Recent decreases in population size of some Sympetrum species (Odonata: Libellulidae) that used to flourish in paddy fields in Satoyama, Japan, are thought to be caused by the development of rice cropping systems. As habitat use is species-specific, some conservation studies have shown that the causes of the decline in darter populations may also be species-specific. While eggs and larvae of lentic species like Sympetrum frequens decrease owing to the effect of pesticides, those of S. pedemontanum elatum, which lives along weakly flowing water, are not influenced by these chemicals, although they have decreased with the modernization of water management in paddy fields. It is also known that drying of the soil surface in no-till farming areas does not reduce the population size of S. infuscatum, whose eggs have a higher resistance to drought than those of other darter species. For the conservation of darters, we should investigate habitat use during the developmental stages of each species, identify the causes of population decline, and maintain a suitable balance within the microhabitats required by each darter species. Such designed habitats will contribute to the conservation of not only darters, but also many other aquatic organisms endangered in the paddy fields of Satoyama.
Invasive alien bivalve species of the genus Corbicula, originally from Asia, were first introduced into North America in the 1920s and have spread around the world, including South America and Europe. In the 1980s, these species also entered Japan. The worldwide expansion of Corbicula species has resulted in negative ecological and economic impacts. To raise public awareness about this issue, I have written two review papers addressing questions such as "What are the scientific names and origins of Corbicula species?", "How and why have they been successfully introduced into new habitats?", "Why have they expanded their distribution areas?", "How have they influenced native ecosystems and local economies?", and "What types of measures should we take against the Corbicula issues?" The present paper is the first review, and it describes the taxonomic problems of Corbicula species and summarizes the information currently available on phylogenetic systematics using morphological and molecular data, which indicate sources, dispersion pathways, and introduction and dispersion mechanisms. Recent results of the phylogenetic systematics can be summarized as follows: 1) species and lineages cannot be identified based solely on morphology; 2) alien Corbicula species are composed of a few freshwater lineages, characterized by hermaphroditic and androgenetic reproduction; 3) considering the very subtle differences in shell morphology and the low genetic distance between the Japanese native species C. leana and the invasive C. fluminea, C. leana is estimated to be closely related to C. fluminea or a lineage within C. fluminea. The origin, introduction routes, and dispersion pathways of alien Corbicula species are being partly revealed by analyzing habitat preference, morphology, karyotype, sperm morphology, mode of reproduction, and genetic mitochondrial and nuclear DNA markers.
Invasive alien species of the genus Corbicula (Bivalvia), originally from Asia, were introduced and spread over North and South America, Europe, and Japan during the 20th century. Dense populations established in the introduced areas have resulted in negative impacts on aquatic ecosystems and local economies. To raise public awareness about this issue, I have written two review papers on the alien Corbicula species: the first summarizes previously published data on taxonomy, native ranges, introduction routes, and dispersion pathways; the present (second) paper summarizes data on biological characteristics, including mode of reproduction, feeding behaviour, physiological tolerance to abiotic changes, impacts of established Corbicula populations on ecosystems and economies, and the response of native ecosystems to the introduced Corbicula species. Future measures and research tasks to resolve the problems associated with Corbicula are also noted. The successful invasive behaviour of Corbicula species can be explained by their reproduction/life-history traits (hermaphroditism, unreduced biflagellate sperm, androgenesis, ovoviviparity, r-strategy with rapid individual growth, early maturity, high fecundity, and high dispersal ability), plasticity in feeding behaviours (suspension- and deposit-feeding), and high filtration rates rather than by their physiological tolerance to abiotic changes (salinity, temperature, oxygen, etc.).