Allodero hylae

Half of all known segmented worms are oligochaetes, and the most well-known example is an earthworm. But aside from the earthworms that might be crawling under your garden, there are a wide variety of oligochaete species living in all kinds of environments, including freshwater habitats, seashore, sewage, and even glacial ice. Considering the range of environments they inhabit, it is a bit surprising that so few oligochaetes have evolved to be internal parasites.

From Figure 2 of the paper

There are only two known genera of endoparasitic oligochaetes - Chaetogaster which lives as internal parasite/symbionts of freshwater molluscs such as snails and mussels, and Dero which lives in frogs and toads, and they both belong to the family Naididae. The species featured today is Allodero (a subgenus of Dero) hylae - it lives out its life in the ureter of the Cuban tree frog Osteropilus septentionalis. Wild frogs can have more than 40 worms in the ureters, which can become dilated due to the parasite load.

The study we are featuring today investigated how these worms get from one frog to another. The researchers knew that the larval worms are pass (or pissed) into the environment via the frog's urine, but they wanted to test whether A. hylae which had been freshly expelled with frog pee can actively infect another frog, and what happens to the worms that don't end up in a frog.

First, they exposed five different species of frog and toads to some A. hylae larvae from a "donor" frog. They observed that A. hylae infect their hosts by swimming up their cloaca, but they are rather picky about whose cloaca they went up. Out of the five species of potential hosts, only the tree frogs ended up being infected. But this is not an entirely one-sided interaction - the researchers also noted that potential hosts can turn the tables on the worms by eating them before they have a chance to swim up their cloaca. If A. hylae enters a frog through its mouth instead of its cloaca, they will simply get digested.

Next they test if an uninfected frog can become infected in the presence of an infected one, and they did so by placing an uninfected frog with a frog carrying A. hylae in either a plastic container or a water-filled bromeliad (for a more naturalistic setting). For good measure, they simulate a predation event on the infected frog to ensure that some worms are expelled. In less technical terms, they scared the piss (and worms) out of an infected frog.

Photo of Allodero lutzi, a related species from southern Brazil
Photo from from Figure 1 of this paper

Sharing a room with a infected room mate is one thing, but to share a room with one that had just pissed themselves and there are parasites in their pee that wants to crawl up your cloaca is probably a bit much (even for reality TV these days). Between 60-73 percent of the tree frogs sharing a container or bromeliad with an infected room mate did end up getting worms in their ureters, showing that fresh pee from an infected frog can be a source of new infection.

Since A. hylae needs to actively seek out a host in the environment, when these worms are born, they start out well-equipped for a life swimming in the water. They have bristles (setae) on their back, well-developed gills, and a fully functional digestive tract - all necessary for making it as a free-living organisms. But once they get in a frog, within 72 hours they undergo a transformation whereby they lose all the those features and become more equipped for a life as a parasite inside a frog's ureters.

But what happens to the worms that do not end up in a frog? For most parasites, not finding a host means death. But it seems that once a larval A. hylae has been away from a frog for long enough, they don't look back. The researchers found that while worms that have been out of a frog for less than a week are attracted to frog BO, those that have been out over two weeks lose their attraction. In addition to being disinterested in frog BO, these older worms retain their bristles, gills, and fully functional digestive tract for good. Unlike their parasitic cousins who have lost all such features once they found a nice frog to settle into, these worms have become used to the outside world and are content to spend their life swimming in the water and foraging for microbes.

Animals like A. hylae, which have not evolved to be fully commit to a parasitic lifestyle, can give insight into how internal parasites have evolved from ancestors that were initially free-living organisms. Depending on its circumstances, A. hylae will end up either living in the ureters of a frog, or out hunting microbes in the water. Allodero hylae doesn't always chose the outside life, sometimes the outside life choses it

Reference:
Andrews, J. M., Childress, J. N., Iakovidis, T. J., & Langford, G. J. (2015). Elucidating the Life History and Ecological Aspects of Allodero hylae (Annelida: Clitellata: Naididae), A Parasitic Oligochaete of Invasive Cuban Tree Frogs in Florida. Journal of Parasitology 101: 275-281.