A bloodworm. Credit: Hans Hillewaert / CC-BY-SA-3.0
A bloodworm. Credit: Hans Hillewaert / CC-BY-SA-3.0

The Venomous Cocktail in a Fisherman’s Bait

Anglers often use bloodworms as bait, and aquarists use them as fish food. These small squirming creatures are named for their red bodily fluids which are visible through their translucent skin. They seem innocuous enough—at least, until they extrude their huge, terrifying proboscis, tipped with four, black fangs. Each one is lined with copper minerals, and connected to a venom gland. They are, in fact, venomous fish bait.

A bloodworm’s bite feels a bit like a bee or wasp sting. The venom can stop the heart of the small crustaceans that these creatures eat, but it’s not strong enough to harm a human. It can, however, occasionally trigger a severe allergic reaction, much like a bee sting.

Now, Björn von Reumont and Lahcen Campbell from the Natural History Museum in London have catalogued the full array of venom-making genes that are active in the bloodworm’s venom gland. They found a ridiculous amount of diversity. One species—Glycera dibranchata—makes 32 different types of toxin, which is “in the ballpark for snake venoms”, according to Ronald Jenner who co-led the study.

Venom research is booming, and for good reason. These substances, and the creatures that inject them, instil a morbid fascination. They’re great case studies for how evolution shapes molecules. And they’ve often been sources of new drugs. Using new sequencing technologies, scientists have teased apart the killer cocktails injected by familiar groups like snakes and spiders, and also less obviously venomous ones like vampire bats, Komodo dragons, shrews, echidnas, and one group of weird cave crustaceans.

But there are still many venomous animals left to analyse. Jenner was looking to study some of these neglected creatures, and settled on the bloodworms.

The word “bloodworm” is also used to describe the maggots of some groups of midges, a couple of species of parasitic nematodes, and the fictional ones that cause vampirism in The Strain. This post isn’t about any of those. The bloodworms that Jenner’s team looked at are annelids, members of a large group of segmented worms that also includes earthworms and lugworms. Two groups of annelids are venomous: the leeches, whose toxins stop blood from clotting, and the bloodworms, which use their venom to overpower their prey.

Some scientists have looked at bloodworm venom before, but only in a piecemeal way. Von Reumont and Campbell did something more comprehensive. When genes are switched on, the information encoded within their DNA is transcribed into another molecule called RNA. These transcripts are then used to build proteins, like those found in venom. The team identified all the RNA transcripts that are produced in the venom glands of three bloodworm species.

They found plenty. Many of them belonged to 30 known groups. Some produce proteins that kill nerve cells, punch holes in cell membranes, or trigger intense pain. But a dozen of these toxin types are a mystery—they don’t match anything we know, and may be unique to bloodworms.

The team also compared the bloodworm transcripts to those form other animal groups, and found some that are shared across many venomous lineages. Some resemble toxins in bees that trigger severe allergic reactions in a minority of people, which may explain why some folks go to hospital with severe inflammation after being bitten by bloodworms. One toxin has only ever been found in discovered in scorpionfish, platypuses and echidnas. And until now, one toxin was thought to be unique to sea anemones, while another was supposedly an invention of predatory snails. Now, we know that bloodworms wield the same chemical weapons.

These similarities between bloodworm venom and those of very distantly related animals exemplify the single most important theme in venom evolution: convergence. Different groups of venomous animals have independently transformed the same kinds of proteins into the same kinds of venom. You’ll see the same toxins in shrews and Gila monsters, or in snakes and cone snails. “Venom toxin evolution is rampantly convergent,” says Jenner.

Why? That’s one of the big questions in the field, says Jenner, and there are probably several answers. Venom proteins have to work in the bodies and bloodstreams of other animals, so they have to be very stable. As such, they tend to be rich in components that form strong bridges between one another, so the proteins don’t lose their shape after leaving their owner’s fang or sting.

Jenner also notes that small genetic changes can radically change the way some proteins work. Maybe some groups of proteins are “poised to be weaponised”. In other words, it only takes a few small changes to change them into toxins, so they more readily evolve that way when the need arises.

Finally, there are only so many ways to kill another animal, and many venoms end up hitting the same targets. Fellow Phenomena blogger Carl Zimmer explains this well in his post on the origin of venom:

“For example, cone snails, scorpions, and anemones have all evolved venoms that attack channels on neurons that pump out potassium. Snakes and bees have evolved the ability to block platelets from clumping together, a crucial step in blood clotting. These results show that there are a limited number of ways to kill your victim quickly. No matter what genes you borrow for the evolution of venom, they will end up very similar to other venoms.”

Ironically, venom—a weapon for destroying animal bodies—is also a wonderful testament to the similarities that bind us together.

Reference: Von Reumont, Campbell, Richter, Hering, Syke, Hetman, Jenner & Bleidorn. 2014. A polychaete’s powerful punch: venom gland transcriptomics of Glycera reveals a complex cocktail of toxin homologs.