Abstract
Seasonal changes in gametogenesis, spawning behaviour and morphometric parameters of Macoma balthica and Mytilus trossulus from two depth zones in the Gulf of Gdańsk (Southern Baltic Sea) were investigated. The studies allowed the authors to make observations of the ecophysiological features of both species and revealed major differences in the weight index between deep and shallow water subpopulations. Both condition and gametogenesis probably depend mainly on the availability of nutrients and on temperature changes. The data showed that Mytilus trossulus spawned earlier at the shallow water station than at the deeper one. No differences were observed between the run of the Baltic clam's reproductive cycle at 10 m and at 75 m depths.