How Crabs Avoid Getting Eaten

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 crabs quickly learn to recognize if an approaching creature is a threat, a mate 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 behavior 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.

The study was conducted in the Research School of Biology at The Australian National University.

The Vision Centre is funded by the Australian Research Council as the ARC Centre of Excellence in Vision Science.

Source: redOrbit (http://s.tt/14ewu)
http://www.redorbit.com/news/science/1112429177/how-crabs-avoid-getting-eaten/index.html

Fiddler crabs keep cool with giant claws

Male fiddler crabs use their giant claws to keep cool as well as attract females, say scientists.

The crabs' oversized limbs are waved to signal strength to potential mates.

US researchers discovered that crabs with an intact major claw also cool faster than those without, showing that the claws help regulate temperature.

Scientists suggest this previously unknown benefit could help offset the energy cost of growing such a large sexual ornament.

There are about 100 species of fiddler crab found in tropical regions around the world.

Males are recognisable by their assymetrical claws, whereas females' claws are equally sized.

To attract the attention of potential mates, males perform elaborate claw-waving displays.

Dr Zachary Darnell and Assistant Professor Pablo Munguia from the University of Texas, US investigated whether the giant claws had a function beyond attracting mates.

Their study focussing on the Gulf coast fiddler crab (Uca Panacea) was recently published in the American Naturalist journal.

In the experiment, researchers shone lamps on crabs with an intact major claw and on crabs that were missing theirs.

They measured the crabs' body temperature every 10 minutes and found that the crabs lacking the large claw took much longer to cool down.

"The major claw likely functions like a heat sink, with heat being transferred from the body to the claw and dissipated into the surrounding air through convective heat transfer," said Dr Darnell.

Although the giant limbs effectively attract the attention of females, they had previously been considered a physical burden.

"The large claw is metabolically costly, it hinders feeding because it is cumbersome for this task, and it reduces endurance capacity when crawling on the sand," Dr Darnell explained.

"Male crabs are both heat-stressed and hungry while on the surface, foraging and performing the claw-waving display."

But Dr Darnell says the team's results are the first evidence to show how the enlarged structure benefits both reproduction and survival.

"With the large claw acting as a heat sink male fiddler crabs can remain on the surface longer, foraging and performing the waving display," he told BBC Nature.

Dr Darnell suggests that the dual function of fiddler crab claws could provide insight for other species.

"A thermoregulatory function of the major claw should change the way scientists think about this structure, and reconsider the costs and benefits associated with the claw and other [sexual] traits."

Ella Davies
Reporter, BBC Nature
http://www.bbc.co.uk/nature/14631028

Baby fiddler crabs seek shelter in seashells

Baby fiddler crabs "move in" to empty snail shells for shelter, scientists have found.


Researchers made this discovery whilst studying salt marsh ecosystems on Tybee Island in Georgia, US.

The finding shows how "resourceful" these primitive crustaceans are, they say, as unlike hermit crabs, fiddlers do not carry seashells on their backs as a permanent home.

It also reveals how crabs can benefit when snails are eaten by predators.

Professor Sophie George from Georgia Southern University in Statesboro, who led the study, explained that fiddler crabs are important in the establishment and maintenance of the salt marsh ecosystem.

In the Journal of Experimental Marine Biology and Ecology, her team explained that the crabs were a source of food for many animals.

"They are eaten by resident and migratory birds, by [other] crabs and by shrimp and fishes," wrote the researchers.


"Small mammals and reptiles feed on fiddler crabs as they move across the salt."

The tiny baby crabs - with a soft shell less than 3mm wide - are particularly vulnerable.

"Settlement of the first fiddler crab stage during high tide exposes them to a variety of habitats but also to predators," said Professor George.

On top of this, the juvenile crabs are too small to dig their own burrows.

Previous studies had found that some animals used shells for shelter.

So to find out if the baby fiddler crabs might also hide in the empty snail shells, the researchers visited the island 20 times to collect the shells.

Out of every 100 empty shells, they found that 79 were occupied by the tiny crabs.


Professor George said: "An abundance of Littoraria irrorata [snail] shells [provide] a welcome refuge for juvenile fiddler crabs."

By Victoria Gill

Science and nature reporter, BBC News

http://news.bbc.co.uk/earth/hi/earth_news/newsid_9166000/9166605.stm