Saphoerster, Julia (2008) The physiology of the blue mussel (Mytilus edulis) in relation to ocean acidification. (Diploma thesis), Christian-Albrechts-Universität, Kiel, 96 pp.

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Abstract

Anthropogenic CO2 emissions have already altered ocean chemistry, leading to lower carbonate saturation states, elevated seawater pCO2 and decreased pH. This scenario will even be enhanced in the future due to rising atmospheric CO2 levels. Recent studies suggested drastic impacts of increased seawater pCO2 values especially on the ecophysiological performance of calcifying organisms. In this study, whole-animal performance of the blue mussel, Mytilus edulis, from the Baltic Sea was related to rising pCO2 levels in order to predict physiological changes on the organism level in a future high CO2 world. Thus, cardiac performance and activity patterns were observed in adult specimens exposed to different pCO2 values (380 ppm [39 Pa; pH 8.0], 560 ppm [57 Pa; pH 7.9], 840 ppm [85 Pa; pH 7.8], 1120 ppm [114 Pa; pH 7.7], 1400 ppm [142 Pa; pH 7.6] and 4000 ppm [405 Pa; pH 7.1]) for two weeks. The results revealed that heart rates and heart rate variability, as well as activity patterns were not significantly affected by CO2 levels applied in this study. The mean instantaneous heart rate (IHR) was determined using infrared light sensors (plethysmographs). Throughout the experiment, IHR fluctuated around a mean value of 12 beats per minute (BPM). There was also no change in the distribution of beat-to-beat intervals (heart rate variability, HRV). This study examined changes in the HRV by comparing different parameters, including parameters so far only used in humans, composed of mean, median, mode, range, skewness, kurtosis, coefficient of variation (CV), standard deviation of beat-to-beat intervals (SDNN), standard deviation of differences between adjacent beat-to-beat intervals (SDdeltaNN) and root mean square successive differences (RMSSD). Valve and siphon activity was investigated using time lapse images recorded with a set of webcams. The valve and siphon diameter in percentage of their maximum opening per mussel varied around 57 % (Valve) and 46 % (Siphon). On average valve and siphon were open about 50 % of the time even when exposed to CO2. Since the food was supplied at a continuous rate, cleared algal cells could be measured using a coulter counter. While mussels from all treatment groups <1400 ppm were able to clear a similar percentage of algae cells in their experimental aquaria (49 %), there was a drastic decline in filtration rate in the 4000 ppm treatment, where the mussels were able to remove only 3 % of the supplied algae cells. Field measurements carried out at the sampling site during the trial period revealed high pCO2 levels of up to 2345 ppm (238 Pa) and low pH values of even 7.4 at some days probably due to upwelling of undersaturated bottom water. Thus, animals are already confronted with extraordinarily high CO2 levels in their natural habitat. As they are probably well adapted to a fluctuating carbonate system, this might partly explain why CO2 levels in this study were not sufficient to trigger a response in cardiac performance and activity patterns. But on the longer term, inhibition of feeding at very high CO2 levels will probably lead to a concomitant decrease in heart rates and valve/siphon activity. Due to its low buffering capacity and observed corrosive upwelling events, critical pCO2 may be reached in the Baltic Sea in the future, if atmospheric carbon dioxide concentrations continue to rise as anticipated.

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