Many animals have extraordinary defence mechanisms, from the sea cucumbers that expel their entrails through their anuses to the exploding ants that blow themselves up to protect the colony. Now we can add an Australian slug that glues down would-be predators to the list.
Biologists have shown that the red triangle slug (Triboniophorus graeffei) produces a special kind of mucus when threatened. Unlike the thin, slippery slime it secretes as it moves, the special defensive mucus is extremely sticky – strong enough to glue down predators for days.
John Gould of the University of Newcastle, Australia, made the discovery when he spotted a green tree frog stuck to a branch right next to a red triangle slug (pictured, above) in the Watagans Mountain Range in New South Wales. After 10 minutes it still hadn’t freed itself, so he took both animals back to the lab.
A day later the frog was still stuck fast. The researchers prised it free, but it still had mucus stuck on its skin and kept getting stuck to the sides of the container it was placed in. Any frog stuck for so long in the wild would probably fall victim to predators or die from lack of water.
At least two other species of slug also produce sticky mucus, but these have only been studied in the lab. “As far as we can tell, no one has actually seen its use in the wild before,” says team member Jose Valdez of Aarhus University in Denmark.
What is unusual about slug glue is that it adheres strongly in wet conditions and loses its stickiness as it dries. That property could be very useful – one team is already developing a glue for treating wounds based on the sticky mucus of the slug Arion subfuscus.
Many animals produce adhesive glue for defence, says Valdez, but in most cases it is thought to merely distract predators as they try to remove the glue from their face or body. Only a few are known to actually glue down predators, including one salamander that can immobilise a snake for up to 48 hours.
One mystery remains: how does the slug avoid getting stuck down by its own glue?
“That is the big question,” says Valdez. “The answer is that no one knows, not only for this species, but all other species which produce adhesives for defence or predation.”
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