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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