The effects of ocean acidification on juvenile Haliotis iris

The world’s oceans are increasing in levels of anthropogenic CO2 resulting in reductions in seawater pH and the availability of carbonate ions which are essential to calcifying marine species. Ocean acidification is considered to be a potential threat to marine populations through changes to survival, growth and calcification. Very little information exists on the effects of reduced seawater pH on abalone molluscs, particularly the New Zealand black-foot abalone Haliotis iris. This thesis aimed to investigate the effects of reduced seawater pH, through elevating the partial pressure of carbon dioxide, on the growth, biomineralisation and respiration of juvenile H. iris. To assess the impacts of acidification, post-settlement (initial length 4-10 mm) and 30–40 mm juvenile H. iris were exposed to long-term (two separate 100 day experiments) pH levels of ambient pH, pH 7.8 and pH 7.6 across autumn/winter (6.4-11.6oC) and spring/summer (13.0-19.5oC) temperatures, and measured; survival, growth, and shell deposition, repair, microstructure, mineralogy and also metabolism in the form of oxygen consumption. The experiments were conducted using a flow-through design, and each pH treatment had three replicate 16 L aquaria containing equal numbers of abalone.

Juvenile survival appeared to be unaffected by acidification, however, other sub-lethal changes were found. After 100 days of experimental exposure, the relative growth rate of shell length was 9%, and wet weight was 50% lower for post-settlement juveniles reared at pH 7.6 in comparison to ambient pH during the autumn/winter. Larger 30-40 mm juveniles exposed to ambient pH had a 1.2% greater RGRSL than pH 7.6 juveniles. During the spring/summer growth trial, growth differences among treatments were more pronounced with post-settlement ambient pH juveniles growing 160% and 1500% in shell length and wet weight respectively, in comparison to only 99% and 800% gains in post-settlement pH 7.6 juveniles. Growth was also significantly greater for 30-40 mm H. iris grown in ambient pH, although differences among treatments were not as pronounced as post-settlement growth. The pH 7.8 treatment also significantly reduced growth in both size-classes, but not as substantially as the pH 7.6 treatment.

Shell deposition and mineralogy were significantly reduced by declines in seawater pH and the sensitivity to acidification was found to decrease with increasing body size. X-ray diffraction results found that ambient pH-reared post-settlement juveniles had a 15% higher calcite weight % in their shell in comparison to juveniles reared at pH 7.6. Scanning electron micrographs and XRD of juvenile shells indicated substantial dissolution of the outer calcitic layer of post-settlement juvenile shells following prolonged low pH exposure, however, larger juveniles were generally unaffected. A shell repair experiment showed that shell damage decreases growth of juvenile H. iris, and when coupled with reduced seawater pH the effects were additive, and in addition scanning electron micrographs indicated shell repair was inhibited by low pH conditions.

No changes to soft-tissue growth or oxygen consumption were found leading to the proposal that juvenile H. iris suffer a growth delay and do not upregulate metabolism under long-term pH stress. The results suggest there is a shift in the energy budget of juvenile H. iris with resources invested into survival and somatic mass, however, juveniles do not upregulate metabolism to maintain normal growth or to combat shell dissolution pressure by low pH seawater. Overall, ocean acidification has the potential to cause detrimental effects to the juvenile stages of abalone, creating concerns for future aquaculture and ecosystem management.

Cunningham S. C., 2013. The effects of ocean acidification on juvenile Haliotis iris. MSc Thesis, University of Otago, 160 p.. Thesis.

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