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Experimental studies revealed that a freshwater ectoparasitic flagellate Ichthyobodo necator (Henneguy, 1883) could survive and reproduce in seawater after infected chum salmon fry, Oncorhynchus keta (Walbaum), were transferred directly from fresh water to 33% seawater. Minor morphological changes (slight reduction in body width, loss of twistlike wrinkles on body surface, and reduction in contractile vacuoles) were observed in the attached form of I. necator following transfer to seawater. The field survey also confirmed that I. necator occurs on chum salmon fry in seawater estuaries (salinity 17-34%) and in freshwater habitats. It was assumed that I. necator acquired salinity tolerance as a result of adapting to the migratory behavior of its anadromous host. Two morphologically similar bodonids, I. necator from chum salmon and Ichthyobodo sp. from marine Japanese flounder, Paralichthys olivaceus (Temminck and Schlegel), were differentiated by cross-infection experiments. Thus, the parasite from marine flounder should be regarded as a separate species from I. necator.

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SynopsisDuring gestation, live-bearing fishes incur physiological energy costs, including provision of energy and respiratory gases to the developing embryos and removal of waste products. Fecundity in the genus Sebastes is high, and the ovaries represent a significant portion of the weight of gestating females. In this study, we compare oxygen consumption of gestating females with non-gestating females and males of kurosoi, Sebastes schlegeli, to estimate these costs. Oxygen consumption by pregnant females is significantly higher than that of males and immature females at similar sizes and weights. We estimate that a 1.5 kg gestating female consumes 68% more oxygen than a non-gestating fish during the 51.5-day period of gestation. Such an increase in oxygen consumption rates may have important implications to the metabolic scope of gestating females. The additional oxygen consumed by gestating females is greater than that predicted for the embryos alone, suggesting that costs of increased gill ventilation, ionic and osmotic regulation and cardiac output are relatively high. Such energetic costs represent a quantifiable expense of the viviparous mode of reproduction in Sebastes as compared with oviparous species.