A special protein allows anemones to choose when and who to sting. Understanding it guides stinging decisions shows how even the tiniest, subtle adaptations can drastically change an organism’s behaviour.
Anemones sting by shooting out venom-covered barbs called nematocysts, which can grow to 20 to 50 times their original size and travel at the speed of a fired bullet, says at Harvard University.
“It’s one of the fastest events in biology,” says Bellono. “It’s very explosive.”
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The barbs can only be discharged once and it takes a lot of energy to make them. As anemones lack a central nervous system, researchers have long been baffled by how the animals manage to avoid wasting their weapons.
Bellono’s team studied a pale anemone (Exaiptasia diaphana) – these animals live in exposed ocean environments and don’t hunt, but get nutrition by housing photosynthetic organisms in return for protection. The researchers first probed the anemones’ tentacles, which immediately caused the creatures to shoot their microscopic bullets indiscriminately. When the researchers dissected the tentacles, they found proteins called calcium ion channels similar to those in human neurons. When activated, these channels permit the flow of calcium ions into neurons so that electrical signals can propagate, before being inactivated. In the pale anemone, however, the channels are only weakly inactivated, meaning its easier to set them off.
These gateways work differently in the related predatory starlet sea anemone (Nematostella vectensis), a burrowing anemone that keeps only its tentacles exposed and ready to sting passing prey. Their ion channels are inactivated much more strongly, meaning it takes a stronger stimulus to open them. In a , these anemones were triggered to sting solely if they simultaneously perceived specific prey chemicals in their environment along with physical touch. Even when the cells that shoot the barbs, called nematocytes, were cloned and placed into other cells – like frog eggs, which wouldn’t normally have these ion channels – they acted this way.
“That means that protein on its own intrinsically behaves this way, which is very strange,” says Bellono. “It’s really a nice example of evolutionary tuning of an existing protein that many other organisms use, which has just been subtly modified to control this decision process.”
Bellono says these findings start to suggest there is a pattern: the nematocytes of predatory anemones tend to have inactive calcium ion channels while those of defensive anemones tend to have active calcium ion channels, to match their lifestyle.
“An ancient organism with a primitive nervous system is able to implement optimal decisions in a way that’s ingrained in their molecular machinery,” says at the University of Genoa in Italy. “This is very exciting because it’s a principle and principles are rare in biology.”
Still, both Seminara and Bellono note there are probably many more factors at play that still need to be fully understood.
at VU Amsterdam in the Netherlands, who was not involved in the research, cautions that it may be hasty to conclude that one small protein makes a big difference in behaviour without understanding its context. He says more research is needed, particularly on the entire organism in the wild, to strengthen the link between molecular machinery and behaviour.
eLife
