|(A) Adult male Corynosoma australe, (B) Adult female C. australe, (C) spiny proboscis of an adult worm
Photos from Fig 4. of the paper
The reason why they need a paratenic host is that seals and sea lions do not usually go rummaging through the the mud for tiny thumbnail-size crustaceans. But there are fish that do, and it is those fish that seals and sea lions eat. By using fish as paratenic hosts, C. australe can bridge the ecological gap between tiny amphipods and seals. But having fish as paratenic hosts also open up other possibilities because pinnipeds are not the only marine animal with a taste for fish. This is where penguins enter the story.
Even though taxonomically, birds and mammals are on very different branches of the vertebrate animal tree, because seals and penguins lead comparable life-styles, sometimes they can also end up with similar (or in this case, the same) parasites. In this case, Magellanic penguins end up with what is usually a seal parasite because they have been eating the same fish that the seal usually feed on, and they are physiologically similar enough to seals and sea lions for C.australe to go "Eh, good enough.". In fact, C. australe seems to be a fairly versatile parasite - it has been reported from 16 different types of marine mammals and birds. However, those previous reports also indicate that the parasite can only produce viable eggs while living in pinnipeds, and in the evolutionary game it all comes to nothing if you can't reproduce. Which means while C. australe can stay alive in those non-pinniped hosts, those other hosts are effectively dead ends.
But, this study shows that not only can C. australe survive perfectly fine in penguins, they can also reproduce while living in a bird host. From the samples that the researchers examined, 19 out of the 20 seals and sea lions they looked at were infected with C. australe. In comparison, only 18 out of the 87 penguins they examined were infected. Female worms grew bigger in the gut of Magellanic penguins, yet at same time they did not produce as much eggs as those living in pinnipeds. Also for some currently unknown reason (s), the sex ratio of C. australe in penguins is highly skewed - whereas seals and sea lion have an almost one-to-one ratio of male versus female worms in their guts, females worms vastly outnumbered male worms in the gut of Magellanic penguins.
Judging from egg production and prevalence, Magellanic penguins are not exactly the most ideal or reliable hosts for C. australe. Pinnipeds remain the hosts with the most for C. australe, but at least penguins can serve as a viable (if not ideal) substitute. For C. australe living in penguins, this might be a case of ecological fitting, whereby an organism can survive and (and even thrive) in a habitat which different to the one that it usually live in because it just so happen to have the right set of adaptations that allows it to survive in this new and novel environment.
But there is another twist to this story. While most species of Corynosoma live in marine mammals, it seems that they had evolved from ancestors that originally lived in aquatic birds. So perhaps Corynosoma already has the latent ability to survive in the gut of a bird, and when circumstances brought them together, C. australe was ready. When it comes to this thorny worm, what is good enough for the sea lion is good enough for the penguin.
Hernández-Orts, J. S., Brandão, M., Georgieva, S., Raga, J. A., Crespo, E. A., Luque, J. L., & Aznar, F. J. (2017). From mammals back to birds: Host-switch of the acanthocephalan Corynosoma australe from pinnipeds to the Magellanic penguin Spheniscus magellanicus. PloS One 12(10): e0183809.