Thyonicola americana

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Thyonicola americana
Scientific classification
Kingdom: Animalia
Phylum: Mollusca
Class: Gastropoda
(unranked):clade Caenogastropoda
Superfamily: Eulimoidea
Family: Eulimidae
Genus: Thyonicola
Species:T. americana
Binomial name
Thyonicola americana
Tikasingh, 1961 [1]

Thyonicola americana is a species of parasitic sea snail, a marine gastropod mollusc in the family Eulimidae. It infests the sea cucumbers Eupentacta quinquesemita and Eupentacta pseudoquinquesemita in Puget Sound and other parts of the northeastern Pacific Ocean. [2]

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 sexes or mating types can produce fertile offspring, typically by sexual reproduction. Other ways of defining species include their karyotype, DNA sequence, morphology, behaviour or ecological niche. In addition, paleontologists use the concept of the chronospecies since fossil reproduction cannot be examined. While these definitions may seem adequate, when looked at more closely they represent problematic species concepts. For example, the boundaries between closely related species become unclear with hybridisation, in a species complex of hundreds of similar microspecies, and in a ring species. Also, among organisms that reproduce only asexually, the concept of a reproductive species breaks down, and each clone is potentially a microspecies.

Parasitism Interaction between two organisms living together in more or less intimate association in a relationship in which association is disadvantageous or destructive to one of the organisms

In evolutionary biology, parasitism is a relationship between species, where one organism, the parasite, lives on or in another organism, the host, causing it some harm, and is adapted structurally to this way of life. The entomologist E. O. Wilson has characterised parasites as "predators that eat prey in units of less than one". Parasites include protozoans such as the agents of malaria, sleeping sickness, and amoebic dysentery; animals such as hookworms, lice, mosquitoes, and vampire bats; fungi such as honey fungus and the agents of ringworm; and plants such as mistletoe, dodder, and the broomrapes. There are six major parasitic strategies of exploitation of animal hosts, namely parasitic castration, directly transmitted parasitism, trophically transmitted parasitism, vector-transmitted parasitism, parasitoidism, and micropredation.

Sea snail common name for snails that normally live in saltwater

Sea snail is a common name for snails that normally live in salt water, in other words marine gastropods. The taxonomic class Gastropoda also includes snails that live in other habitats, such as land snails and freshwater snails. Many species of sea snails are edible and exploited as food sources by humans.



The adult female Thyonicola americana bears little resemblance to a mollusc, having no shell and a coiled worm-like form. [3] 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. They are benthic and move by crawling. [2]

Lumen (anatomy) cavity within an organ

In biology, a lumen is the inside space of a tubular structure, such as an artery or intestine. It comes from Latin lumen, meaning 'an opening'.

Benthos community of organisms which live on, in, or near the seabed

Benthos is the community of organisms that live on, in, or near the seabed, also known as the benthic zone. This community lives in or near marine sedimentary environments, from tidal pools along the foreshore, out to the continental shelf, and then down to the abyssal depths.

Life cycle

A female T. americana larva settles on a suitable 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 200 mm (8 in) 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. [2] 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. [2]

Host (biology) organism that harbours another organism

In biology and medicine, a host is an organism that harbours a parasitic, a mutualistic, or a commensalist guest (symbiont), the guest typically being provided with nourishment and shelter. Examples include animals playing host to parasitic worms, cells harbouring pathogenic (disease-causing) viruses, a bean plant hosting mutualistic (helpful) nitrogen-fixing bacteria. More specifically in botany, a host plant supplies food resources to micropredators, which have an evolutionarily stable relationship with their hosts similar to ectoparasitism. The host range is the collection of hosts that an organism can use as a partner.

Metamorphosis profound change in body structure during the postembryonic development of an organism

Metamorphosis is a biological process by which an animal physically develops after birth or hatching, involving a conspicuous and relatively abrupt change in the animal's body structure through cell growth and differentiation. Metamorphosis is iodothyronine-induced and an ancestral feature of all chordates. Some insects, fishes, amphibians, mollusks, crustaceans, cnidarians, echinoderms, and tunicates undergo metamorphosis, which is often accompanied by a change of nutrition source or behavior. Animals that go through metamorphosis are called metamorphoses. Animals can be divided into species that undergo complete metamorphosis ("holometaboly"), incomplete metamorphosis ("hemimetaboly"), or no metamorphosis ("ametaboly").

In animal anatomy, a cloacakloh-AY-kə is the posterior orifice that serves as the only opening for the digestive, reproductive, and urinary tracts of many vertebrate animals, opening at the vent. All amphibians, birds, reptiles, and a few mammals have this orifice, from which they excrete both urine and feces; this is in contrast to most placental mammals, which have two or three separate orifices for evacuation. Excretory openings with analogous purpose in some invertebrates are also sometimes referred to as cloacae. Mating by cloaca is known as cloacal copulation, mostly referred to as cloacal kiss.


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, [2] and also to the closely related Eupentacta pseudoquinquesemita ; in Puget Sound, 38% of the latter were found to be parasitised. [3] 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. [2]

<i>Eupentacta quinquesemita</i> species of echinoderm

Eupentacta quinquesemita is a species of sea cucumber, a marine invertebrate with an elongated body, a leathery skin and tentacles surrounding the mouth. It is commonly known as the stiff-footed sea cucumber or white sea cucumber, and occurs on rocky coasts in the northeastern Pacific Ocean.

Puget Sound sound along the northwestern coast of the U.S. state of Washington

Puget Sound is a sound along the northwestern coast of the U.S. state of Washington, an inlet of the Pacific Ocean, and part of the Salish Sea. It is a complex estuarine system of interconnected marine waterways and basins, with one major and two minor connections to the open Pacific Ocean via the Strait of Juan de Fuca—Admiralty Inlet being the major connection and Deception Pass and Swinomish Channel being the minor.

Evisceration is a method of autotomy involving the ejection of internal organs used by animals as a defensive strategy. Sea cucumbers (Holothuroidea) eject parts of the gut in order to scare and defend against potential predators such as crabs and fish. The organs are regenerated in a few days by cells in the interior of the sea cucumber.

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