Population size structure, spatial distribution, and life cycle of the hydromedusa Aglantha digitale (O.F. Müller, 1766) in the White Sea

Seasonal variations of the abundance, size structure, and vertical distribution of the population of the hydromedusa Aglantha digitale are studied using the materials collected during multiyear observations in the White Sea from 1961 to 2003. The wide distribution of the species in the Basin of the White Sea and in Kandalaksha, Dvina, and Onega bays is shown. Usually, the maximum bell height of the specimens did not exceed 12 mm, although a few individuals up to 22 mm high were found. The maximum abundance of A. digitale is observed during the reproductive period, which lasts from the second half of June to August. The reproduction starts when the water temperature near the surface reaches 9–11°C. By the onset of winter, specimens with a height of 4–5 mm representing a new generation dominate. By the beginning of the subsequent spring, their average size reaches 5–6 mm, and, by the end of June, specimens with bell heights of 8 mm become mature. During the spring and the summer, the A. digitale population concentrates in the upper 0-to 25-m water layer; during the autumn and winter, it concentrates below the 50-m depth. At the end of the autumn and in the winter, the population is spread over the water column and inhabits all the water layers. At the end of the winter, in April, the ascent of the population starts. An analysis of the seasonal dynamics of the population abundance and size structure indicates that A. digitale in the White Sea has a one-year life cycle.