|Left: taxonomic drawing of an adult Polypipapiliotrema stenometra from Fig. 2 of the paper.
Right: Pink, swollen Porites coral polyps infected with Polypipapiliotrema larvae (photo by Greta Aeby).
In this study, researchers examined 26 species of butterfly fishes collected from the French Polynesian Islands, and O'ahu, Hawai'i, and found 10 species which were infected with Polypipaliliotrema. Upon examining the DNA and the physical features of those flukes, they discovered that what was thought to be a single species turns out to be at least FIVE different species of coral-infected flukes, and there are variations in their geographical distribution.
Butterfly fish species that are found across different locations were sometimes found to have different species of Polypipapiliotrema at each location, so it seems some fluke species were localised to particular island groups. This means there might be more unique species of coral-infected flukes that remain undiscovered and undescribed from other coral reefs around the world.
In order for Polypipalliotrema to complete its life cycle, it needs the host polyp to be eaten by a butterfly fish. While coral polyps are stable food for some fish, they can be small and finicky to handle - you have to be quick and precise in picking the coral polyp lest it retreats back into its skeleton. Also, corals usually occur in vast colonies composing of hundreds and thousands of polyps, so the chances that the infected polyp would be among the ones eaten by a butterfly fish would be quite slim. On top of that, the polyps of Porite is consider to be poor quality food for most coral-eating fishes - their polyps are tiny and quick to retracts into its skeleton - so even fish that feed almost exclusively on coral polyps prefer species other than Porites.
But Polypipalliotrema has a clever way of stacking the odds in its favour, and it does what many parasites do - by manipulating its host. Coral polyps infected with Polypipalliotrema become swollen and bright pink, in complete contrast to the tiny uninfected polyps. Not only does the colouration draws the attention of butterfly fish, the swollen polyp also can't retract into the coral skeleton, making it easier to the butterfly fish pick them up and get more coral flesh for every mouthful.
But why should the butterfly fish eat something that is filled with parasites? Shouldn't they try to avoid parasitised prey, especially when the infected polyps are so easy to distinguish? Since this fluke is commonly found in butterfly fish, it is clear that they make no attempt at avoiding the fluke-laden polpys.
This could be that while Polypipapiliotrema is technically a parasite, it doesn't really harm the fish host that much, and because of what the fluke larvae do to coral polyps, the fish have an easier time getting its meal. As such, the relationship between Polypipapiliotrema and butterfly fishes is closer to a form of mutualism - by altering the coral polyp, the fluke helps butterfly fish get more to eat for less effort, and for its side of the bargain, butterfly fish allows the fluke to complete its life cycle.
Martin, S. B., Sasal, P., Cutmore, S. C., Ward, S., Aeby, G. S., & Cribb, T. H. (2018). Intermediate host switches drive diversification among the largest trematode family: evidence from the Polypipapiliotrematinae n. subf.(Opecoelidae), parasites transmitted to butterflyfishes via predation of coral polyps. International Journal for Parasitology 48: 1107-1126.