Friend and foe? How crabs avoid getting eaten

Nov 28, 2011

Despite their simple compound eyes crabs have evolved a smart way to tell the difference between friend and foe, new scientific research has revealed.

Scientists from The Vision Centre have found that fiddler quickly learn to recognise if an approaching creature is a threat, a mate or or a harmless passer-by - according to its direction of approach.

“Fiddler crabs have extremely poor sight, with no depth perception and no ability to see in detail,” says Ms Chloe Raderschall, a researcher from The Vision Centre and The Australian National University. “In a situation where every ‘blob’ that moves in the environment can be a threat, they have to strike a balance between succumbing to paranoia – and ending up as bird feed.

“Crabs achieve this through a process called habituation where they learn from repeated events to differentiate threats from harmless objects. Humans too use habituation: for instance we learn to ignore the sound from an air conditioner once we grow accustomed to it.

‘We found that crabs have a very selective and finely tuned habituation response – instead of relying solely on the physical appearance of an object, they associate the object with its past behaviour in their living environment, such as its direction of approach.”

In the study, the researchers used dummy predators to approach groups of fiddler crabs from two different compass directions.

“We did two dozen runs of a dummy approaching from direction A without attacking the crabs, and within five runs, the crabs started to ignore it,” Ms Raderschall explains. “When we switched to another dummy coming from direction B, the crabs were scared witless and headed straight to their burrows.”

When the researchers switched back to direction A, they found the crabs did not attempt to escape, indicating that they clearly distinguish between the dummies approaching from the two directions, she says.

“As both dummies were identical and there was no difference in the timing of their movements, we conclude that the crabs used the direction of approach to determine whether an approaching object was a threat or not.”

Ms Raderschall explains that this finding confirms that crabs have an extremely specific habituation response. This contradicts previous assumptions found in most text books that habituation is a simple learning mechanism based mostly on physical appearances.

“Their identification of a dangerous or harmless object is closely associated with their memory of how the object behaves, rather than how it looks.

“Apart from very simple visual cues, they don’t really have other ways to detect predators, and this study provides clues as to how animals with relatively poor vision can adapt and survive over time.”

The paper ‘Habituation under natural conditions: model predators are distinguished by approach direction’ by Chloe A. Raderschall, Robert D. Magrath and Jan M. Hemmi was published on 23 November 2011 in The Journal of Experimental Biology.

Explore further: Telling the time of day by color

Related Stories

Blurry-eyed beachcombers beat birds

Apr 14, 2011

(PhysOrg.com) -- A study from The Australian National University has revealed for the first time how an animal sees and responds to predatory attacks in its natural environment.

Danger lurking below the sand

Aug 01, 2011

A voracious predator that devours prey larger than itself has been found lurking beneath Queensland's golden sandy beaches.

Size doesn't matter to fighting fiddler crabs

Dec 19, 2007

A person’s home may be their castle and in the world of the fiddler crabs having the home advantage makes it a near certainty that you’ll win a battle against an intruder – regardless of your opponent’s size.

Chinese takeaway in the Wadden Sea

Sep 25, 2007

Shore crabs catch their food at food-rich spots and subsequently eat it elsewhere. With this takeaway strategy the crabs maximize their food uptake and keep competing crabs at a distance, says Dutch researcher Isabel Smallegange.

Recommended for you

Telling the time of day by color

21 hours ago

Research by scientists at The University of Manchester has revealed that the colour of light has a major impact on how the brain clock measures time of day and on how the animals' physiology and behavior adjust accordingly. ...

Aphrodisiac for fish and frogs discovered

Apr 17, 2015

A supplement simply added to water has been shown to boost reproduction in nematodes (roundworms), molluscs, fish and frogs – and researchers believe it could work for humans too.

Evolution puts checks on virgin births

Apr 17, 2015

It seems unnatural that a species could survive without having sex. Yet over the ages, evolution has endowed females of certain species of amphibians, reptiles and fish with the ability to clone themselves, ...

Humans can't resist those puppy-dog eyes

Apr 16, 2015

When humans and their four-legged, furry best friends look into one another's eyes, there is biological evidence that their bond strengthens, researchers report.

User comments : 0

Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.