Parorchis acanthusi is a parasitic flatworm of the class Trematoda. It is a parasitic castrator of the common periwinkle Littorina littoria. The resulting decline in the numbers of that mollusc can lead to more ungrazed algae, with corresponding increases in invertebrate populations and a greater diversity of smaller Littorina snails. The increased numbers of small prey items makes the affected rocky coasts more attractive to birds such as the purple sandpiper and Eurasian rock pipit.