'' Parorchis acanthus'' is a

parasitic Parasitism is a close relationship between species, where one organism, the parasite, lives on or inside another organism, the host, causing it some harm, and is adapted structurally to this way of life. The entomologist E. O. Wilson has c ...

flatworm The flatworms, flat worms, Platyhelminthes, or platyhelminths (from the Greek πλατύ, ''platy'', meaning "flat" and ἕλμινς (root: ἑλμινθ-), ''helminth-'', meaning "worm") are a phylum of relatively simple bilaterian, unsegment ...

of the

class Class or The Class may refer to: Common uses not otherwise categorized * Class (biology), a taxonomic rank * Class (knowledge representation), a collection of individuals or objects * Class (philosophy), an analytical concept used differentl ...

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 is a parasitic castrator of the

common periwinkle The common periwinkle or winkle (''Littorina littorea'') is a species of small edible whelk or sea snail, a marine gastropod mollusc that has gills and an operculum, and is classified within the family Littorinidae, the periwinkles.Reid, David ...

''Littorina littorea''. Unlike many trematode species it encysts on hard surfaces and not inside a second

intermediate host In biology and medicine, a host is a larger organism that harbours a smaller organism; whether a parasitic, a mutualistic, or a commensalist ''guest'' (symbiont). The guest is typically provided with nourishment and shelter. Examples include a ...

. Free-living cercariae are released from the snail hosts to encyst on hard surfaces, generally the shells of

bivalves Bivalvia (), in previous centuries referred to as the Lamellibranchiata and Pelecypoda, is a class of marine and freshwater molluscs that have laterally compressed bodies enclosed by a shell consisting of two hinged parts. As a group, bival ...

. The release is encouraged by warmer temperatures and increased light intensity. Encystment on bivalves is most successful when they enter the mussel with the inhalation current first. They then actively emerge, using their suckers and then encyst on the shell. If the bivalve is then eaten by a bird ''P. acanthus'' will be transmitted to the

definitive host In biology and medicine, a host is a larger organism that harbours a smaller organism; whether a parasitic, a mutualistic, or a commensalist ''guest'' (symbiont). The guest is typically provided with nourishment and shelter. Examples include a ...

. Survival and successful encystment can be reduced by the presence of epibionts, such as the barnacle Semibalanus balanoides, on the shell. Castration of the snail molluscan host can result in a decline in host numbers and, in turn, can lead to more ungrazed

alga Algae (; singular alga ) is an informal term for a large and diverse group of photosynthetic eukaryotic organisms. It is a polyphyletic grouping that includes species from multiple distinct clades. Included organisms range from unicellular mi ...

e, with corresponding increases in

invertebrate Invertebrates are a paraphyletic group of animals that neither possess nor develop a vertebral column (commonly known as a ''backbone'' or ''spine''), derived from the notochord. This is a grouping including all animals apart from the chordate ...

populations and a greater diversity of smaller ''

Littorina ''Littorina'' is a genus of small sea snails, marine gastropod molluscs in the family Littorinidae, the winkles or periwinkles.WoRMS (2011). Littorina Férussac, 1822. Accessed through: World Register of Marine Species at http://www.marinespe ...

'' snails. The increased numbers of small prey items makes the affected rocky coasts more attractive to birds such as the

purple sandpiper The purple sandpiper (''Calidris maritima'') is a small shorebird in the sandpiper family Scolopacidae. This is a hardy sandpiper that breeds in the arctic and subarctic regions of Eurasia and North America and winters further south on the Atlant ...

and

European rock pipit The European rock pipit (''Anthus petrosus''), or just rock pipit, is a species of small passerine bird that breeds in western Europe on rocky coasts. It has streaked greyish-brown upperparts and buff underparts, and is similar in appearance to ...

. This is likely to facilitate transmission of ''P. acanthus'' to its definitive host.

References

Parasites of molluscs Molluscs described in 1907 Plagiorchiida {{Trematoda-stub