What will happen to calcifying organisms as anthropogenic CO2 is absorbed by the oceans? Will they survive? And how will organisms with internal calcified structures, such as fish and cephalopods, cope with these environmental changes?
Many studies on marine calcifiers, such as corals, mussels and echinoderms (Fabry et al. 2008), have shown depressed calcification rates at increased CO2 levels and experiments with marine fish larvae have shown toxic effects of CO2 on certain stages during the development with higher mortality in CO2 treatments (Kikkawa et al. 2003, 2004).
Recent studies have shown exceptions to expectations (see EUROCEANS Research Highlight on ‘coccolithophore cells grow larger under high CO2 conditions’ and counter-argument by Riebesell et al., 2008). In the latest issue of Science (26 June 2009), Checkley and colleagues demonstrate a similar example from a higher trophic level. Young fish will not only be able to maintain, but even enhance calcification under ocean acidification scenarios. Checkley et al. show increased otolith growth in sea bass larvae under elevated CO2 conditions. Whether enlarged otoliths in fish larvae affect their performance and survival remains to be determined. However, these results suggest that fish, even in their most vulnerable egg and larval stage, might not be greatly affected by ocean acidification, due to their good acid-base regulating abilities.
Similar to Checkley et al.’s findings, a study on juvenile cuttlefish by Gutowska et al. (2008) found that animals reared at the highest CO2 levels (6000ppm) incorporated significantly more CaCO3 into their cuttlebone compared to the control animals, with no obvious behavior changes. While this might not be a benefit in the long term, the animals do seem to be functionally normal.
Own investigations on larval squid statoliths, calcified structures analog to fish otoliths, have shown counter results to those of Checkley et al. Preliminary results indicate that elevated CO2 levels (1400ppm) lead to malformations in larval statoliths, while highest CO2 levels (4000ppm) inhibit the formation of statoliths completely.
Role of Otoliths
Because these structures are laid down pre-hatch when the acid-base regulatory ability in the larvae are not yet completely functional, this stage is most vulnerable to its environment and the larvae will not be able to compensate calcification as they mature. Structural abnormalities, asymmetries and most of all absence of statoliths result in extreme behavioral responses. Statoliths (like otoliths in fish) are used for orientation by giving clues to gravity and angular acceleration via attaching hair cells. Abnormally formed or missing statoliths will not be able to provide the necessary stimuli to these hair cells and therefore impair the animals’ ability to perform basal tasks such as swimming and food capture, thus proving lethal to the animals.
These are just first steps to understanding the effects of high CO2 levels on internal calcification in marine organisms and much more research in this direction is needed. The mechanisms of how calcification is altered in the endolymph need to be investigated to understand how otolith formation is affected by a rise in CO2. To achieve this, the saturation state of aragonite in the endolymph, along with the acid-base status needs to be determined. Furthermore, the protein matrix surrounding the otolith should be examined, as it is thought to control the shape of the otolith.
Last but not least, the long-term effects of otolith calcification on growth, behaviour and survival needs to be examined in detail.
Review for EUROCEAN Research Highlights by Andrea FROMMEL, IFM-GEOMAR AF is EPOCA (European Project on Ocean Acidification)-funded postgraduate student examining the effects of changes in ocean pH on the development, growth and physiology of marine fish eggs, larval fish and cephalopods.
Checkley DM, Dickson AG, Motomitsu T, Radich JA, Eisenkolb N, Asch R. (2009) Elevated CO2 enhances otolith growth in young fish. Science (26 June 2009)
Fabry VJ, Seibel BA, Feely RA, Orr J (2008) Impacts of ocean acidification on marine fauna and ecosystem processes. ICES J Mar Sci 65: 414-432
Gutowska MA, Poertner HO, Melzner F (2008) Growth and calcification in the cephalopod Sepia officinals under elevated seawater pCO2. Mar Ecol Prog Ser 373:303-309
Kikkawa T, Ishimatsu A, Kita J (2003) Acute CO2 tolerance during the early developmental stages of four marine teleosts. Environ Toxicol 18: 375-382
Kikkawa T, Kita J, Ishimatsu A (2003) Comparison of the lethal effect of CO2 and acidification on red sea bream (Pagrus major) during the early developmental stages. Parine Pollution Bulletin 48: 108-110