You know, it’s a crab-eat-crab world out there. And that’s not just a saying: crabs do sometimes eat other crabs, especially when the other crab is of a different species.
I think life is particularly tough for the small, soft-shelled crabs out there. Consider mud crabs (Panopeus herbstii): they’re about the size of a silver dollar, and their best defense is hiding.
But how do they know when to hide?
The waters where mud crabs live, estuaries in Georgia for example, are very murky. I don’t think sight is part of the equation.
The sense the mud crabs use is smell: their own underwater version. Small hairs on their antennae, which, when swept back and forth, can detect chemical scents in the water around them.
Such as the scent of danger. That sounds like it’s just a dramatic description, but it’s a real thing. In this case, the scent of danger comes from another mud crab that’s already been eaten and metabolized.
A Metabolized And Eaten Mud Crab
Scientists studied the urine of a common mud-crab predator: the blue crab, Callinectes sapidus. They were able to compare the chemical profiles of blue crab urine after different meals, and identified over six-hundred chemicals.
It probably comes as no surprise that the ones found most alarming to mud crabs – trigonelline and homarine – spike after the blue crab has just eaten a mud crab. It gives us some interesting insight into the chemical cues produced by predator-prey interactions underwater.
Thank you to Julia Kubanek of the Georgia Institute of Technology for reviewing this episode’s script!
Sources And Further Reading:
- Klein, Joanna. “The Invisible Underwater Messaging System in Blue Crab Urine.” The New York Times. January 8, 2018. Accessed June 19, 2018.
- Milne, Ruth. “How Crabs Smell.” The Royal Society via Phys.org. January 20, 2016. Accessed June 19, 2018.
- Poulin, Remington X. Laovie, Serge. Siegel, Katherine. Gaul, David A. Weissburg, Marc J. Kubanek, Julia. “Chemical encoding of risk perception and predator detection among estuarine invertebrates.” PNAS. January 23, 2018. 115 (4) 662-667; published ahead of print January 8, 2018. https://doi.org/10.1073/pnas.1713901115