It’s a crab-eat-crab world out there.
Crabs do sometimes eat other crabs, especially when the other crab is of a different species. Life is particularly tough for the small, soft-shelled crabs out there. Consider mud crabs: they’re about the size of a silver dollar, and their best defense is hiding.
How do they know when to hide?
They can't always rely on seeing a predator coming. The waters where mud crabs live -- estuaries in Georgia for example -- are very murky. Sight isn't really a 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.
Including the scent of danger. But where does this smell come from?
In this case, the scent of danger comes from another mud crab that’s already been eaten and metabolized. Scientists studied the urine of a common mud-crab predator: the blue crab. 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.
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Sources
- New York Times - The Invisible Underwater Messaging System In Blue Crab Urine
- Phys Org - How Crabs Smell
- PNAS - Chemical Encoding Of Risk Perception And Predator Detection Among Estuarine Invertebrates
D: You know, it’s a crab-eat-crab world out there, Yaël.
Y: That’s true, Don. 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: they’re about the size of a silver dollar, and their best defense is hiding.
D: How do they know when to hide? When they see a predator coming?
Y: Not quite. 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.
D: Such as the scent of danger.
Y: Exactly.
D: But where does the scent of danger come from, Yaël? A dangerous situation?
Y: You’re right on the money, Don. In this case, the scent of danger comes from another mud crab that’s already been eaten and metabolized. Scientists studied the urine of a common mud-crab predator: the blue crab. 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.