The Effect of Desalination on the Growth, Coenosarc Pulsations, and Hydroplasm Movement in the Colonial Hydroid Dynamena pumila (L., 1758)

The article describes the assessment of the primary (i.e., occurring before the onset of adaptation) reaction of coenosarc pulsations in the colonial hydroid Dynamena pumila (L., 1758) to decreased salinity and the associated changes in the growth and functioning of the distribution system. Time-lapse video microscopy has been used to register coenosarc pulsations of D. pumila stolons and stems in seawater with a salinity of 26‰ (control) and in seawater serially diluted to 20, 15, and 10‰. Video microscopy of colony coenosarc pulsations made it possible to monitor the colony’s reaction to salinity changes within approximately 2 h. D. pumila colonies did not show any distress in a salinity range of 26 to 20‰ at a temperature of 14°C, and the growth increment of stolons and stems, both in 1 h and in one pulsation, was the greatest under these conditions. The increment per pulsation by decreased 1.5 times at 15‰. Signs of stress, such as the cessation of stolon growth and a significant deceleration in stem growth, were observed at a salinity of 10‰. The period of growth pulsations of stem tips and stolons was doubled at 10‰. The reactions of the growth pulsation amplitude of the stolons and stems at 10‰ were different: the amplitude did not change in the stems but decreased by approximately ten times in the stolons. The resting phase of lateral pulsations and hydroplasm flows in both stems and stolons increased with decreasing salinity, which may be indicative of a decrease in propulsatory activity. Dissimilar changes in the rate of hydroplasm movement in the coenosarc module closest to the growth tip occurred in the stems and stolons: it decreased slightly in the stolons and significantly in the shoots.