Effects of ocean acidification on a calcifying marine animal, the abalone Haliotis rubra x laevigata

Carbon dioxide (CO2) is produced by industrial and other anthropogenic activities. Almost one half of anthropogenically produced CO2 has been taken up by the ocean, with associated decline in sea water pH and this is expected to continue in the future. This might have a detrimental impacts on animals with calcium carbonate skeletons or shells. This research study demonstrates that ocean acidification markedly alters the shell growth of abalone, which are important commercial shellfish, broadly distributed around the world. In the first experiment, juvenile abalone (Haliotis rubra x laevigata) were exposed to predicted future conditions of elevated temperature and lowered pH. The experimental design included two temperatures and two levels of pH in an orthogonal design, where the control represented present day conditions. In experimental treatments, sea water pH was reduced to 7.6 by injecting CO2, and the temperature was increased to 4 °C above ambient water, to reflect environmental conditions predicted for 2100. Growth rate was measured by comparing initial to final shell lengths after 100 days. At the ambient temperature, shell growth ceased and the shells shrunk by 1.43 % in the low pH treatment. Abalone in this treatment ceased growth, and shell erosion was evident in their size and colour. At the high temperature, there was no significant effect between the low and high pH treatments. Fouling tubeworms had also been lost from the shells. The cross-sections of abalone shells close to the growing edge were examined with SEM. The thickness of various of shell layers on the shell were measured using ImageJ software on the SEM images. Although the nacre thickness did not show a significant difference in the treatments, the prismatic thickness were significantly affected by the lower pH and the higher temperature while, interestingly, the combination of these two factors did not show a significant effect. We found a structure not described previously, which was a triangular shape in the prismatic layer. The new structure we called "Zagros", height, and its distribution in the shell were affected by the stresses introduced to the abalone which were lower pH and/or higher temperature. However, the nacre thickness of shells exposed to both these factors combined, was not significantly different from that of the controls. The last measurement shell structural component was the ratio of prismatic-to-nacre layer thickness. Temperature did not affect the ratio of prismatic to nacre thickness, but pH and its interaction with temperature were both statistically significant. At the ambient temperature, the low pH caused a 31% reduction in the layer ratio, whereas at the higher temperature there was no difference between the pH treatments. All, the data suggest a pattern in which decreasing pH will cause a dissolution of the shell, while the higher temperature will lead to faster growth in abalone shell. Since our studied species is cultured in high amount for food source for human population, it is extremely important to study the effect of ocean acidification accompanied by heavy metal (Cu) pollution on this species and investigate the level of heavy metal intake in abalone tissue. The second experiment was designed to answer the question. In the second experiment, juvenile abalone were exposed to the environmental conditions of elevated temperature and lowered pH predicted for 2100, together with a common marine toxicant (i.e. copper) to evaluate the effects of this additional stressor on abalone subject to these future ocean conditions. Abalone were subjected to the addition of the 40μg/L of copper (Cu) exposure for 48 hours, and then transferred to clean sweater in the relevant pH and temperature regimes and sampled after two weeks. The experimental design included two temperatures and two levels of pH, and either the presence or absence of Cu exposure in an orthogonal design, where the control represented present-day conditions in terms of temperature and pH and contained no Cu. The elemental composition of abalone shells and gills were measured using Inductively Coupled Plasma Mass Spectrometry (ICP-MS) (Agilent Technologies, Model 4500 series 300). The proportions of Cu, Ca, Na, Mg, Zn and K were calculated from the total known mass of the abalone shells and gills and the dilution factor. In abalone shells Mg was the only element showing significant difference with the intraction of all stressors of pH and temperature and Cu when analysed by analysis of variance P= 0.040. In abalone gills Mg and Na concentration each had a significant interaction of pH and temperature (P=0.021, 0.030 respectively). None of the other studied elements showed significance for any factors or interactions. However, analysing the data by pairwise Tukey comparisons on the shell and gill data did not show any significant differences among any pairs of treatments. The results demonstrate that lower pH increased dissolution of CO2 and higher temperatures as predicted with in future oceans will affect the growth of wild abalone. The results also have implications for the maintenance of ideal culturing conditions in the farming industry under a changing climate