The sound of a starfish 'seeing'
The sounds of science
- MP3 (00:00:05; 91 Kb)
Crown of thorns starfish don't just sniff out coral — they look for it too.
That thankfully short-lived tone is the sound of a crown of thorns starfish seeing a bright light. It's not squealing from the glare, that's the electrical signal generated by photoreceptors in one of its eyes.
Eyes? Yep. Starfish have got eyes — one on the tip of every arm. Like compound eyes in insects they're light sensitive, but don't have any lenses.
Apparently scientists have known about the eyes for a couple of hundred years. But it was only very recently discovered that they can use their eyes to find their way around.
"Everybody thought starfish only used smell for orientation", says researcher Dr Ronald Petie from the University of Copenhagen.
"But now we know this is not true for the crown of thorns starfish and one more species," says Petie.
Normally the crown of thorns starfish walk straight towards a reef, but with their eyes removed, Petie found they walked in random directions.
"This shows that vision and not smell is the cue they use for detecting reefs at short distance," he says.
To find out what colour light the starfish were most sensitive to, he used microelectrodes to measure the electrical response of neurons in the eye to different colours of light. (It's that signal converted to an audible tone that you can hear above).
Not surprisingly for a reef-based creature, the crown of thorns was most sensitive to blue light, so the coral boulders they feed on would appear as a dark patch on a bright background.
Petie is currently on Lizard Island testing the animals' response to colour in the wild.
"I've done experiments where I attracted animals to a black sheet in the middle of a sand flat and got animals to walk away from a reef when I covered it with a white sheet".
Before this work, and earlier work on the blue star by his colleague Anders Garm, it was thought that starfish vision was limited to phototaxis — allowing them to move towards or away from light.
But the new findings show that vision is at least as important as smell for navigation on reefs.
"We need to update our text books on this one!"
Petie is hoping to eventually construct a bait to concentrate and capture the crown of thorns starfish, which are devastating coral reefs.
But for that to be effective more needs to be known about their sense of smell, which is what his collaborator at the Australian Institute of Marine Science in Townsville is working on at the moment.