Asian Scientist (Jul. 12, 2013) – An international team of marine scientists has discovered that baby fish, no more than a few millimeters in length, avoid getting lost and eaten in the vast ocean and navigate their way home using a ‘sun compass’.
When baby fish hatch from eggs on a coral reef they are swept away by currents into the open ocean. To seek safety, these tiny creatures have to find their way back to their home reef, or another reef nearby.
“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 Professor Mike Kingsford, a member of the team.
“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.”
In their study, published in PLOS One, the scientists tested their theory using a small plastic swimming pool and baby cardinal fish at One Tree Island on Australia’s Great Barrier Reef.
Almost immediately after being released the fry turned and headed in a south southeasterly (SSE) direction – and kept on heading that way even when the scientists turned their pool. 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.
When the scientists ‘clock shifted’ the little fish six hours back in time, they were fooled by the position of the sun and began automatically to swim in the opposite direction. Clock shifting involves putting the fish in a dark room and using artificial lights to reset their body clocks to a time six hours earlier.
According to the researchers, this shows that the fish have an internal clock, known as zeitgeber, that they use as part of a time-compensated sun compass to maintain their SSE heading.
“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,” said Prof Kingsford.
“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.”
Because the time-compensation required for a sun compass needs to be learned (the exact movement of the sun varies greatly with season and latitude), the scientists believe it is likely that this learning takes place soon after the fish larvae drift off their home reef.
However, certain birds and sea turtles are born with an inbuilt sense of direction, so it is also possible that baby cardinal fish can inherit the instinct to head SSE – towards home.
The article can be found at: Mouritsen et al. (2013) Sun Compass Orientation Helps Coral Reef Fish Larvae Return To Their Natal Reef.
Source: ARC Centre of Excellence in Coral Reef Studies; Photo: babasteve/Flickr.
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