''Accacoelium contortum'' is a parasitic flatworm of the class

Trematoda Trematoda is a Class (biology), class of flatworms known as flukes. They are obligate parasite, obligate internal Parasitism, parasites with a complex biological life cycle, life cycle requiring at least two Host_(biology), hosts. The intermedia ...

. It lives in the gills and oral cavity of the

ocean sunfish The ocean sunfish or common mola (''Mola mola'') is one of the largest bony fish in the world. It was misidentified as the heaviest bony fish, which was actually a different species, ''Mola alexandrini''. Adults typically weigh between . The spe ...

, '' Mola mola'', and was first described by Swedish-born naturalist Karl Rudolphi in 1819. ''Accacoelium contortum'' is the type-species of the family Accacoeliidae and the only known species of the genus ''Accacoelium.'' ''Accacoelium contortum'' is one of the most common sunfish parasites and has been reported in the

Mediterranean Sea The Mediterranean Sea is a sea connected to the Atlantic Ocean, surrounded by the Mediterranean Basin and almost completely enclosed by land: on the north by Western and Southern Europe and Anatolia, on the south by North Africa, and on the ea ...

, the Northeast

Atlantic Ocean The Atlantic Ocean is the second-largest of the world's five oceans, with an area of about . It covers approximately 20% of Earth's surface and about 29% of its water surface area. It is known to separate the " Old World" of Africa, Europe ...

and the

South Pacific The Pacific Ocean is the largest and deepest of Earth's five oceanic divisions. It extends from the Arctic Ocean in the north to the Southern Ocean (or, depending on definition, to Antarctica) in the south, and is bounded by the continen ...

. In a study in Spain, 47.2% of 106 sunfish examined were found to host ''Accacoelium contortum.'' Most trematodes are internal parasites (endoparasites), but ''Accacoelium contortum'' is also found on the outside of its host. It has an elongated body with oral and ventral suckers and papillate anterior. It lacks the hooks and clamps found on most other external parasites, and instead attaches using the ventral sucker. The parasite also developed a strong ventral musculature in the hindbody which it uses like a prehensile tail to grasp onto other parasites, aiding in the formation of larger clusters of the parasite. It is found in clusters in the mouth, gills,

pharynx The pharynx (plural: pharynges) is the part of the throat behind the mouth and nasal cavity, and above the oesophagus and trachea (the tubes going down to the stomach and the lungs). It is found in vertebrates and invertebrates, though its struc ...

, and pharyngeal teeth, but can also survive in the gastrointestinal tract. ''Accacoelium contortum'' induces a strong immune response in its host when infecting the oropharyngeal chamber, leading to

inflammation Inflammation (from la, wikt:en:inflammatio#Latin, inflammatio) is part of the complex biological response of body tissues to harmful stimuli, such as pathogens, damaged cells, or Irritation, irritants, and is a protective response involving im ...

hyperplasia Hyperplasia (from ancient Greek ὑπέρ ''huper'' 'over' + πλάσις ''plasis'' 'formation'), or hypergenesis, is an enlargement of an organ or tissue caused by an increase in the amount of organic tissue that results from cell proliferati ...

and

necrosis Necrosis () is a form of cell injury which results in the premature death of cells in living tissue by autolysis. Necrosis is caused by factors external to the cell or tissue, such as infection, or trauma which result in the unregulated dige ...

at the site of attachment. In some cases, the parasites can end up encased in host tissue. This might actually work to the parasite's advantage because now it is enclosed in a small flesh bag that is tightly secured to the host body. In high infections with large numbers of the parasite, if a wide gill area is damaged it can prevent normal gas exchange, which can compromise the survival of the ocean sunfish. Strangely, when infecting areas in the stomach and esophagus the parasite are scattered rather than clustered and are not correlated to changes are damage in the epithelium near the site of infection. ''Accacoelium contortum'' also infects the right gill at significantly higher rates than the left gill. This is believed to be caused by the host's tendency to swim in the water column and rest at the surface with its left side up, which exposes it to more sunlight, UV radiation, and air.

References

{{Taxonbar, from=Q2087774 Plagiorchiida Taxa named by Karl Rudolphi