Even animals without hands can display handedness--or, at least, a preference to do things with one side of the body rather than the other. Animals ranging from primates to birds to invertebrates have been shown to favor their left or right side. Fish might reveal that preference by choosing to swim right, for example, when avoiding a predator. Don't get too charmed by the idea of left-handed and right-handed fish, though: In a warming world, they may disappear.
A new study by researchers in Italy and Australia looked at young coral reef fish, Neopomacentrus azysron, which often display a preference for the right or left side. (Humans are unusual in that almost all of us prefer the right side. In other animals, individuals might be biased toward one side or the other, but the population as a whole tends to be evenly distributed.) To measure the reef fishes' preference, the researchers put them in a very elementary maze: It was shaped like a T, and fish had to swim either right or left at the end.
As some hard-working fish wranglers put 70 baby reef fish through the maze 10 times each, the researchers recorded the preference of each individual fish to turn right or left. They also performed a random simulation of fish going through the maze, to see what the results would have been like if fish were choosing their direction arbitrarily. There was a significant difference between real fish and simulated fish: This meant the baby reef fish were displaying a true preference for the right or left side, or "lateralization."
Another group of baby reef fish were kept for four days in water with an elevated level of carbon dioxide. As CO2 in the atmosphere increases, causing global warming, more of it is also absorbed by the oceans. The carbon dioxide level in the water that housed these baby fish, the authors say, is a level our oceans are predicted to reach between the years 2050 and 2100.
After being kept in the high-CO2 water, these reef fish also took their turn in the T-shaped maze. Unlike the fish raised in normal water, though, high-CO2 fish showed no preference for turning right or left. Their results were indistinguishable from the random simulation.
The increased CO2 seems to have affected the neural development of these animals. But what does it matter if climate change wipes out right- or left-handed fish? Preferring one side over the other, in fish or in humans, isn't just a biological fluke. The authors say reef fish may choose the right or left side because one of their eyes is stronger than the other. Having a bias toward one side allows fish to make a quick decision when avoiding a predator: In a previous study, fish from areas with high predation were found to have stronger lateralization, suggesting that preferring one side or the other is an evolutionary advantage.
It's not just about turning left or right. Lateralization in animals' behaviors reflects the underlying asymmetry in our brains. It's thought that dividing tasks between the two sides of the brain makes us more efficient, like a computer using parallel processing. Fish with a right or left preference have been found to perform better in various cognitive and physical challenges.
In a warming world--and an acidifying ocean--will ecosystems fall apart as fish and other animals lose the skills evolution has selected? A study published last year found that clownfish raised in elevated CO2 levels chose to swim toward the smell of a predator, rather than away. Their sense of smell seems to have been damaged by living in acidified water. Or maybe they were suicidal because they couldn't decide which fin they preferred to write with. Either way, the future is looking stormy for ocean ecosystems.
Domenici, P., Allan, B., McCormick, M., & Munday, P. (2011). Elevated carbon dioxide affects behavioural lateralization in a coral reef fish Biology Letters DOI: 10.1098/rsbl.2011.0591
