July 8, 2013
Juvenile Reef Fish Use The Sun Compass To Guide Them To Safety
Brett Smith for redOrbit.com - Your Universe Online
When reef fish hatch from their eggs, they are immediately swept out to sea -- sometimes miles from the safety of their coral homes. Once they are able to swim, these fish must begin the arduous trek back or risk getting devoured in the open waters.
The tiny fishes' uncanny ability to locate their home reef has captivated scientists and a new study in the journal PLoS ONE revealed these fish use the sun's position in the sky to navigate their way to safety, also referred to as a sun compass.
"Failure to get back to a reef spells death for baby fish, and we've known for some time that they use their senses of hearing and smell to locate the reef and head back to it," said coauthor Mike Kingsford, a marine biology professor at Australia's James Cook University. "The fact that we've shown they also have a sun compass in their tiny heads and can orient themselves according to the sun's position through the day provides the missing link in their navigational toolkit."
Using a small plastic swimming pool and baby cardinal fishes from Australia's Great Barrier Reef, the researchers watched as the fish immediately began swimming in the pool in a south-southeasterly direction -- even when the pool was turned.
"The currents that sweep the baby fish off the reef generally set in a north-northwesterly direction, so to get back to it the fish have to swim SSE," Kingsford said. "The big question was: how did they know where that point of the compass lay, and keep to it?
"Though smaller than a good many insects, baby fish are surprisingly strong swimmers and they can push up against the current for several days, covering distances of [12 miles] or even more," he added. "The mystery was how they maintained a correct orientation during this life-or-death journey."
To see if it was indeed the sun guiding the fish home, the researchers used a dark room and artificial lighting to send "the sun" six hours back in time, fooling the fish into swimming the opposite direction -- to the NNW.
"Since they are swept too far from the home reef to smell or hear it, this provides strong evidence they steer mainly by the sun, making compensatory allowances as it moves across the sky," Kingsford said. "This is a complicated task which quite a few humans would struggle to perform -- but which baby coral reef fish seem to accomplish with few difficulties."
The team also looked at the fishes' ability to navigate on a cloudy day, and found overcast weather did indeed hamper their directional capabilities. The marine biologists also found the fish had the most difficulty around noon, when the sun was directly overhead.
"The tests ... demonstrate that the fish have an internal clock ("zeitgeber") that they use as part of a time-compensated sun compass to maintain their SSE heading," the authors wrote. "Since the time-compensation required for a sun compass needs to be learned [due to seasonal changes], it is likely that this learning takes place during the early dispersal phase."
Some species of birds and turtles have also shown the ability to navigate via the sun, the researchers noted.