Evolution and adaptation of the eels, genus Anguilla

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We made expeditions to a total of 12 countries for 540 days during 1995-1998 fiscal years and obtained the following results. 1. Adaptive significance : (1) Physiological characteristics : Immunohistochemical analysis showed two types of chloride cells in the gill lamella and gill fillament of Anguilla japonica. The former was activated in freshwater habitat and the latter, in the marine habitat following the downstream migration to the sea. (2) Migratory behavior : The survey on the inshore migration of tropical glass eels during 1995-1998 revealed that three species (A.celebesensis, A.marmorata and A.bicolor pacifica) occurred at the mouth of Poigar River, Sulawesi Island, Indonesia. The former two migrated almost throughout the year with the peak season in June and the last occurred in Jan., Mar., Apr., Oct. and Dec.. Analyses on otolith microstructure and Sr/Ca ratio suggested that tropical eels metaborphose at 90-160 days after hatching while teperate species, A.australis and A.di … More effenbachi collected in New Zealand, metamorphosed at older ages, 204 and 248 days, respectively. Age at recruitment to freshwater was also older in temperate eels (268 and 297 days in mean) than those in tropical eels (116-195 days). These differences in early life history of eels might be an adaptation which was caused on the evolutional process from local migaration in tropical eels to long distance migration in temperate species. 2. Evolition : (1) Phylogenetic analysis : Phylogenetic analysis on mtDNA data set suggested that Anguilla originated in tropical area in the Cretaceous and that the ancestor of A.anguilla and A.rostrata would dispersed from the Indo-Pacific into the Atlantic Ocean through the Tethys Sea at latest 35 Ma ago. (2) Parasitological analysis : Two new species of Pseudodactylogyrus, monogenean parasites, were found on the gills of the two Australian eels, A.australis and A.reinhardti. They appear to be endemic to Australia, distinct morphologi- cally and genetically from their nearest relatives infecting the Japanese eel, indicating that Pseudodactylogyrus has coevolved with eels during the isolation of A.japonica in eastern Asia from the two species in Australia. Less