If you wanted to find a venomous animal, you could do far worse than picking up a random arthropod—the group of animals that includes spiders, scorpions, centipedes, ants, bees and wasps. The group includes hundreds of thousands of venomous members, who inject their debilitating chemical weapons via fangs and stings.
Within this toxic dynasty, one of the major arthropod groups—the crustaceans—sticks out. There’s no such thing as a venomous crab or lobster, prawn or shrimp. There are some 70,000 species of crustaceans and, until recently, it seemed that all of them were venom-free.
The only exceptions live in coastal caves, which are connected to the ocean by underground tunnels. The dark, salty worlds are home to blind, white, sinuous creatures called remipedes. Although they look a lot like white centipedes, that’s just a coincidence. They’re actually crustaceans, and possibly close relatives of the insects.
The remipedes were first discovered in the 1980s, and named after the Latin for “oar-footed” because of their many pairs of swimming legs. Observant scientists soon noticed that on either side of their head, behind their jaws, they had a pair of fangs —sharp, hollow-tipped and connected to glands. Others noticed them eating other crustaceans in the wild. Connecting the dots, it looked as if these creatures were venomous.
Now, Bjorn von Reumont from the Natural History Museum in London has proved as much. His team has thoroughly described the fangs of the remipede Speleonectes tulumensis, and characterised the cocktail of toxins in its venom. These creatures are undoubtedly venomous crustaceans, and perhaps the only ones on the planet.
Von Reumont showed that the remipede’s venom system is very sophisticated. One set of muscles contracts the creature’s glands, pumping venom into its fangs. A second set of muscles stabs the fangs forwards, while squeezing a duct to prevent the venom from flowing backwards.
Remipede venom consists of big enzymes like peptidases, which destroy other proteins, and chitinases, which break down the chitin in the external skeletons of arthropods. Together, these substances combine to soften the hard shells of the remipede’s prey, and to digest their innards.
But before a remipede can liquefy its meal, it must first capture it. It probably does that with another chemical—a single unique neurotoxin that’s similar to others found in spider venom. The team thinks that the toxin causes the victim’s motor neurons to fire continuously, paralysing it through its own spasms. “We had to do some work to confirm this, but that was the coolest finding,” says study leader Ronald Jenner. “It makes sense for a blind, aquatic, cave-dwelling predator to have a paralysing toxin so that prey can be instantly overwhelmed.” In a dark, largely empty cave, second chances don’t come often.
Still, the remipede’s venom is weird. Other arthropods, like scorpions or spiders, mainly rely on small proteins that poison nerve cells. The remipede has just the one neurotoxin and relies instead on beefy digestive enzymes. If anything, its venom is more similar to that of vipers and rattlesnakes—a clear case of convergent evolution, where different life-forms independently turn up to the party with the same outfits.
Why? “I don’t know,” says Jenner. It may be that they live in water, while other venomous arthropods are land-lubbers. What works on land may not work in water. There’s also the fact that creatures like centipedes and scorpions have powerful mouthparts for chewing up their prey. Remipedes seem to feed more like spiders—they liquefy they prey and suck the juices through the shell.
And why are they the only crustaceans to have evolved venom? Again, it’s not clear. Jenner notes that most crustaceans scavenge off debris or feed on small particles in the water. There aren’t many of them that specialise in killing larger prey. “If you want to do that, you either need power or a trick,” says Jenner. The pugilistic mantis shrimps went for power. Remipedes use venom as their trick.
“There might be a few more instances of venomous crustaceans, for which current evidence is anecdotal,” says Jenner. The branchiurans, for example, are a group of fish lice that stab through the skin of their hosts with a sharp spine. That causes heavy bleeding, which “can wreak havoc on the fish and be a real burden on aquaculture operations,” says Jenner. “It would be cool to have a closer look at those.”
Reference: Von Reumont, Blanke, Richter, Alvarez, Bleidorn & Jenner. 2013. The first venomous crustacean revealed by transcriptomics and functional morphology: remipede venom glands express a unique toxin cocktail dominated by enzymes and a neurotoxin. Mol Biol Evol http://dx.doi.org/10.1093/molbev/mst199