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Garden snails and aphids are venomous animals. This is the startling outcome of a new study that argues for a radical shift in how we think about venom. It proposes a wider definition that would also include animals that inject toxins into plants and members of their own species.

The number of venomous species in the world has just increased by the tens of thousands.

This is because researchers have expanded how we should be thinking about venom. They have shown that animals that inject toxins into plants utilize the same molecular processes as those which inject venom to kill other animals.

As a result, venom expert Dr. Ronald Jenner and his colleagues Nicholas Casewell and Eivind Undheim are arguing that any species that manipulates the body of another organism through, for example, suppressing the immune system, should be classified as venomous.

Redefining what has historically been seen as an animal injecting saliva as venom could provide a radical shake up for zoologists by bringing together two major fields of research.

This is because it opens the door for sap-sucking insects such as cicadas, aphids and shield bugs to join the ranks of snakes and scorpions. It would also see species that use toxins to manipulate their mates during sex, such as garden snails and slugs, being reclassified as venomous.

"If an animal starts manipulating a living recipient with an injected cocktail of toxins, then that saliva is a venom," says Ronald, who has this study in the journal Trends in Ecology & Evolution. "This is where we draw the line."

"This is one of these things that is obvious when you think about it, but not obvious when you don't."

What is venom?

Intrinsically, it may seem straightforward. Venom is a substance that one animal injects into another to kill it. This would include the toxins produced by animals such as snakes, spiders and scorpions.

But ask different people, and this definition seems to shift. For example, in a medical setting, venom might only apply to those creatures which harm humans, which would exclude . Others might talk about venom as a substance that is used specifically for predation or defense, even though those are . What's more, it is often assumed that venom is specifically used by animals on other animals.

But as a venom researcher, Ronald has often found these definitions lacking.

"I'm an evolutionary biologist," he explains, "and I've always wondered where the world of venom starts and ends. It's clearly a very useful adaptation because it's been invented again and again, often for defense and predation, but also competition."

"So then the question is what actually are the hallmarks of venom?"

In about 2009, a helped to change how researchers were thinking about venom. It suggested that some species of blood-sucking animals, such as mosquitoes and leeches, should be classed as venomous too.

This is because as these animals feed, they deliver toxins into their prey which reduce pain and prevent blood-clotting and inflammation. As these substances alter the physiology of their victim, the authors argued that this should be classed as a type of venom.

"If you look at what the proboscis of a mosquito does when it's in your skin, it injects toxins that suppress the immune system so that the animal can safely take a blood meal without being swatted away," explains Ronald. "On a molecular level it shows a lot of similarities to ."

This reframing of how venom was viewed got Ronald and his co-authors thinking. Taking things back to basics, he questioned these historical definitions and wondered if, even with the addition of blood-sucking organisms, they were still thinking too narrowly.

When does saliva become a venom?

One of the big divides has been the difference between saliva and venom. This was the case with mosquitoes and leeches, which were previously talked about as injecting their saliva rather than injecting venom.

But by building on this change in perspective for the blood suckers, Ronald and his co-authors began looking at what other animals are known to inject their prey with saliva. They came across the sap suckers.

Sap suckers are a large group of insects that, as the name suggests, suck the sap from host plants. But when Ronald started to read the literature on how these insects do this, it became clear that there were huge parallels with his work on venomous animals.

When, for example, a shield bug pierces the stem of a plant it is feeding on, that bug will inject saliva which inhibits the immune system of the plant. It is trying to prevent the plant from mounting a chemical defense against its feeding. This is almost the same process that is going on with mosquitos, which are now classed as venomous.

"If you look at it conceptually, researchers working on venom and toxic saliva are working on exactly the same system," says Ronald. "They work on a conflict arena between two organisms that is mediated by injected toxins. And that's venom."

"So it became clear this is the hallmark of the biological essence of venom and if you apply that, you can see that aphids and other sap suckers must be venomous too."

Zootoxic and phytotoxic venom

This new definition of one organism altering the physiology of another by injecting toxins means that any animal feeding on plants by injecting them with toxins is venomous. As too are any animals that to manipulate the chances of paternity, such as slugs and snails.

This new way of thinking about toxins massively expands the number of animals that could be classed as venomous. For example, there are at least 80,000 species of sap suckers alone. Ronald suggests that we should be talking now about "zootoxic" and "phytotoxic" venom to refer to whether it is directed towards animals or plants—and in some cases, both!

The next steps will be to see how the insect-plant experts react to this radical new thinking. But Ronald is hopeful that soon they can organize a conference so that the different groups of researchers can come together and exchange their knowledge, ideas and work.

"I'm pretty psyched about this," says Ronald. "I want to get people from these different communities—the sexual conflict, plant-insect interactions, and traditional venom researchers—together."

"I want to see what the common ground is, and what the unique things are about these different types of venom. I am curious what people will say in these other fields."

This story is republished courtesy of Natural History Museum. Read the original story here