|photo of Philometroides paralichthydis from here.
But, mere common sense is not science and whatever our expectations might be, they must be tested against the real world. To find out just what effects this muscle-dwelling parasite can have, researchers collected a group of flounders from estuaries in South Carolina and put them through a series of physical exercises in a custom-built swim track and an aquarium filled with sand and water. It was a blinded experiment; the researchers did not know the parasite load of the flounders before or during the trials, so they would not be subconsciously biased about the outcome one way or the other. During the trials, they observed and recorded the flounders' ability to perform actions that are key to their survival - swimming, accelerating, and burying themselves in sand - all of which require the use of the dorsal and anal fins.
After those physical trials, they counted the number of worms found in each flounder and in contrary to what "common sense" may lead us to believe, the presence of P. paralichthydis did not seem to make much difference. Regardless of where they are located in the host, they did not compromise the flounder's ability to accelerate, or cover themselves with sand. The parasite was found to have some affect on swimming speed, but only in smaller juvenile fish and not adult fish. On average, juvenile fish with P. paralichthydis swum thirty percent slower than uninfected fish of the same size class.
So why does this parasite only affect juveniles? Perhaps fully-grown fish are better able to compensate for any pathology incurred by the parasite simply by being larger, which provides a buffer against any damage caused by the parasite. It seems that once they are large enough, the flounders are fairly safe from the damaging effects of this muscle parasite. Juvenile fish are generally more vulnerable to the injurious effects of parasites which can contribute significantly to juvenile mortality in fish and affect recruitment by making them more vulnerable to predators or simply through the pathology they can cause (see this post on how juvenile reef fish run the parasite gauntlet before settling down.)
There are two caveats to this study that we need to consider before drawing any final conclusions. It is possible that the researchers had only collected those fish that had managed to survive well enough despite having the parasite, and fish with greater morbidity from P. paralichthydis had naturally died from starvation or predation and were not represented in the sample. Also, none of the flounders in the trial were infected with more than twelve nematodes. Maybe there was simply not enough worms to have a noticeable affects on the flounder's behaviour. Even with Curtuteria australis, there needs to be a critical number of larval flukes in the cockle's foot before its burrowing ability is impaired.
Umberger, C. M., de Buron, I., Roumillat, W. A., & McElroy, E. J. (2013). Effects of a muscle‐infecting parasitic nematode on the locomotor performance of their fish host. Journal of Fish Biology 82: 1250–1258