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Gigantorhynchus is a genus of spiny-headed (or thorny-headed) worms from the family Gigantorhynchidae that parasitize marsupials, anteaters, and a possibly a baboon by attaching themselves to the intestines using their hook-covered proboscis. The intermediate host includes termites. This genus is characterized by a cylindrical proboscis with a crown of robust hooks at the apex followed by numerous small hooks on the rest of the proboscis, a long body with pseudo-segmentation, filiform lemnisci, and ellipsoid testes. Gigantorhynchus is considered a well-supported monophyletic group with the related Mediorhynchus genus in the Gigantorhynchidae family based on genetic analysis. There are six species in this genus distributed across Central and South America and possibly Zimbabwe.

Taxonomy
The name Gigantorhynchus was chosen based on the large size and characteristic proboscis in this genus of Acanthocephala. Phylogenetic analysis has been conducted on only one species in the genus, G. echinodiscus, using the gene for 28S ribosomal RNA and confirms that this species forms a monophyletic group with the related Mediorhynchus genus in the Gigantorhynchidae family. The type species is G. echinodiscus.

Description
The genus Gigantorhynchus is characterized by the presence of a cylindrical proboscis with a crown of robust hooks at the apex followed by numerous small hooks on the rest of the proboscis. The body is long with pseudo-segmentation, the lemnisci are filiform, and the testes are ellipsoid. Species of Gigantorhynchus are distinguished based on the number and size of hooks on the crown of the proboscis, the type of pseudosegmentation, and size of the ellipsoid eggs. Males of all species possess eight cement glands which are used to temporarily close the posterior end of the female after copulation. There is pronounced sexual dimorphism in the trunk lengths which are often two or more times longer in the female than the male.

Species
There are six valid species in the genus Gigantorhynchus, although one species, G. pasteri, appears to be incorrectly assigned.


 * Gigantorhynchus echinodiscus (Diesing, 1851)

G. echinodiscus is a tropical parasite of anteaters including the giant anteater (Myrmecophaga tridactyla), the southern tamandua (Tamandua tetradactyla) and the silky anteater Cyclopes didactylus. It has been found in Brazil, Venezuela, Panama, and Trinidad Island. Intermediate hosts include two species of termites from Brazil (Labiotermes emersoni and Orthognathotermes heberi) that showed head shape abnormalities and discolouration when infested with G. echinodiscus in an encysted larval state. It is the first species to be described in the genus Gigantorhynchus and is the type species.

Morphological traits used to distinguish the species include a cylindrical proboscis with a crown having 18 large hooks followed by 21 to 23 small rootless spines arranged in two longitudinal rows. The first row has six hooks measuring between 0.16 and 0.23 mm from the tip of the hook to the root. The second row has 12 hooks in pairs which are smaller than first row measuring between 0.18 and 0.19 mm from the tip of the hook to the root. The crown is separated from numerous small, rootless spines by a short space without hooks. Twenty-one to twenty-three small, rootless spines are arranged in longitudinal rows 0.05 to 0.08 mm long. The lemnisci reach the middle of the trunk and are sometimes bent back on themselves. Other traits include a lateral papilla at the base of the proboscis, a small region (2.24 to 3.21 mm long) after the proboscis with no segmentation, a ringed with no complete segmentation, large testes, eight cement glands in pairs occupying a region measuring between 0.98 and 2.13 mm long and between 0.45 and 0.76 mm wide, and a non-segmented region in the posterior end of the body.

Females have a body length over twice that of males (75.45 mm compared to 31.53 mm). The proboscis receptacle is between 0.48 and 0.64 mm long and between 0.21 and 0.32 mm wide in the male and between 0.63 and 0.74 mm long and between 0.23 and 0.31 mm wide in the female. The male has two ellipsoid testes that are narrow and in tandem. The posterior end after the anterior testes without a segmentation region measures between 5.45 and 8.53 mm. In the female, the gonopore is subterminal and the vagina has a sinuous lateral region in a “guitar” format. The genital pore including the vagina, uterus, and uterine bell is between 0.69 and 0.97 mm long. The eggs contain three membranes.


 * Gigantorhynchus lopezneyrai Diaz-ungria, 1958

G. lopezneyrai has been found parasitizing the Southern tamandua (Tamandua tetradactyla) in Venezuela. The trunk is slightly segmented and no female measurements are known. There are 12 hooks on the proboscis (4 in the first circle each around 0.235 mm long, and 8 in the second circle each around 0.106 mm long). The eight cement glands are organized in pairs. There are doubts about the validity of this species raised by Amato (2014) who suggests that the hook number and arrangement is an incorrect observation that needs to be revisited as no drawings of the proboscis showing the hook formation was published. This species is named in honour of Carlos Rodríguez López-Neyra de Gorgot, a Spanish parasitologist.


 * Gigantorhynchus lutzi Machado, 1941

G. lutzi has been found parasitising a Bare-tailed woolly opossum (Caluromys philander) and the Common opossum (Didelphis marsupialis) in Pará, Brazil and Huanuco, Peru. It was the second species in the genus Gigantorhynchus to be described. The body is ringed with no complete segmentation. There are 12 hooks on the proboscis (six in the first circle each measuring 0.285 by 0.165 mm, and six in the second circle measuring 0.225 by 0.135 mm). The eight cement glands are organized in pairs.


 * Gigantorhynchus ortizi Sarmiento, 1954

G. ortizi has been found infesting the intestines of the Brown four-eyed opossum (Metachirus nudicaudatus) and possibly a White-bellied slender opossum (Marmosops cf. noctivagus) both between Iquitos and Nauta in Peru. Another survey found nearly 100% of the Brown four-eyed opossum were infected with this parasite in the Darien Province of Panama and the Departments of Chocó, Meta, and Nariño in Colombia. It was named in honour of Dr. Javier Ortiz de La Puente, a Peruvian ornithologist from the Museum of the University of San Marcos, Lima, Peru who collected the brown four-eyed opossum from La Merced, Junin, Peru, which later was determined to have been infested with this new species of acanthocephalan. The trunk pseudosegmented or slightly segmented. There are 12 hooks on the proboscis (six in the first circle each measuring 0.160 by 0.10 mm, and six in the second circle measuring 0.140 by 0.09 mm). The proboscis receptacle is 0.750 to 0.920 mm long. The eight cement glands are organized in groups.


 *  Gigantorhynchus pasteri Tadros, 1966

G. pasteri was recorded from an unknown baboon species in Rhodesia (now Zimbabwe). Gomes (2019) considers this species to be incorrectly assigned due to a lack of information including missing registration number and deposit of specimen in a collection, missing type host species, and the description being based on only two immature females. Of the two immature female specimens: the proboscis has 4 hooks and the proboscis receptacle was 0.75 mm by between 0.18 and 0.2 mm in size.


 * Gigantorhynchus ungriai Antonio, 1958

G. ungriai has been found parasitizing a Southern tamandua (Tamandua tetradactyla) in Guayo, Delta Amacuro, Venezuela. The body is ringed and has a cylindrical shape with a complete segmentation consisting of a union in dorsal and ventral regions. The anterior end without segmentation measures 2 to 2.6 mm long. The retractable proboscis has 18 hooks arranged in two circular rows. The first row has six hooks that are 0.140 to 0.2 mm long and the second row has 12 hooks that are 0.104 to 0.180 mm long. The eight cement glands occupy a space of 0.869 by 0.1896 mm. The female genital tract made of an ovary-uterus extends throughout the length of the body. The male genitals occupies one quarter of the length of the body and contains elliptical testicles and eight peripheral prostate glands. The species name was named after Carlos Díaz Ungría.

Hosts
Gigantorhynchus species infest marsupials and myrmecophagids (anteaters) in Central and South America and possibly a baboon from Africa. They are found in the intestines. The intermediate host includes termites.