Introduction
Climate changes represent a potential limiting factor to aquaculture, directly interfering with survival and behavior of bivalves. Severe fluctuations in temperature along with periods of heavy rainfall or periods of drought that significantly change the salinity1, can promote physiological stress of bivalves, resulting in changes in physiological and behavioral responses and, in extreme cases, leading to high mortalities2,3.
European clam (Ruditapes decussatus) represents one of the most relevant emergent bivalve species from the aquaculture sector in Europe with high economic value. Considering the predicted climate changes and given the economic and ecological importance of R. decussatus, the current study was designed to evaluate the effects of different combinations of salinities (0-40) and temperatures (5-35°C) on mortality and feeding behaviour of juveniles and adults of R. decussatus.
Material and methods
Juveniles and adults of clams R. decussatus, captured in Ria Formosa lagoon, were exposed to combined ranges of temperature and salinity. Considering the annual temperature and salinity variations in this coastal area over the last years, it was established a control temperature range between 15°C and 23°C, inclusive, and salinity 35.
Three and two trials of 120 h were performed for adults and juveniles, respectively.
Mortality and feeding behaviour were evaluated and registered every 24 hours. Individuals were considered dead every time they were unable to close their valves after mechanical stimulus4, being then removed from the beakers. Feeding behaviour was evaluated through the absence or presence of faeces, scored as 0 or 1, respectively. Median lethal time (Lt50) and median lethal temperature (LC50) were calculated.
Results
Low salinities and high temperatures caused higher mortality rates, in both juveniles and adults, after 120 hours of exposure. In the control temperature range (15°C - 23°C), juveniles revealed to be more sensitive to the salinity variations. As salinity increased, juveniles’ mortality decreased. In adults, the mortality rate reached 60% of the individuals at salinities lower than 15. Above the upper limit of the control temperature range (23°C) and salinities inferior to 15, the observed mortality was almost always extremely high, for both size classes. In more saline waters, juveniles seemed to be more resistant than adults, mainly at extreme temperatures (32°C and 35°C), where occurred 100% of mortality of the adult population. For salinities equal or below 15, LC50 varied between 20.28°C and 22.29°C for juveniles and adults, respectively, for salinities higher than 15, LC50 was set at 31.08°C for juveniles and 30.08°C for adults. In temperatures higher than 23°C and salinities less or equal to 15, Lt50 substantially decreased as temperature increased, with juveniles more sensitive to higher temperatures (32°C and 35°C). At higher salinities, juveniles seemed to be more tolerant than adults, however, the mortality of 50% of adults’ population would never be expected to occur below 30 hours of exposure.
Feeding behavior seemed to be highly influenced by salinity, regardless the temperature, for both juveniles and adults. At salinities lower than 15, it appeared that individuals did not feed. From this point and at control temperature range, the feeding behavior of juveniles and adults was maximum. Out of this range, juveniles were less active at lower temperatures and more active at extreme temperatures. On the other hand, adults showed the opposite pattern. Mortality revealed a positive correlation with temperature while salinity had a strong positive influence on feeding behaviour.
Discussion
Our results suggested salinity 15 as “turning point” below occurred a change in mortality and feeding behavior, which is in accordance with previous studies for venerid species 5,6. Salinity fluctuations have a direct impact on feeding behavior, while temperature has a major impact on survival of clams. Low salinities and high temperatures are lethal to R. decussatus. Juveniles are less resistant to the increase of temperature in less saline environments, however, more resistant than adults to extremely high temperatures under more saline conditions.
Considering the predicted climate changes scenarios, juveniles will die faster when exposed to low salinities and high temperatures, consequences of heavy rainfall and heatwaves, never reaching the adult stage, thus compromising reproduction and consequently, bivalve recruitment. The failure in bivalve recruitment will have a strong negative impact on the economic sector due to the decrease in the available biomass for harvesting.
Acknowledgements
This work was supported by the projects CERES – Climate change and European aquatic RESources, funded by European Union’s Horizon 2020 research and innovation program (grant agreement no. 678193), AIM (MAR – 02.01.01 – FEAMP – 0060) and SNMB-Sul-IV (MAR – 02.01.02 – FEAMP – 0221) funded by PO MAR2020. Ph.D. grant (SFRH/BD/147215/2019) was granted to A.R. by the Portuguese Foundation for Science and Technology (FCT).
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