'' Parorchis acanthus'' is a

parasitic Parasitism is a close relationship between species, where one organism, the parasite, lives (at least some of the time) on or inside another organism, the host, causing it some harm, and is adapted structurally to this way of life. The ent ...

flatworm Platyhelminthes (from the Greek language, Greek πλατύ, ''platy'', meaning "flat" and ἕλμινς (root: ἑλμινθ-), ''helminth-'', meaning "worm") is a Phylum (biology), phylum of relatively simple bilaterian, Segmentation (biology), ...

of the

class Class, Classes, 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 d ...

Trematoda Trematoda is a class of flatworms known as trematodes, and commonly as flukes. They are obligate internal parasites with a complex life cycle requiring at least two hosts. The intermediate host, in which asexual reproduction occurs, is a mol ...

. 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. This is a ...

''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 ...

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

bivalves Bivalvia () or bivalves, in previous centuries referred to as the Lamellibranchiata and Pelecypoda, is a class of aquatic molluscs (marine and freshwater) that have laterally compressed soft bodies enclosed by a calcified exoskeleton consis ...

. 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 ...

. Survival and successful encystment can be reduced by the presence of epibionts, such as the

barnacle Barnacles are arthropods of the subclass (taxonomy), subclass Cirripedia in the subphylum Crustacean, Crustacea. They are related to crabs and lobsters, with similar Nauplius (larva), nauplius larvae. Barnacles are exclusively marine invertebra ...

Semibalanus balanoides ''Semibalanus balanoides'' (common barnacle, common rock barnacle, or northern rock barnacle) is a common and widespread boreo-arctic species of acorn barnacle. It is common on rocks and other substrates in the intertidal zone of north-western ...

, on the shell. Castration of the snail

mollusc Mollusca is a phylum of protostome, protostomic invertebrate animals, whose members are known as molluscs or mollusks (). Around 76,000 extant taxon, extant species of molluscs are recognized, making it the second-largest animal phylum ...

an host can result in a decline in host numbers and, in turn, can lead to more ungrazed

alga Algae ( , ; : alga ) is an informal term for any organisms of a large and diverse group of photosynthetic organisms that are not plants, and includes species from multiple distinct clades. Such organisms range from unicellular microalgae, suc ...

e, with corresponding increases in

invertebrate Invertebrates are animals that neither develop nor retain a vertebral column (commonly known as a ''spine'' or ''backbone''), which evolved from the notochord. It is a paraphyletic grouping including all animals excluding the chordata, chordate s ...

populations and a greater diversity of smaller ''

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

'' 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 Atlan ...

and

European rock pipit The European rock pipit (''Anthus petrosus''), or simply 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 ...

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

References

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