Effects of Ocean Acidification on Otolith Growth of the Mummichog

By the year 2100, it is projected that atmospheric CO₂ concentrations will rise to approximately 1000 ppm under a business-as-usual model. As a result, oceanic pH levels are expected to drop 0.5 units, causing ocean acidification. Although the process of ocean acidification is well documented, the implications it has on marine organisms and their ability to adapt are not well known.  Scientists are becoming alarmed at the possible corrosive effects of ocean acidification on marine ecosystems and especially on calcifying, shell-forming organisms. Most research to date has focused on calcifiers, such as coccolithophorids, corals and mollusks, while very little has been dedicated to fish.   Thus far, three studies have examined the link between an increasingly acidic ocean and otolith growth in larval fish. Contrary to expectations that otoliths would show a decline in growth, similar to other calcifying organisms, Checkley et al. (2009) demonstrated that otoliths of larval white sea bass exhibited a growth in otolith size with increasingly acidic environments.  However, Munday et al. (2011) found no increased growth in otolith size in larval damselfish under similar conditions.  To better understand the consequences of ocean acidification, we are determining the impact of elevated pCO₂ levels on the estuarine species, the mummichog (Fundulus heteroclitus), which incubates its eggs in open air in contrast to other species studied.  We are growing mummichog larvae under three pCO₂ concentrations: the present atmospheric level of 400 ppm; the year 2100 projected level of 1000 ppm, and 2500 ppm.   If a change in otolith size is observed in either of the elevated pCO₂ treatments, auditory behavioral studies may be conducted to test the effects of varying otolith size in fish.