|Adult female G. batrachiensis on the left, adult male G. batrachiensis on the right|
Left photo is from Fig. 1 of this paper and the right photo is from Fig. 1 of this paper
By far the most important task that a parasite needs to accomplish during its limited time in the host is reproduction. Gyrinicola batrachiensis can reproduce in two different ways: (1) the asexual way, which result in thick-shelled eggs that are release to the outside world and infect other tadpoles, or (2) via sexual reproduction which produce a mix of both thick-shelled eggs and thin-shelled eggs. Those thin-shelled eggs never leave the tadpole, instead they are "autoinfective" - which means they hatch right there in the tadpole's gut and starts growing. So while those thin-shelled eggs won't survive the rigours of the outside world, but are good for filling up the tadpole's gut with more worms in a relatively short period. Each of those egg types have their own purposes, so how does G. batrachiensis balance between producing those two different types of eggs?
Of the five different species of frogs and toads that the researchers examined, one species stood out as being the best host for G. batrachiensis - the tadpoles of the Southern leopard frog (Rana sphenocephala). Leopard frog tadpoles are much larger than those of other four species they looked at, and it takes between 8 to 13 weeks for the tadpole to reach adulthood, comparing with the tadpoles of the other species which can complete development in as little as 4 weeks. With more space and time to grow, the pinworms living in leopard frog tadpoles could afford to invest time and resources towards growing bigger instead of rushing to pump out eggs before their time runs out. In the long run, bigger worms can produce more eggs - but the pinworms living in the tadpoles of those other frog species don't have that luxury.
Additionally the researchers found that only the pinworms in leopard frog tadpoles produced the autoinfective thin-shelled eggs. While pinworms in the tadpoles of other frog species have to focus on producing thick-shelled eggs to infect new tadpoles before their limited time run out, those in the gut of leopard frog tadpoles have more time and room to work with - so they might as well make the most of it by producing some autoinfective, thin-shelled eggs to fill up the tadpole's gut with more of its own offspring and get a head start on producing the next generation.
But while the leopard frog tadpole seems to provide G. batrachiensis with the ideal environment, it is not the species which is most commonly infected with G. batrachiensis. Once those thick-shelled eggs leave the tadpole, they sink to the bottom of ponds where they wait to get sucked up by an unwary tadpole - and they don't get to chose which tadpole they end up in. For this study, the researchers found that pinworms were most commonly found in the tadpoles of Blanchard's cricket frog (Acris blanchardi). In contrast, the tadpoles of the narrow-mouthed toad (Gastrophryne olivacea) found in the same pond managed to stay worm-free.
So why does one species seem to be a pinworm magnet while the other manage to stay clean even though they are living in the same environment? This might something to do with how they eat. Tadpoles of the Blanchard cricket frog feed by scrapping algae off the bottom of ponds with their mouth. In the process, they also suck up some of those thick-shelled pinworm eggs that are lurking amidst the muck. In contrast, the tadpoles of narrow-mouthed toad feed by slurping tiny plants and animals off the water's surface, so they don't come anywhere near those pinworm eggs. While G. batrachiensis might not always end up in their ideal host, they always try to make the most of it.
Pierce, C. C., Shannon, R. P., & Bolek, M. G. (2018). Distribution and reproductive plasticity of Gyrinicola batrachiensis (Oxyuroidea: Pharyngodonidae) in tadpoles of five anuran species. Parasitology Research 117:461-470.