Tongue worms have been successfully mooching off their hosts for 425 million years.
We'd usually think that getting devoured is bad news for whoever's being eaten. But for the parasites known as tongue worms—or pentastomids in scientific parlance—being part of a meal is the easiest way to sneak into the bodies of their hosts.
These wormlike moochers live on the milliscale, and infect the respiratory systems of vertebrates including humans (cue: gross video of one being extracted from an eyeball). According to a forthcoming paper in Current Biology, this strategy has been working for at least 425 million years, as evidenced by the fortuitous discovery of a fossilized crustacean infected with primordial tongue worms, whose name comes from the tonguelike shape of some species.
The specimen was found in Herefordshire, England, and is one of the oldest examples of a fossilized parasite interacting with its host. "Examples of fossil parasites are exceedingly rare," lead author and paleontologist David Siveter told me over email.
"There are older possible examples of host/parasite interaction," he continued. However, he said that the specimen described in the paper "is the oldest and the only known example involving tongue worms. Tongue worms are exceedingly rare in the fossil record, and they are previously known from only a handful of juvenile specimens; we have adults in our find."
Indeed, the beleaguered crustacean that was parasitized by these tongue worms all the way back in the Silurian Period was being colonized on multiple fronts. Not only did these "worms"—while they've actually been grouped with crustaceans for some time, there's serious debate over what pentastomids should actually be classified as—set up firm roots inside the host's shell next to its eggs, they were also found clinging to its exterior. This is yet another way in which the specimen discovered by Siveter's team is extraordinary.
"Our examples that are attached to the external surface of the host (an ostracod crustacean) is unique for any fossil or living tongue worm," he told me. "Tongue worms have to find a way of entering a host. We surmise that the external examples we record were possibly on their way to do that, so to speak."
The fact that this kind of configuration has never been seen between a pentastomid and its host, even among the 140 species of extant tongue worms around the world today, further highlights the sheer unlikeliness of such a find.
"The chances of specimens being preserved are very slim indeed, especially compared to, say, an animal that has bones or shells as a part of their anatomy," Siveter said. "Then, generally you need some special preservational factor (i.e., geochemical conditions) for such exceptional preservation to occur."
"In our example the fossils are preserved in a 425 million-year-old volcanic ash," he continued, "which rained down on the animals living on the seafloor, and led to their death and preservation."
Needless to say, these fossilized worms represent an entirely new genus of tongue worm, which Siveter and his colleagues have dubbed Invavita, meaning "ancient intruder." This particular species was given the badass name Invavita piratica, a reference to its pirate-like evolutionary strategy.
Moving forward, Siveter would like to explore the deep evolutionary history of these parasites, which have soldiered through dozens of geological ages and several mass extinction events since the Silurian Period. "This discovery affirms that tongue worms were external parasites on marine invertebrate animals at least 425 million years ago," Siveter said.
A modern tongue worm specimen. Image: Dennis Tappe & Dietrich W. Büttne
"Today they are known from about 140 species, nearly all of which are parasitic on vertebrate animals, particularly reptiles and including humans," he added. "It is still an intriguing question as to whether ancient tongue worms also were hosted by some kind of vertebrate [such as fish] or not."
Regardless of the answer to that question, the Invavita piratica specimen has provided an unprecedented glimpse into the development of parasites in Earth's early oceans. Death by volcano isn't usually regarded in a positive light, but we are lucky that in this case, volcanic ash preserved one of the oldest examples of the troubled bond between parasite and host.
Top image: Computer renders of the newly-discovered parasite (two individuals in orange) and host (structures represented by other colors, the shell is in gray.) The left shows host and parasite positioning complete with shell, while the right has the shell removed; the middle shows a render of one of the parasite individuals found.