''Halipegus eccentricus'' is a monoecious, digenea

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 ...

trematode 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 ...

commonly found in

true frog True frogs is the common name for the frog family Ranidae. They have the widest distribution of any frog family. They are abundant throughout most of the world, occurring on all continents except Antarctica. The true frogs are present in North Am ...

s in North America. It was first described in 1939. ''H. eccentricus'' is mainly found in the

Eustachian tube In anatomy, the Eustachian tube, also known as the auditory tube or pharyngotympanic tube, is a tube that links the nasopharynx to the middle ear, of which it is also a part. In adult humans, the Eustachian tube is approximately long and in ...

s of a variety of frog species, its definitive

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 ...

, although its life cycle involves other hosts, as is common for trematodes. Earlier research proposed that its life cycle involved two other species of hosts (

ostracod Ostracods, or ostracodes, are a class of the Crustacea (class Ostracoda), sometimes known as seed shrimp. Some 70,000 species (only 13,000 of which are extant) have been identified, grouped into several orders. They are small crustaceans, typic ...

s and

snail A snail is, in loose terms, a shelled gastropod. The name is most often applied to land snails, terrestrial pulmonate gastropod molluscs. However, the common name ''snail'' is also used for most of the members of the molluscan class G ...

s); however, subsequent research has revealed that the

nymph A nymph ( grc, νύμφη, nýmphē, el, script=Latn, nímfi, label=Modern Greek; , ) in ancient Greek folklore is a minor female nature deity. Different from Greek goddesses, nymphs are generally regarded as personifications of nature, are ty ...

form of the

damselfly Damselflies are flying insects of the suborder Zygoptera in the order Odonata. They are similar to dragonflies, which constitute the other odonatan suborder, Anisoptera, but are smaller and have slimmer bodies. Most species fold the wings alo ...

is also involved.

Population dynamics and distribution

There is a positive correlation between numbers of ''H. eccentricus'' and frog size and age. The maximum infrapopulation density that a single frog can support is twelve, although most infected frogs have only five or six worms. Seasonal patterns of prevalence of the parasite in one of its intermediate hosts, ''

Physa gyrina ''Physa'' is a genus of small, left-handed or sinistral, air-breathing freshwater snails, aquatic pulmonate gastropod mollusks in the subfamily Physinae of the family Physidae.MolluscaBase eds. (2021). MolluscaBase. Physa Draparnaud, 1801. Acc ...

'', have been observed to peak in May through July and to decline in autumn, which coincides with the decline of leaf litter.

Morphology

The adult ''H. eccentricus'' is an orange-brown elongated worm approximately long and wide, with the greatest width at the posterior end. It has a subterminal oral sucker and a short esophagus. When laid, the eggs are fully embryonated, straw colored, and operculated. Each egg contains a single recurved filament. The cercariae are cystophorous, while the metacercariae have two

flame cell A flame cell is a specialized excretory cell found in the simplest freshwater invertebrates, including flatworms, rotifers and nemerteans; these are the simplest animals to have a dedicated excretory system. Flame cells function like a kidney, re ...

s, a large acetabulum, and a spiny cuticula with eight pen-shaped apical hooks, which are used to penetrate the snail intestines. ''H. eccentricus'' can be differentiated from the related species '' H. occidualis'' depending on where the adult is found inside of frogs; ''H. eccentricus'' is found primarily in the Eustachian tubes whereas ''H. occidualis'' is found primarily in the esophagus and under the tongue. ''H. eccentricus'' overwinter in their amphibian hosts.

Life cycle

Metamorphosed anurans become infected with ''H. eccentricus'' by feeding on metacercariae-infected

damselflies Damselflies are flying insects of the suborder Zygoptera in the order Odonata. They are similar to dragonflies, which constitute the other odonatan suborder, Anisoptera, but are smaller and have slimmer bodies. Most species fold the wings along ...

or microcrustaceans (''

Cypridopsis ''Cypridopsis'' is a genus of ostracods, belonging to the family Cyprididae. The genus was described in 1867 by George Stewardson Brady. The genus has cosmopolitan distribution. Species include: * ''Cypridopsis bamberi'' Henderson, 1986 * ''Cy ...

'' sp.). Adult worms are released in the stomach of anurans, migrate to the Eustachian tubes within 32–39 days post-exposure (DPE), and release eggs into the feces 50-60 DPE. ''Physa gyrina'' or ''

Planorbella trivolvis ''Planorbella trivolvis'' is a species of freshwater air-breathing snail, an aquatic pulmonate gastropod mollusk in the family Planorbidae, the ram's horn snails, or planorbids, which all have sinistral or left-coiling shells. Description All ...

'' snails subsequently ingest the eggs. Inside of the snail intestine, the eggs undergo a series of development stages culminating in the cystophorous cercariae (32–35 DPE). The cercariae pass into the feces and infect copepod and ostracod microcrustaceans, the second intermediate hosts, where they develop in the hemocel into metacercariae. 15 to 19-day-old metacercariae from ostracods are infective to both damselfly larvae and metamorphosed anurans.

References

{{DEFAULTSORT:Halipegus Eccentricus Plagiorchiida Animals described in 1939