The effects of reduced seawater pH on the early-life history of three key echinoderm (Echinodermata) species.

Abstract

Ocean acidification, the reduction in ocean pH as a result of the uptake of anthropogenic atmospheric carbon dioxide (CO2) by surface waters, has emerged over recent years as a foremost area of research in environmental physiology, molecular biology, ecology and marine science. Due to the evident sensitivity of the early life history stages of marine organisms, it is important to recognize the effects of ocean acidification during embryo and larval development. These stages are considerably important to recruitment, and are able to directly influence population dynamics and can impose negative latent effects later in an organism’s life history. Subsequently, the deleterious effects of ocean acidification are likely to have profound broad-scale impacts on marine ecosystems. This is the first study to investigate the effects of ocean acidification on the physiological responses of the early life history stages of three key echinoderm species, two temperate echinoids (Evechinus chloroticus and Pseudechinus huttoni) and one polar asteroid (Odontaster validus). The effects of experimentally reduced seawater pH levels on embryo and larval morphometrics and the activity of the important ion-transporter Na+/K+-ATPase was evaluated. Furthermore, the effects on aerobic respiration and the genetic expression of the α-subunit of Na+/K+-ATPase were additionally assessed in E. chloroticus. The CO2-adjusted pH levels considered for this investigation includes ambient (pH 8.1) and levels predicted for 2100 (pH 7.8) and 2250 (pH 7.6). Morphology of the early life history stages of E. chloroticus and P. huttoni are negatively impacted by reduced seawater pH/hypercapnia, with larvae being significantly smaller, and developing heterogeneously. Conversely, the development of the polar O. validus appeared robust to the effects of near-future ocean acidification which may be attributed to pre-adaptation. There were no significant differences in the activity of Na+/K+-ATPase for all three species. Moreover, the mRNA transcript abundance for Na+/K+-ATPase decreased significantly early in development (fertilisation to two days) in E. chloroticus cultivated at pH 7.8 and pH 7.6. However, this trend was not sustained, such that, later in development (four- six days) there was an increase in Na+/K+-ATPase expression when qPCR data was normalized using copy number over wet weight. There was a significant increase in larval aerobic respiration in E. chloroticus cultivated in pH 7.6. Based on the impacts on the physiology and genetics in the two temperate echinoids (E. chloroticus and P. huttoni) and the polar asteroid (O. validus), chronic exposure to elevated pCO2 and reduced pH could result in marked physiological changes in larvae during their pelagic stage. Nonetheless, the physiological mechanisms responding to ocean acidification are species specific, and may be attributed to taxonomic differences, such as evolutionary history, physiology and genetic make-up. The implications for these key life-history stages are likely to be substantial in terms of the distribution, abundance and population dynamics for the three echinoderm species. Further research is necessary in order to evaluate the physiological responses of marine invertebrates to multiple climate stressors. Moreover, in order to assess ecosystem wide impacts, additional investigations that involve the quantification of responses of complex ecosystems processes are required.