"Otoliths are key sensory structures in fish," Dave Checkley of Scripps told environmentalresearchweb. "Humans with malfunctioning otoconia – analogues of otoliths in our semicircular canals – experience vertigo. Fish larvae live in a fluid environment and thus their ability to sense orientation and acceleration is key to their survival."

At present, it's unclear how or whether a larger otolith structure will affect the fish, although it is known that possessing asymmetric otoliths causes problems. The structures consist of layers of protein and aragonite, a form of calcium carbonate.

"We wish to investigate whether there is an effect of larger otoliths on fish behaviour, growth, and survival," said Checkley. "Our premise is that fish have evolved with otoliths of a certain size, shape and mass, and that any alteration of otoliths due to enhanced carbon dioxide may alter their sensory use by fish."

Seven- to eight-day-old white sea bass that were grown under 993 and 2558 µatm of carbon dioxide had otoliths 7–9% and 15–17% larger, respectively, than fish grown under 380 µatm. The presence of more carbon dioxide in the atmosphere alters seawater chemistry, increasing the concentration of hydrogen and bicarbonate ions and decreasing the concentration of carbonate ions.

"Among marine vertebrates, fish eggs and larvae may be particularly susceptible to changes in ocean chemistry due to their small size and thus exposure to the environment," said Checkley. "Otoliths in juvenile and adult fish are shielded from variations in the environment by a circulatory system with blood. Gases such as carbon dioxide move within eggs and larvae largely by diffusion."

Checkley believes it's important to understand the mechanism by which elevated carbon dioxide affects marine organisms and ecosystems, rather than just measuring its net effect. "To understand, and hence predict, the effects of elevated carbon dioxide on net calcification requires elucidation of its effects on both formation and dissolution," he said.

Now, as well as examining the effect of larger otoliths on fish, the team plans to research whether other species of fish and fish in other environments, such as at higher latitudes in cooler water, respond similarly. The researchers would also like to investigate the mechanism behind the effect. "We hope to perform experiments in which we measure the chemistry in the otic vesicle containing the otoliths under a range of conditions of seawater carbon dioxide," said Checkley.

The researchers reported their work in Science.