Otolith size and the vestibulo-ocular reflex of larvae of white seabass Atractoscion nobilis at high pCO2

We investigated vestibular function and otolith size (OS) in larvae of white seabass Atractoscion nobilis exposed to high partial pressure of CO2 (pCO2). The context for our study is the increasing concentration of CO2 in seawater that is causing ocean acidification (OA). The utricular otoliths are aragonitic structures in the inner ear of fish that act to detect orientation and acceleration. Stimulation of the utricular otoliths during head movement results in a behavioral response called the vestibulo-ocular reflex (VOR). The VOR is a compensatory eye rotation that serves to maintain a stable image during movement. VOR is characterized by gain (ratio of eye amplitude to head amplitude) and phase shift (temporal synchrony). We hypothesized that elevated pCO2 would increase OS and affect the VOR. We found that the sagittae and lapilli of young larvae reared at 2500 µatm pCO2 (treatment) were 14 to 20% and 37 to 39% larger in area, respectively, than those of larvae reared at 400 µatm pCO2 (control). The mean gain of treatment larvae (0.39 +/- 0.05, n = 28) was not statistically different from that of control larvae (0.30 +/- 0.03, n = 20), although there was a tendency for treatment larvae to have a larger gain. Phase shift was unchanged. Our lack of detection of a significant effect of elevated pCO2 on the VOR may be a result of the low turbulence conditions of the experiments, large natural variation in otolith size, calibration of the VOR or mechanism of acid?base regulation of white seabass larvae.