ALTERNATIVE LIFE-HISTORY STYLES OF JAPANESE FRESH-WATER SCULPINS REVISITED

Japanese freshwater sculpins consist of 6 Cottus and 1 Trachidermus species, with various types of life-cycles, namely, catadromous, amphidromous, lacustrine land-locked and fluvial land-locked. Among them, C. amblystomopsis and C. nozawae have been regarded as sibling species. Because of very similar morphological and ecological adult characteristics they were formerly classified as a single species. C. amblystomopsis mainly inhabits the lower courses of rivers and produces many small eggs from which pelagic larvae are formed. In contrast, C. nozawae lives in the middle or upper courses of rivers and deposits few, large eggs, from which well-developed benthic young emerge, well on the way to the definitive phenotype as fully formed juveniles. These two species have distinctly different life-cycles: amphidromous for C. amblystomopsis and fluvial for C. nozawae. A comparison of the early ontogeny shows that, among Cottus species, the small-egg, amphidromous C. amblystomopsis is altricial, while the large-egg, fluvial C. nozawae is precocial. Assuming that C. nozawae has been derived phylogenetically from C. amblystomopsis or its ancestral relatives, it is reasonable to consider that the fluvial land-locked life-history style of C. nozawae has evolved from the ancestral arnphidromous one through adapting to the upstream habitat by an increase in egg size and consequent truncation of the larval period. © 1990 Kluwer Academic Publishers.