2010 Volume 71 Issue 3 Pages 241-254
The relationship between the water temperature regime and life history variations was studied in a pontogeneiid gammaridean, Sternomoera japonica (Tattersall, 1922), in the streams of a spring-fed interconnected lake system (Koikuchi-no-ike water system in the Tsugaru-Juniko Lakes) of an intermountain deciduous forest in northern Japan, where the seasonal variation of water temperature was distinctly amplified downstream. S. japonica reproduced around the year in two springs and in streams located in the upper course of the water system where the water temperature was stable at around 10°C throughout the year. On the other hand, no reproduction occurred during summer periods in streams of the middle course, nor was any distribution found in the two lowermost streams. A temperature-survival experiment demonstrated that S. japonica could not survive for long under a temperature higher than 25 °C, and the distribution and seasonal variations in water temperature regimes supported those results. In the two successive midstreams, precopulating formations commenced synchronously with a water temperature that fell below around 12 °C in late autumn. This finding suggested that the distribution and life history patterns of S. japonica were regulated by two kinds of temperatures, each of which affected survival and reproduction, indicating that S. japonica is probably distributed in locations where the temperature was no higher than 25 °C and fell below 12 °C during its life history. Among such temperature regimes, reproduction might be interrupted during periods when the water temperature rose higher than 12 °C. Seasonal changes in the population structure suggest that S. japonica reproduces only once in its life cycle, and could reproduce reaching its smallest size among three Sternomoera species in Japan.
