Schertenleib, Katrin S. H. (2018) Effects of acute temperature stress on the feeding of the subtropical marine grazer Arbacia lixula (Linnaeus, 1758). (Master thesis), Universität Rostock, Rostock, Germany, 97 pp.

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Abstract

Ocean warming and more intense, more frequently occurring marine heat waves may alter marine community structures and ecosystem functions, since water temperature directly influences metabolism in marine ectotherms. How physiological temperature-driven changes in high-impact benthic species such as sea urchins affect population dynamics, interaction strengths and community structure is not yet well understood. I investigated the effect of water temperature on consumption rates and the gonad index of the sea urchin Arbacia lixula on Madeira Island, to estimate the consequences of ocean warming for this population’s feeding pressure and reproduction. The test animals were acclimated for five days to laboratory conditions at the ambient water temperature of the sampling site, and then for up to ten days to different target temperatures. During temperature acclimation, temperature was increased by 1°C per day successively in the different treatment levels, so that treatment groups reached target temperature on the same day. After exposing the individuals to their respective target temperatures for four days, I conducted a two-day feeding assay and calculated each animal’s consumption rate by standardising the amount of food consumed according to the sea urchin’s dry weight and per day. Pellets made from agar-agar and Ulva rigida powder were provided as food to the individuals. After ten days at target temperature, gonads were extracted and gonad indices calculated as biomass percentages. The experiment was split into two parts, which began on 21.6.2017, and 11.8.2017, respectively. In the first part, temperatures of 21.5°C, 22.9°C, 24.9°C, 27.2°C, 29.3°C and 31.1°C were tested, in the second part temperatures of 22.8°C, 23.7°C, 26.3°C, 27.2° and 28.3°C were applied. At 31.1°C, no replicate survived until the end of the experiment, and mean time to death in this temperature level was significantly different from the other temperature levels tested. An ANCOVA revealed that, in addition to temperature, consumption rates in A. lixula were influenced by the time when the experiment was conducted, an individual’s diameter, and its sex. Sea urchins consumed significantly more pellet material in the second part of the experiment than in the first, and females of almost all body sizes generally fed more than males. Smaller females always consumed more food per unit of biomass than larger female individuals. Male A. lixula followed this pattern only at higher temperatures >23.7°C, while at lower temperatures consumption rates increased with size. The highest median consumption rate occurred at 23.7°C. The gonad indices were significantly higher in the second part of the experiment than in the first, and the maximum median gonad index was found at 22.8°C. With ocean warming as predicted by the IPCC (2014), it can be expected that A. lixula will feed at maximum consumption rates for longer periods in the year than currently. Furthermore, it is likely that the population will increase its reproductive output, and thus ill grow in density.

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