|Sunfish gill with arrows indicating the location of A. contortum
From Fig. 1 of the paper
Scientists in Spain examined the parasites of 106 sunfish which were caught as bycatch and found that almost half (47.2%) were infected with A. contortum. Most of the flukes were found on the gills, some in the back of the throat near the pharyngeal teeth (which are a set of teeth that ray-finned fishes have at the back of their throat) and only a few were in the stomach. They noticed that usually the gills on the fish's right side are more heavily infected - this asymmetrical distribution is similar to what has been observed for other parasites, where they tend to congregated towards one side of the host, though in this case it's not entirely clear why they do this. In addition to preferentially hanging out on one side of the host, they also tend to congregate in clusters comprised of dozens of individuals, with those in the pharynx forming larger groups than those on the gills.
While A. contortum seems to do quite well in its rather unusual (for a trematode) habitat, the basic body plan for trematode is that of an internal parasite. So how does one modify a body plan for living inside the cosy confine of a host's body to a life clinging on to the more exposed parts of the host? Fortunately for A. contortum there are some functional overlaps between living in a fish's intestine versus living on its gills. While it lacks the hooks and sucker clamps of monogenean flatworms which are specialised for ectoparasitism, A. contortum has co-opted its large and muscular ventral sucker for hanging on to the sunfish's gills. Other trematode species use their ventral sucker to attach to the intestinal wall. In A. contortum it also function as the main attachment organ, but on a different part of the host's body. Additionally, this fluke's hind body appears to be long and prehensile, which it might to able to use to grip like a chameleon's tail (to a limited degree).
|Left: Anatomical drawing of A. contortum, Right: Scanning electron microscope photo of the parasite's front
Image from Fig. 2. of the paper
The way that A. contortum apparently manipulates the sunfish's tissue is rather reminisce of how gall wasp induces their host plants to form a protective gall. While those galls protect their inhabitant against predators, in this case A. contortum, the flesh bag that it induces provide the parasite with a shelter on an otherwise exposed and turbulent location.
Ahuir-Baraja, A. E., Padrós, F., Palacios-Abella, J. F., Raga, J. A., & Montero, F. E. (2015). Accacoelium contortum (Trematoda: Accacoeliidae) a trematode living as a monogenean: morphological and pathological implications. Parasites & Vectors 8: 540.
P.S. I recently wrote a post about prehistoric/fossil parasites (which you can read here). On a related note I also wrote an article for The Conversation which focuses specifically on the fossil evidence for (non-avian) dinosaur parasites - you can check it out here.