Overview

Brief Summary

Biology

Thorny-headed worms, or acanthocephalans, are parasites that live in the gut of vertebrates and - earlier in their life cycle - within invertebrates. The thorny protrusible proboscis is globular or cylindrical; the body also may bear spiny thorns. These parasitic worms anchor themselves with their proboscis and body spines to the gut wall of a host.
  • Margulis, L.; Schwartz, K.V. (1998). Five Kingdoms: an illustrated guide to the Phyla of life on earth. 3rd edition. Freeman: New York, NY (USA). ISBN 0-7167-3027-8. xx, 520 pp.
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Source: World Register of Marine Species

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The roughly 1100 species of described acanthocephalans are obligate intestinal parasites of vertebrates. Teleost (bony) fishes are especially well represented in the list of acanthocephalan hosts, but known hosts also include birds, mammals, amphibians, and reptiles. Larval development takes place in intermediate arthropod hosts. The name "Acanthocephala" is derived from Greek roots meaning "spiny head" and refers to the set of recurved hooks on the eversible proboscis at the anterior end of the worm. Most acanthocephalans are less than 20 cm long, although a few exceed 60 cm. Females are generally larger than males. As a consequence of their lifestyle, the digestive tract has been completely lost and most other organ systems are notably reduced, with the exception of the reproductive system. (Brusca and Brusca 2003; Margulis and Chapman 2010)

Adult acanthocephalans attach to their host's intestinal wall with their retractable proboscis hooks, which can be pulled back into pockets like the claws of a cat. Nutrients, gases, and waste products are exchanged through the body wall. Acanthocephalans are dioecious (i.e., sexes are separate). Males have a set of six or eight cement glands, the secretions of which plug the female genital pore after copulation, which is accomplished with an eversible penis that the male inserts into the female's vagina. The vagina leads to an elongate uterus that terminates in a complex open funnel known as the uterine bell. Much of the early development of acanthocephalans takes place within the female's body cavity. Sufficiently developed embryos pass through the uterus and out the genital pore, eventually exiting to the outside world in the host's feces. Once outside the definitive host, the developing acanthocephalan must be ingested by an arthropod intermediate host to continue its life cycle.

Known intermediate hosts for acanthocephalans with terrestrial life cycles include insects (especially Coleoptera and Orthoptera), terrestrial isopods, and millipedes (e.g., Richardson 2006). Intermediate hosts for acanthocephalans with aquatic life cycles are typically decapods or other crustaceans. Although no acanthocephalan life cycles are known to include more than one intermediate arthropod host, there may sometimes be a paratenic vertebrate host in addition to the definitive vertebrate host. (In contrast to an intermediate host, a paratenic host is not required for parasite development; the definitive host is the host in which the parasite matures and reproduces.) In the paratenic host, the worm penetrates the intestinal wall and localizes in the mesenteries or viscera, where it remains in the infective cystacanth stage. Although the paratenic vertebrate host is apparently not necessary for the acanthocephalan's development, it may nevertheless play an important role in moving the parasite to the next trophic level so that it can complete its life cycle. Acanthocephalan species vary in their degree of host specificity, but in at least some cases, taxa that appear to be very catholic in their choice of hosts may actually represent species complexes (Stein et al. 2007). (Brusca and Brusca 2003; Margulis and Chapman 2010)

As with many parasites, hosts infected by certain acanthocephalans may exhibit modified behavior that increases the likelihood of being eaten (e.g., cockroaches inected with the acanthocephalan Moniliformis moniliformis move more slowly than uninfected cockroaches) (Margulis and Chapman 2010).

The Acanthocephala were formerly believed to be sister to the Rotifera. A variety of analyses now strongly suggest that the Acanthocephala are in fact a clade of parasitic rotifers, most likely sister to the free-living bdelloids (Garcia-Varela and Nadler 2006; Min and Park 2009; Witek et al. 2009).

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Comprehensive Description

The roughly 1100 species of described acanthocephalans are obligate intestinal parasites of vertebrates. Teleost (bony) fishes are especially well represented in the list of acanthocephalan hosts, but known hosts also include birds, mammals, amphibians, and reptiles. Larval development takes place in intermediate arthropod hosts. The name "Acanthocephala" is derived from Greek roots meaning "spiny head" and refers to the set of recurved hooks on the eversible proboscis at the anterior end of the worm. The remainder of the body forms a cylindrical or flattened trunk, often bearing rings of small spines. Most acanthocephalans are less than 20 cm long, although a few exceed 60 cm. Females are generally larger than males. As a consequence of their lifestyle, the digestive tract has been completely lost and most other organ systems are notably reduced, with the exception of the reproductive system. (Brusca and Brusca 2003; Margulis and Chapman 2010)

Adult acanthocephalans attach to their host's intestinal wall with their retractable proboscis hooks, which can be pulled back into pockets like the claws of a cat. Nutrients, gases, and waste products are exchanged through the body wall. Acanthocephalans are dioecious (i.e., sexes are separate). Males have a set of six or eight cement glands, the secretions of which plug the female genital pore after copulation, which is accomplished with an eversible penis that the male inserts into the female's vagina. The vagina leads to an elongate uterus that terminates in a complex open funnel known as the uterine bell. Much of the early development of acanthocephalans takes place within the female's body cavity. Eventually a shelled "acanthor larva" develops. Developing embryos are apparently "sorted" by the muscular funnel of the uterine bell, which permits only embryos that are sufficiently developed to pass through the uterus and out the genital pore, eventually exiting to the outside world in the host's feces. Once outside the definitive host, the developing acanthocephalan must be ingested by an arthropod intermediate host to continue its life cycle. The acanthor larva penetrates the gut wall of the intermediate host and enters the body cavity, where it eventually develops into an encapsulated form known as a cystacanth.

Known intermediate hosts for acanthocephalans with terrestrial life cycles include insects (especially Coleoptera and Orthoptera), terrestrial isopods, and millipedes (e.g., Richardson 2006). Intermediate hosts for acanthocephalans with aquatic life cycles are typically decapods or other crustaceans. Although no acanthocephalan life cycles are known to include more than one intermediate arthropod host, there may sometimes be a paratenic vertebrate host in addition to the definitive vertebrate host. (In contrast to an intermediate host, a paratenic host is not required for parasite development; the definitive host is the host in which the parasite matures and reproduces.) In the paratenic host, the worm penetrates the intestinal wall and localizes in the mesenteries or viscera, where it remains in the infective cystacanth stage. Although the paratenic vertebrate host is apparently not necessary for the acanthocephalan's development, it may nevertheless play an important role in moving the parasite to the next trophic level so that it can complete its life cycle. For example, a Centrorhynchus acanthocephalan may be consumed by an intermediate arthropod host which is then eaten by a paratenic insectivore host such as a shrew which is eaten, in turn, by a bird of prey that serves as the definitive host. If the intermediate host is ingested by an appropriate definitive host, the cystacanth attaches to the intestinal wall of the host and matures into an adult. Acanthocephalan species vary in their degree of host specificity, but in at least some cases, taxa that appear to be very catholic in their choice of hosts may actually represent species complexes (Stein et al. 2007). (Brusca and Brusca 2003; Margulis and Chapman 2010)

As with many parasites, hosts infected by certain acanthocephalans may exhibit modified behavior that increases the likelihood of being eaten (e.g., cockroaches inected with the acanthocephalan Moniliformis moniliformis move more slowly than uninfected cockroaches) (Margulis and Chapman 2010).

The Acanthocephala were formerly believed to be sister to the Rotifera, with the two groups together forming a clade that has been referred to as the Syndermata. A variety of analyses now strongly suggest that the Acanthocephala are in fact a clade of parasitic rotifers, most likely sister to the free-living bdelloids (Garcia-Varela and Nadler 2006; Min and Park 2009; Witek et al. 2009), making the name Syndermata a junior synonym of Rotifera (Sørensen and Giribet 2006). Based on a large-scale analysis of molecular data (amino acid sequences), Witek et al. (2009) concluded that Syndermata and Gnathostomulida together comprise a monophyletic clade known as the Gnathifera. The Gnathifera was originally proposed as a monophyletic clade based on jaw morphology and may also include the Micrognathozoa and, conceivably, the Cycliophora (Funch et al. 2005; Sørensen and Giribet 2006).

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Molecular Biology and Genetics

Molecular Biology

Statistics of barcoding coverage

Barcode of Life Data Systems (BOLD) Stats
                                        
Specimen Records:428Public Records:334
Specimens with Sequences:351Public Species:15
Specimens with Barcodes:252Public BINs:45
Species:19         
Species With Barcodes:18         
          
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Source: Barcode of Life Data Systems (BOLD)

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Barcode data

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Source: Barcode of Life Data Systems (BOLD)

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Locations of barcode samples

Collection Sites: world map showing specimen collection locations for Acanthocephala

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Source: Barcode of Life Data Systems (BOLD)

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Genomic DNA is available from 3 specimens with morphological vouchers housed at Florida Museum of Natural History and Raffles Museum, Singapore
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© Ocean Genome Legacy

Source: Ocean Genome Resource

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