AcanthocephalaDJ Richardson, Quinnipiac University, Hamden, Connecticut, USA

Based in part on the previous version of this eLS article ‘Acanthocephala’(2001) by DWT Crompton.

The phylum Acanthocephala is comprised of more than

1000 species of pseudoceolomic helminths, which, as

adults, occur exclusively in the vertebrate small intestine.

The most commonly parasitised definitive hosts are bony

fishes, followed by birds, mammals and rarely amphibians

and reptiles. Acanthocephalans are characterised by the

possession of a head called a proboscis bearing hooks and

spines that enable them to attach to the intestinal wall of

their definitive host. Acanthocephalans are dioecious and

exhibit sexual dimorphism. As an adaptation to parasit-

ism, acanthocephalans have secondarily lost their

digestive system and acquire their nutrients by direct

absorption across the body wall. Acanthocephalans are

primarily osmoconformers, and respiration and excretion

occur primarily by diffusion across the body wall. All

acanthocephalans exhibitan indirect life cycle utilisingan

arthropod intermediate host. Despite their sometimes

large size, acanthocephalans cause relatively little path-

ology. Although very rare, human infection does occur.

Molecular evidence suggests that Acanthocephalans are

phylogenetically most closely aligned with the rotifers.

Introduction

The phylum Acanthocephala is comprised of approxi-mately 1200 valid species of intestinal worms (Monks andRichardson, 2011). As adults, acanthocephalans occurexclusively in the vertebrate small intestine. Themajority ofspecies occur as adults in bony fish, followed by mammals,reptiles and amphibians. All acanthocephalans utilise anindirect life cycle in which an arthropod serves as inter-mediate host. Acanthocephalans are variable in size withadults ranging from 2mm to nearly 1m long. See also:Parasitism: The Variety of Parasites

Structure and Function

Acanthocephalans (Greek acantha, spine or thorn+kephale, head) get their name from the conspicuous pro-boscis or head that is covered with hooks and/or spines.Figure 1 illustrates the diversity of form exhibited by pro-boscides of various acanthocephalans. The retractable andeversible head or proboscis of an acanthocephalan func-tions as a holdfast structure,which anchors theworm to theintestinal wall of the vertebrate host. The number andarrangement of hooks and shape of the proboscis are theprimary characters used to distinguish one species fromanother. Some acanthocephalans possess spines on thetrunk that presumably aid in gripping the gut wall andlocomotion. Acanthocephalans are dioecious and sexuallydimorphic.The proboscis, proboscis receptacle (muscular sac into

which the proboscis is withdrawn) and the paired lemnisciconstitute the praesoma, whereas the portion of the bodyexposed to the host’s intestinal lumen is known as themetasoma. Acanthocephalans have no mouth or digestivesystem, although it is presumed that they arose from anancestor with a digestive system. Like tapeworms, theyhave secondarily lost their digestive system as an adap-tation to parasitism. They absorb predigested nutrientsdirectly across their bodywall, called a tegument. In adults,the body wall consists of a network of cavities called thelacunar system, which presumably serves as a transportsystem.The most conspicuous organs within the acantho-

cephalan body are the reproductive organs. Reproductiveorgans are containedwithin and supported by the ligamentsac or sacs.Male worms possess paired testes and a cementgland or glands. At copulation, the male extrudes a bursa,which encloses the posterior of the female, sperms aretransferred and a copulatory cap of secretions from thecement gland(s) is deposited over the genital opening of thefemale.In the female, ovarian tissue grows and divides so that by

maturity the body cavity contains numerous free-floatingovarian balls. Each ovarian ball consists of an oogonialsyncytium giving rise to mature oocytes and a supportingsyncytium providing nutritional and mechanical supportfor the germ cells and zygotes. Fertilisation occurs in theovarian ball. Zygotes develop there until they are shed into

Introductory article

Article Contents

. Introduction

. Structure and Function

. Classification

. Life Cycle

. Pathology and Immunology

. Evolutionary Relationships

Online posting date: 15th March 2013

eLS subject area: Evolution & Diversity of Life

How to cite:Richardson, DJ (March 2013) Acanthocephala. In: eLS. John Wiley &Sons, Ltd: Chichester.

DOI: 10.1002/9780470015902.a0001595.pub2

eLS & 2013, John Wiley & Sons, Ltd. www.els.net 1

Figure 1 Proboscides of: Centrorhynchus robustus from a northern spotted owl Strix occidentalis, upper left, bar=250 mm. Redrawn from Richardson and

Nickol (1995). Polymorphus cucullatus from a hooded merganser Lophodytes cucullatus, upper right, bar5500 mm. Redrawn from Van Cleave and Starrett

(1940). Oligacanthorhynchus tortuosa from a Virginia opossum (Didelphis virginiana), centre, bar5100 mm. Redrawn from Richardson (2006). Mediorhynchus

centurorum from a red-bellied woodpecker Centurus carolinus, lower left, bar5220 mm. Redrawn from Nickol (1969). Plagiorhynchus cylindraceus from an

American robin Turdus migratorius, lower right, bar51 mm. Redrawn from Schmidt and Olsen (1964). & The American Society of Parasitologists.

Acanthocephala

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the body cavity, where embryogenesis is completed.Female acanthocephalans possess a uterine bell, whichsorts the varying stages of developing eggs. From the bellapparatus, sometimes called the selector apparatus, fullyformed eggs pass down the uterus and are released throughthe genital opening, whereas immature eggs are returned tothe body cavity (pseudocoel). Spermatozoa reach oocytesvia the uterus and uterine bell.The nervous system consists of a cerebral ganglion

located in the proboscis receptacle. Nerves extend from thecerebral ganglion to the proboscis and the ventral anddorsal sides of the metasomal body wall. Males contain agenital ganglion that is assumed to be associated withcopulatory function.Distinct excretory and respiratory organs are lacking,

and excretion and gas exchange occur across the bodywall.A few species possess protonephridia, which may play arole in excretion. For the most part, acanthocephalans areosmoconformers.Basic anatomy of a male and female acanthocephalan is

shown in Figure 2 and Figure 3, respectively.

Classification

Monks and Richardson (2011) recognised four classes inthe phylumAcanthocephala. Although other classificationschemes exist for the Acanthocephala, the one given hereinis currently the most widely accepted. The classificationpresented is based primarily on Meyer (1931) as modifiedby Van Cleave (1936), Golvan (1994) and Amin (1987). Aspointed out by Monks and Richardson (2011), the classi-fication of the Acanthocephala is currently a matter ofcontention and will likely undergo major changes in thenear future as more molecular, morphological and eco-logical data become available. Currently recognised classesare as follows:Archiacanthocephala (Meyer, 1931) – Trunk spines

absent, usually eight cement glands, single layer in theproboscis sheath, dorsal and ventral ligament sacs andthick egg shells are present, some species possess protone-phridia, large body size and infect terrestrial vertebratesand insects (occasionally millipedes).

PR

PRM

L

T

CG

SP

CB

P

Figure 2 Male individual of Plagiorhynchus cylindraceus, a common

acanthocephalan of American robins T. migratorius. P, proboscis; PR,

proboscis receptacle; PRM, proboscis retractor muscle; L, lemnisci; T, testes;

CG, cement glands; SP, Saefftigen’s pouch; CB, copulatory bursa.

Bar51 mm.

UB

BA

U

S

VGO

Figure 3 Posterior end of Centrorhynchus microcephalus, from a groove-

billed ani Crotophaga sulcirostris showing the female reproductive system.

UB, uterine bell; BA, bell (selector) apparatus; U, uterus; S, sphincter; V,

vagina; GO, genital opening. Bar51 mm. After Richardson et al. (2010).

&Helminthological Society of Washington.

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Eoacanthocephala (Van Cleave, 1936) – Trunk spinespresent or absent, two to eight cement glands, usually twomuscle layers in the proboscis sheath, single ligamentsac that ruptures and thin egg shells are present, protone-phridia is absent, body size is variable and they mainlyinfect aquatic and some terrestrial vertebrates andcrustaceans.Palaeacanthocephala (Meyer, 1931) – Trunk spines

present or absent, usually one cement gland, single musclelayer in the proboscis sheath, dorsal and ventral ligamentsacs which rupture and thin egg shells are present. They aresmall in size and infect aquatic vertebrates and crustaceans.Polyacanthocephala (Amin, 1987) – Trunk spined,

proboscis claviform with many rows of hooks, elongatecement glands, oval eggs with radial sculpturing at rightangles to surface. They are parasites of fishes andcrocodilia.

Life Cycle

Adult male and female acanthocephalans mate in the ver-tebrate small intestine. Eggs (Figure 4) containing a larvalform, known as an acanthor (Figure 5), are passed in thefaeces of the definitive host. When ingested by a suitableintermediate host, the eggs hatch in the arthropod intestinereleasing the acanthor, which uses hooks to penetrate theintestine. The larval worm comes to lie in the haemocoel ofthe intermediate host where it develops into the cystacanth,which is the infective stage to the vertebrate definitive host.A suitable definitive hostmay become infected by ingesting

the cystacanth contained within an infected intermediatehost. Figure 6 shows a representative life cycle of anacanthocephalan.No acanthocephalan is known to require more than the

single arthropod intermediate host to complete develop-ment to an infective cystacanth, although two other path-ways of infection have been identified. In some instances, aparatenic hostmay become involved in the life cycle. In thisinstance, a second host may become infected by ingestingthe cystacanth contained in an infected arthropod inter-mediate host. However, in a paratenic host, the cystacanthpenetrates the intestinal wall and re-encysts, usually in themesentery, where it remains in the form of a cystacanth. Asuitable definitive host may become infected by ingestingthe cystacanth containedwithin a paratenic host.Althoughthe paratenic host is not physiologically required forcompletion of the life cycle, it may be required to bridge atrophic level in the life cycle (Richardson and Richardson,2009). An example of an acanthocephalan life cycle thatutilises a paratenic host is shown in Figure 7. Anotheravenue of transmission sometimes exhibited by acantho-cephalans is postcyclic transmission (Richardson andAbdo, 2011). In this instance, one definitive host is eaten byanother, and intestinal worms may be transferred directlyfrom definitive host to definitive host.

Figure 4 Infective egg (shelled acanthor) of M. moniliformis from a rat.

Length approximately 100 mm. Photomicrograph by JR Georgi.

Figure 5 Acanthor of M. moniliformis. Length approximately 250 mm.

Photomicrograph by RHF Holt.

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Pathology and Immunology

Despite their occasional large size, there is little overtpathology associated with acanthocephalan infections.Most of the pathology appears to be the result of a chronicinflammatory response to mechanical trauma resultingfrom injury caused by the proboscis, often with subsequentfibrosis and nodule formation, not unlike the inflammatoryresponse elicited by an inanimate irritating body(Richardson and Barnawell, 1995). Although, epizooticssometime occur, occasionally with extensive mortality.Richardson and Nickol (2008) provided a review ofthe pathogenesis and pathology of acanthocephalaninfection. Although acanthocephalans may evoke aprofound humoral immune response, this responseappears not to be protective (Richardson et al., 2008).Although rare, human infection with acanthocephalans

occasionally occurs. Species most commonly infectinghumans are the common acanthocephalans of rats, Mon-iliformis moniliformis and representatives of the genusMacracanthorhynchus, which normally occur as adults inswine and raccoons. Humans may become infected withM. moniliformis or Macracanthorhynchus sp. by theintentional or accidental ingestion of cystacanth contained

within the cockroach or beetle intermediate hosts,respectively.Richardson andBrink (2011) reviewedhumaninfection and treatment.

Evolutionary Relationships

Recent comparative morphological and molecular ana-lyses, based primarily on 18s ribosomal deoxyribonucleicacid (rDNA), suggest that the closest living relatives ofacanthocephalans are rotifers in the class Bdelloidea(Garey et al., 1996, 1998;Giribert et al., 2000;MarkWelch,2000; Segers, 2011). Subsequently, it has been proposedthat the Acanthocephala be included within the phylumRotifera (Sorenson andGiribert, 2006) or that rotifers andacanthocephalans be lumped into a single phylum, Syn-dermata (Giribert et al., 2000), although neither proposalhas been widely accepted. The same molecular studies thatalign acanthocephalans with rotifers also demonstrate thatthe Acanthocephala, which are dramatically distinct fromrotifersmorphologically, constitute amonophyletic group.Other phyla that are closely alliedwith theAcanthocephalaand rotifers include Gnathostomulida (unsegmentedmarine worms) and Cycliophora, the most recently

Millipede intermediate hostingests ova.

Larvae mature in body cavity (hemocoel)and develop into cystacanths.

Ova (with acanthors) are passed in feces;prepatent period 9 weeks.

Adults live and matein small intestine

of opossumdefinitive host.

Opossum becomes infected afteringesting cystacanth within millipede.

Figure 6 Life cycle of O. tortuosa, an acanthocephalan of the Virginia opossum D. virginiana. Drawing by LM Duclos.

Acanthocephala

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described phylum in the animal kingdom consisting ofsymbionts found on the mouth appendages of crustaceansin the northern hemisphere. Most contemporary acan-thocephalan biologists support the retention of the phylumAcanthocephala, although further work is clearly neededto resolve higher level of systematic issues associated withboth acanthocephalans and rotifers. Such studies willundoubtedly lead to major taxonomic revisions. See also:Cycliophora; Gnathostomulida (Unsegmented MarineWorms); Rotifera

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Aschelminthes, and Entoprocta: The Pseudocoelomate Bilateria.

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McGraw Hill.

Petrochenko VI (1956) Acanthocephala of Domestic and Wild

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Roberts LS and Janovy J Jr (2009)GeraldD. Schmidt andLarry S.

Roberts’ Foundations of Parasitology, 8th edn. Boston:

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Van Cleave HJ (1953) Acanthocephala of North American Mam-

mals. Urbana, Illinois: University of Illinois Press.

Yamaguti S (1963)SystemaHelminthum:Vol. V, Acanthocephala.

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