There are some 70,000 species of crustacean—the group that includes crabs, lobsters, shrimp and crayfish—in the world, and until very recently, none of them were known to be venomous. This has made them the odd animals out in their phylum, Arthropoda, which contains tens of thousands of venomous species across its other three subphylums.
Now, biologists have found an exception to the trend. Meet the remipede Speleonectes tulumensis. These tiny critters look like white centipedes, with long segmented bodies and scores of swimming legs, and call the underground caves of Mexico and Central America home. They were first scientifically described in the early 1980s, but studied little because the cave networks they swim through are difficult to navigate and often dangerous to work in. One group of researchers that kept at it, though, discovered that the remipedes, while blind, are formidable predators and feed on the shrimp that share their underground pools.
On closer examination, biologists from the Natural History Museum in London discovered that the remipedes had a needle-like fang on either side of their head, each attached to a reservoir surrounded by muscle that could pump fluid out through the needle point. Deep inside the crustacean’s body, they also found a gland that produces venom for storage in the reservoirs.
Remipede venom is odd compared to that of other arthropods. While some spiders, for example, use venom consisting of tiny neurotoxic proteins, the remipede’s toxic cocktail is dominated by larger enzymes that break down the exoskeletons of their prey and destroy proteins in their bodies, softening their defenses and making their insides more easily digestible. In its makeup, the researchers say, remipede’s venom is more like that of vipers than any of its arthropod cousins. There is one familial similarity, though: a neurotoxin that paralyzes the remipede’s victims and is nearly identical to one found in spiders.
With so many crustaceans out there, why is the remipede the only one to become venomous? The researchers think that because the group has such varied diets—some are filter feeders, some are scavengers—none of them really needed a potent weapon for taking down large prey. While remipedes have also been seen filtering small bits of food from the water, their environment and lifestyle pressured them into going toxic.