africa through Eurasia and even to Japan, has been regarded as a single cosmopolitan species with wide morphological variations. However, the morphological and molecular analyses conducted by me revealed that it was a hotchpotch of multiple species. If a regional endemic species has been misidentified as a broadly-distributed one (to sum up, this is the most problematic matter of Longhurst’s classification), then any zoogeographical results based on this would be far from the nature; and as such I opted to verify its species definition. Invasive alien species of tadpole shrimp Triops were first introduced into rice fields of Japan in the 1910s and have spread around Japan. The expansion of Triops has resulted in negative ecological and potential economic impact. Regarding their establishment or extinction patterns, I built a simple mathematical competition model among two self-fertile and one related sexual Triops species of Japan, incorporating reproductive interference. Intense reproductive interference drove the competition to sexual exclusion of self-fertile species. Nonetheless, even under intense reproductive interference, the Allee effect (which reduces the population growth rate of sexual species with low density) allowed self-fertile species to survive when the competition started with the lower densities; conversely sexual species to be left alive when the competition started with the higher densities.