''Thyonicola americana'' is a

species In biology, a species is the basic unit of classification and a taxonomic rank of an organism, as well as a unit of biodiversity. A species is often defined as the largest group of organisms in which any two individuals of the appropriate s ...

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 ha ...

sea snail, a marine gastropod mollusc in the

family Family (from la, familia) is a group of people related either by consanguinity (by recognized birth) or affinity (by marriage or other relationship). The purpose of the family is to maintain the well-being of its members and of society. Idea ...

Eulimidae Eulimidae is a family of very small parasitic sea snails, marine gastropod mollusks in the superfamily Vanikoroidea. Description These small parasitic snails live on (or in some cases in) the bodies of echinoderms such as sea cucumbers ...

. It infests the sea cucumbers '' Eupentacta quinquesemita'' and '' Eupentacta pseudoquinquesemita'' in

Puget Sound Puget Sound ( ) is a sound of the Pacific Northwest, an inlet of the Pacific Ocean, and part of the Salish Sea. It is located along the northwestern coast of the U.S. state of Washington. It is a complex estuarine system of interconnected ma ...

and other parts of the northeastern Pacific Ocean.

Description

The adult female ''Thyonicola americana'' bears little resemblance to a mollusc, having no shell and a coiled worm-like form. While living in the visceral tissues of its host, its central cavity is connected to the large intestine lumen of its host by a thin stalk, inside which is a tubule lined with cilia. The larvae are recognisable as mollusc larvae and have a shell and foot but no

velum Velum may refer to: Human anatomy * Superior medullary velum, anterior medullary velum or valve of Vieussens, white matter, in the brain, which stretches between the superior cerebellar peduncles ** Frenulum of superior medullary velum, a sligh ...

. They are benthic and move by crawling.

Life cycle

A female ''T. americana'' larva settles on a suitable

host A host is a person responsible for guests at an event or for providing hospitality during it. Host may also refer to: Places * Host, Pennsylvania, a village in Berks County People *Jim Host (born 1937), American businessman * Michel Host ...

and undergoes metamorphosis into a juvenile which makes its way into the host's gut, possibly through the cloaca, and penetrates the gut wall. The adult female is a worm-like organism which when uncurled can be up to long. The male larva enters the central cavity of the adult female and undergoes metamorphosis into a dwarf adult; it then atrophies apart from its testicular tissues which fertilise the eggs produced by the female. After taking about six months to mature, the parasite can reproduce continually, reaching peak reproduction in late summer. As it matures, the ovaries develop and the interior cavity begins to accumulate egg capsules containing eggs and developing larvae. There may be about 500 capsules each containing 75 to 150 larvae. The larvae are liberated into the host's gut sequentially as they mature. In the autumn, most host sea cucumbers eviscerate, growing a new gut in the spring. Not all individuals eviscerate, but when this occurs, the parasite is expelled (due to its attachment to the gut wall) and dies, and any remaining egg capsules are liberated into the open sea. The parasite seems to be present only in sea cucumbers with entire guts, and more than one parasite may be present in one host.

Ecology

The adult ''T. americana'' lives attached to the hind third of the intestine of the sea cucumber '' Eupentacta quinquesemita'', a common holothurian in the Pacific Northwest, and also to the closely related '' Eupentacta pseudoquinquesemita''; in

, 38% of the latter were found to be parasitised. ''E. quinquesemita'' exhibits a seasonal evisceration, expelling its guts in the autumn and growing a new set in the spring. However evisceration of the host results in the death of the parasite, and when this happens many individuals will not have reached full maturity and will fail to complete their life cycles.

References

{{Taxonbar, from=Q4938421 Eulimidae Gastropods described in 1961