Topsy-Turvy world
High CO2 level confuses fish
Reef-dwellers such as this Australian damselfish may find their brain chemistry confused by dropping pH levels in the ocean.
A new study may explain how rising carbon dioxide concentrations and the ocean acidification they induce can cause topsy-turvy changes in the behavior of fish. Like a flipped switch, the normal response of nerve cells can reverse as acidifying seawater perturbs how a fish regulates acids and bases in its body, including the brain. "This could be a big deal," says neurobiologist Andrew Dittman of the National Oceanic and Atmospheric Administration's Northwest Fisheries Science Center in Seattle. Dittman, who was not affiliated with the study, says the new findings could go a long way toward explaining curious sensory changes observed in fish exposed to acidifying waters. The scary scent of predators, for example, can suddenly become alluring. For the new study, published online January 15 in Nature Climate Change, Göran Nilsson of the University of Oslo and his colleagues homed in on brain chemistry. The idea emerged after Philip Munday of James Cook University in Townsville, Australia, told Nilsson of behavioral quirks his laboratory fish were exhibiting in a highcarbon dioxide environment conditions exemplifying ocean waters a half-century or more from now. Nilsson, a neurophysiologist, speculated that a connection between nerves and chemistry might be involved. "It was very much an 'Aha' moment," Munday says. Once excited, nerve cells need a chemical to calm them down. A compound known as GABA does this by unlocking a gate on the cells' outer membrane, allowing ions of chloride and bicarbonate to enter and quiet the cell. Nilsson speculated that when a fish's body attempts to maintain chemical balance in the face of rising ocean acidification, chloride and bicarbonate concentrations could become higher inside nerve cells than outside them. Later, when GABA gates opened, chloride and bicarbonate would then rush out of the cell instead of into it. That would excite the cell instead of calming it, essentially reversing the nerve's response to a stimulus.
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