June 26, 2009

Rising CO2 Levels Lead To Bigger Fish Ears

For years scientists have observed the deleterious effects of rising levels of carbon dioxide in the oceans on shellfish and corals.  Now, a new study by the Scripps Institution of Oceanography has provided evidence that the physiological development of fish ears is also impacted by the gas.

In the June 26 edition of the journal Science, researchers from the San Diego-based institute published a short paper outlining the results of experiments in which young white seabass were continuously exposed to high levels of CO2.  The most dramatic change observed was the aberrant enlargement of the fish's ear bone, or otolith, which plays a critical role in helping the animals to sense their surroundings and swim upright.

The results were particularly surprising to the researchers, whose initial hypothesis had predicted the shrinking of the otolith in response to elevated carbon dioxide levels.  In general, there was no increase in the overall size of the fish, only in the relative size of their otolith.

"At this point one doesn't know what the effects are in terms of anything damaging to the behavior or the survival of the fish with larger otoliths," explained lead author of the study, David Checkley.

"The assumption is that anything that departs significantly from normality is an abnormality and abnormalities at least have the potential for having deleterious effects."

As carbon dioxide levels around the planet are on the rise, ostensibly due to human activities, one of the many effects has been an increased acidification of the world's oceans. 

Environmentalists and oceanographers alike have watched with horror as falling pH levels in the oceans have brought on the massive erosion of coral reefs and a dramatic shrinking of plankton populations.

"An important observation is that the effect of CO2 in atmosphere, and therefore in the ocean, includes not only [global] warming and making the ocean more acidic. There are other effects on the biology and ecology that merit study," explained Checkley in a telephone interview.

The Scripps group says that they intend to follow-up their study with expanded research aimed at determining whether other species fish besides the white seabass are similarly affected and whether the larger otoliths will have a negative function on the fish's chances of survival, while also attempting to shed light on the physiological mechanism leading to abnormal otolith growth.

Checkley spoke of the particularly critical significance of determining potential effects of changes in the otolith size on survival rates of fish.

"If fish can do just fine or better with larger otoliths then there's no great concern. But fish have evolved to have their bodies the way they are. The assumption is that if you tweak them in a certain way it's going to change the dynamics of how the otolith helps the fish stay upright, navigate and survive."

Checkley explained that there is some evidence indicating that enlarged otoliths may cause fish to be more lethargic, but he emphasized that more research is needed before any definitive claims can be made.

Otoliths are not only associated with orientation and acceleration, but also help reveal physical characteristics of fish. As the tiny bones grow in a manner similar to the rings of a tree, scientists are able to determine the age of fish by counting the concentric layers of growth.


Image 1: This is a side view of an otolith imaged with a scanning electron microscope. The top is smooth (oriented downward) and the bottom is pitted. The holes are approximately 1-2 microns in diameter. Credit: Scripps Institution of Oceanography, UC San Diego

Image 2: These are fertilized eggs of white seabass, each containing an embryo with an attached yolk sac and oil globule (droplet). Credit: Hubbs SeaWorld Research Institute


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