“Walking nose” crabs smell predator pee

Mud crabs sense danger by detecting chemicals in the urine of another crab species of crab that preys on them. 

Mud crabs (Scylla serrata) are small crustaceans native to coastal marshes, and a favourite food  of larger blue crabs (Callinectes sapidus). Compounds in blue crab urine trigger an alarm response in the prey species, causing them to stop foraging and remain still until the blue crabs head off elsewhere or switch prey.

A study in the journal PNAS has identified the two chemicals in the blue crab urine that trigger the alarm response as the digestive metabolites trigonelline and homarine.

“Mud crabs react most strongly when blue crabs have already eaten other mud crabs,” said Julia Kubanek, who co-led the study with Marc Weissburg at the Georgia Institute of Technology in the US. 

“A change in the chemical balance in blue crab urine tells mud crabs that blue crabs just ate their cousins.” 

To isolate and identify trigonelline and homarine from the hundreds of chemicals in blue crab urine, Kubanek and colleagues applied technology and methodology from an emerging field known as metabolomics, which is used in medical research to identify small biomolecules.  

The target chemicals are not pheromones – chemical signals used by members of the same species to communicate status or readiness to mate. 

“In this case, the mud crabs have evolved to chemically eavesdrop on the blue crabs’ pee. You might call trigonelline and homarine fear-inducing cues,” Kubanek says. 

The mud crab is particularly attuned to picking out these cues from their ocean environment. 

“Most crustaceans are walking noses,” Weissburg explains. “They detect chemicals with sensors on their claws, antennae and even the walking legs. The compounds we isolated are pretty simple, which suggests they might be easily detectable in a variety of places on a crab. 

“This redundancy is good, because it increases the likelihood that the mud crabs get the message and not get eaten.”

Chemical signalling between species such as the mud and blue crabs in coastal marshes has practical applications in the management of crab and oyster fisheries. 

“These chemicals are staggeringly important,” Weissburg says. “The scent from a blue crab potentially affects a large number of mud crabs, all of which stop eating oysters, and that helps preserve the oyster populations.”