Abstract
Populations of Sphaeroma hookeri from Copenhagen Harbour (Sydhavn) and S. rugicauda from Selsø (about 40 km west of Copenhagen) were studied during the summer of 1969. Tests were carried out at 5, 15, and 25°C, at salinities from 0 to 60%; a greater percentage of adults of S. hookeri survived longer at lower salinities (0, 2, 5%) at the three temperatures; at these lower salinities females of S. hookeri produced more young which survived longer. The depressing effect of low salinities on reproduction in S. rugicauda is considered to be a significant factor limiting the distribution and abundance of this species in the Baltic.

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