Background
Bignami et al. write that "the days- to month-long pelagic larval period is an ecologically vital ontogenetic phase in marine fishes because it constitutes the primary mode of dispersal in many species (Cowen and Sponaugle, 2009) and represents the life stage most susceptible to mortality (Houde, 1997)," and they thus go on to say that "during this phase, the sensory abilities of larval fishes are important determinants of survival (Montgomery et al., 2006) and ultimately influence the persistence of viable populations."

What was done
The five researchers, as they describe it, "used new 3D microcomputed tomography to conduct in situ analysis of the impact of ocean acidification on otolith (ear stone) size and density of larval cobia (Rachycentron canadum), a large, economically important pantropical fish species that shares many life history traits with a diversity of high-value, tropical pelagic fishes."

What was learned
At an atmospheric partial pressure of 2100 ppm CO2, there was a significant increase in otolith size (up to 49% greater volume and 58% greater relative mass), as well as a 6% increase in otolith density, while the estimated relative mass of larval cobia otoliths in an end-of-century 800 ppm CO2 treatment was 14% greater.

What it means
Armed with these experimental observations, Bignami et al. go on to demonstrate that "these changes could affect auditory sensitivity including a ~50% increase in hearing range at 2100 ppm CO2." And they say that "this is a potentially optimistic result, indicating some resistance to acidification and suggesting that under near-future scenarios these impacts may be most relevant in habitats already experiencing high pCO2 levels."

References
Cowen, R. and Sponaugle, S. 2009. Larval dispersal and marine population connectivity. Annual Review of Marine Science 1: 433-466.

Houde, E. 1997. Patterns and trends in larval-stage growth and mortality of teleost fish. Journal of Fish Biology 51 (Supplement A): 52-83.

Montgomery, J.C., Jeffs, A., Simpson, S.D., Meekan, M. and Tindle, C. 2006. Sound as an orientation cue for the pelagic larvae of reef fishes and decapod crustaceans. Advances in Marine Biology 51: 143-196.