The combination of low salinity and low temperature can limit the colonisation success of the non-native bivalve Rangia cuneata in brackish Baltic waters

We studied the behavioural (burrowing and valves gaping) and metabolic responses of Atlantic rangia Rangia cuneata (G.B. Sowerby I, 1832) at different salinities and temperatures. Three salinities (S = 0.5, 7.0, 10.0) were tested at 20 °C, with an additional control of 2.4. The control salinity (S = 2.4) was also tested at 10 °C to examine responses specific to colder temperatures. Clams exposed to the lowest (S = 0.5, T = 20 °C) and the highest salinity (S = 10.0, T = 20 °C), as well as lower temperature (T = 10 °C, S = 2.4) significantly (p < .05) reduced the rate of routine metabolism. At the lowest salinity (S = 0.5, T = 20 °C) and temperature (T = 10 °C, S = 2.4), they were also more abundant on the surface of the sediment, however the differences in relation to control conditions (S = 2.4, T = 20 °C) were not significant (p > .05). Clams kept their shells open in the lowest salinity, but closed in the highest salinity and at lower temperature. The closure time was longer in salinity of 10.0 than at temperature of 10 °C, where the highest reduction in the metabolic rate occurred in relation to the control conditions. This latter fact, in combination with shell closure, may indicate the activation of an anaerobic pathway. Thus, in the long term, lower temperatures may adversely affect R. cuneata, especially at very low salinity (S = 0.5, 2.4), in which this hyper-osmoregulating species has to strongly pump water with ions and oxygen to compensate for energy demand. In salinity of 10.0, where the species is isosmotic to the environment, changes in behaviour (moving to the sediment surface and closing the shell) with a simultaneous reduction in the metabolic rate may be due to lower costs of osmoregulation. Our results may help explain the strong inter-annual fluctuations in the population size of this subtropical species in the Baltic coastal waters resulting from the mass mortality of adults after long winters.