The Acanthocephala were long thought to be a discretephylum. Recentgenome analysis has shown that they are descended from, and should be considered as, highly modifiedrotifers.[10] This unified taxon is sometimes known asSyndermata, or simply as Rotifera, with the acanthocephalans described as a subclass of a rotifer class Hemirotatoria.[11]
The earliest recognisable description of Acanthocephala – a worm with a proboscis armed with hooks – was made by Italian authorFrancesco Redi (1684).[1] In 1771,Joseph Koelreuter proposed the name Acanthocephala.[1]Philipp Ludwig Statius Müller independently called themEchinorhynchus in 1776.[1]Karl Rudolphi in 1809 formally named them Acanthocephala.
Acanthocephalans are highly adapted to a parasitic mode of life, and have lost many organs and structures through evolutionary processes. This makes determining relationships with other higher taxa through morphological comparison problematic. A 2016phylogenetic analysis of the gene order in the mitochondria suggests that Seisonidea and Acanthocephala are sister clades and that the Eurotatoria are the sister clade to this group, producing the cladogram below.[14]
Acanthocephalans lack a mouth oralimentary canal. This is a feature they share with thecestoda (tapeworms), although the two groups are not closely related. Adult stages live in theintestines of their host and uptake nutrients which have beendigested by the host, directly, through their body surface. The acanthocephalans lack an excretory system, although some species have been shown to possessflame cells (protonephridia).
The most notable feature of the acanthocephala is the presence of ananterior, protrudableproboscis that is usually covered with spiny hooks (hence the common name: thorny or spiny headed worm). The proboscis bears rings of recurved hooks arranged in horizontal rows, and it is by means of these hooks that the animal attaches itself to the tissues of its host. The hooks may be of two or three shapes, usually: longer, more slender hooks are arranged along the length of the proboscis, with several rows of more sturdy, shorter nasal hooks around the base of the proboscis. The proboscis is used to pierce the gut wall of the final host, and hold the parasite fast while it completes its life cycle.
Like the body, the proboscis is hollow, and its cavity is separated from the body cavity by aseptum orproboscis sheath. Traversing the cavity of the proboscis aremuscle-strands inserted into the tip of the proboscis at one end and into the septum at the other. Their contraction causes the proboscis to be invaginated into its cavity. The whole proboscis apparatus can also be, at least partially, withdrawn into the body cavity, and this is effected by two retractor muscles which run from the posterior aspect of the septum to the body wall.
Some of the acanthocephalans (perforating acanthocephalans) can insert their proboscis in the intestine of the host and open the way to the abdominal cavity.[16]
The size of these animals varies greatly, ranging from a few millimetres in length toMacracanthorhynchus hirudinaceus, which measures from 10 to 65 centimetres (3.9 to 25.6 in). A curious feature shared by both larva and adult is the large size of many of the cells, e.g. the nerve cells and cells forming the uterine bell.Polyploidy is common, with up to 343n having been recorded in some species.
The body surface of the acanthocephala is peculiar. Externally, the skin has a thintegument covering theepidermis, which consists of asyncytium with nocell walls. The syncytium is traversed by a series of branchingtubules containing fluid and is controlled by a few wandering,amoeboidnuclei. Inside the syncytium is an irregular layer of circular muscle fibres, and within this again some rather scattered longitudinal fibres; there is noendothelium. In their micro-structure the muscular fibres resemble those ofnematodes.
Except for the absence of the longitudinal fibres the skin of the proboscis resembles that of the body, but the fluid-containing tubules of the proboscis are shut off from those of the body. The canals of the proboscis open into a circular vessel which runs round its base. From the circular canal two sac-like projections called thelemnisci run into the cavity of the body, alongside the proboscis cavity. Each consists of a prolongation of the syncytial material of the proboscis skin, penetrated by canals and sheathed with a muscular coat. They seem to act as reservoirs into which the fluid which is used to keep the proboscis "erect" can withdraw when it is retracted, and from which the fluid can be driven out when it is wished to expand the proboscis.
The central ganglion of the nervous system lies behind the proboscis sheath or septum. It innervates the proboscis and projects two stout trunks posteriorly which supply the body. Each of these trunks is surrounded by muscles, and this nerve-muscle complex is called aretinaculum. In the male at least there is also agenitalganglion. Some scatteredpapillae may possibly be sense-organs.
Acanthocephalans have complex life cycles, involving a number of hosts, for bothdevelopmental and resting stages. Complete life cycles have been worked out for only 25 species.[4]
The Acanthocephala aredioecious (an individual organism is either male or female). There is a structure called thegenital ligament which runs from the posterior end of the proboscis sheath to the posterior end of the body. In the male, twotestes lie on either side of this. Each opens in avas deferens which bears threediverticula orvesiculae seminales. The male also possesses three pairs of cement glands, found behind the testes, which pour their secretions through a duct into the vasa deferentia. These unite and end in apenis which opens posteriorly.
In the female, theovaries are found, like the testes, as rounded bodies along the ligament. From the ovaries, masses ofova dehisce into the body cavity, floating in its fluids forfertilization by male's sperm. After fertilization, each egg contains a developingembryo. (These embryos hatch into first stagelarva.) The fertilized eggs are brought into theuterus by actions of theuterine bell, a funnel like opening continuous with the uterus. At the junction of the bell and the uterus there is a second, smaller opening situateddorsally. The bell "swallows" the matured eggs and passes them on into the uterus. (Immature embryos are passed back into the body cavity through the dorsal opening.) From the uterus, mature eggs leave the female's body via heroviduct, pass into the host's alimentary canal and are expelled from the host's body withinfeces.
Having been expelled by the female, the acanthocephalan egg is released along with the feces of the host. For development to occur, the egg, containing theacanthor, needs to be ingested by anarthropod, usually acrustacean (there is one known life cycle which uses amollusc as a first intermediate host). Inside the intermediate host, the acanthor is released from the egg and develops into an acanthella. It then penetrates the gut wall, moves into the body cavity, encysts, and begins transformation into the infectivecystacanth stage. This form has all the organs of the adult save the reproductive ones.
The parasite is released when the first intermediate host is ingested. This can be by a suitable final host, in which case the cystacanth develops into a mature adult, or by aparatenic host, in which the parasite again forms a cyst. When consumed by a suitable final host, the cycstacantexcysts, everts its proboscis and pierces the gut wall. It then feeds, grows and develops its sexual organs. Adult worms then mate. The male uses the excretions of itscement glands to plug thevagina of the female, preventing subsequent matings from occurring. Embryos develop inside the female, and the life cycle repeats.
Thorny-headed worms begin their life cycle inside invertebrates that reside in marine or freshwater systems. One example isPolymorphus paradoxus.Gammarus lacustris, a small crustacean that inhabits ponds and rivers, is one invertebrate thatP. paradoxus may occupy; ducks are one of thedefinitive hosts.
This crustacean is preyed on by ducks and hides by avoiding light and staying away from the surface. However, infection byP. paradoxus changes its behavior and appearance in a number of ways that increase its chance of being eaten. First, infection significantly reducesG. lacustris's photophobia; as a result, it becomes attracted toward light and swims to the surface.[17] Second, an infected organism will even go so far as to find a rock or a plant on the surface, clamp its mouth down, and latch on, making it easy prey for the duck.[17] Finally, infection reduces the pigment distribution and amount inG. lacustris, causing the host to turn blue; unlike their normal brown colour, this makes the crustacean stand out and increases the chance the duck will see it.[18]
Experiments have shown that altered serotonin levels are likely responsible for at least some of these changes in behaviour. One experiment found that serotonin induces clinging behavior inG. lacustris similar to that seen in infected organisms.[19] Another showed that infectedG. lacustris had approximately 3 times as manyserotonin-producing sites in its ventral nerve cord.[20] Furthermore, experiments in closely-related species ofPolymorphus andPomphorhynchus infecting otherGammarus species confirmed this relation: infected organisms were considerably more attracted to light and had higher serotonin levels, while the phototropism could be duplicated by injections of serotonin.[21]
Polymorphus spp. are parasites ofseabirds, particularly theeider duck (Somateria mollissima). Heavy infections of up to 750 parasites per bird are common, causingulceration to the gut, disease and seasonal mortality. Recent research has suggested that there is no evidence ofpathogenicity ofPolymorphus spp. to intermediate crab hosts. The cystacanth stage is long lived and probably remains infectious throughout the life of the crab.[22] In freshwater systems, acanthocephalans have also been shown to accumulate organic micropollutants at concentrations far higher than their crustacean hosts, potentially reducing host pollutant burdens and acting as biological pollutant sinks.[23]
Acanthocephalosis, a disease caused byAcanthacephalus infection, is prevalent in aquaculture, occurring inAtlantic salmon,rainbow andbrown trout,tilapia, andtambaqui.[24] Increasing occurrence in Brazilian farming of tambaqui has been reported,[25] and in 2003Acanthacephalus was first reported in culturedred snapper in Taiwan.[26]
The life cycle ofPolymorphus spp. normally occurs between sea ducks (e.g.eiders andscoters) and small crabs. Infections found in commercial-sizedlobsters in Canada were probably acquired from crabs that form an important dietary item of lobsters. Cystacanths occurring in lobsters can cause economic loss to fishermen. There are no known methods of prevention or control.[27]
Although they rarely infect humans, worms in the phylum Acanthocephala cause the diseaseacanthocephaliasis in humans. The earliest known infection was found in a prehistoric man inUtah.[28] This infection was dated to 1869 ± 160 BC. The species involved was thought to beMoniliformis clarki which is still common in the area.
The first report of an isolate in historic times was byLambl in 1859 when he isolatedMacracanthorhynchus hirudinaceus from a child inPrague. Lindemann in 1865 reported that this organism was commonly isolated inRussia. The reason for this was discovered by Schneider in 1871 when he found that an intermediate host, the scarabaeid beetle grub, was commonly eaten raw.[29]
The first report of clinical symptoms was by Calandruccio who in 1888 while inItaly infected himself by ingesting larvae. He reported gastrointestinal disturbances and shed eggs in two weeks. Subsequent natural infections have since been reported.[30]
^Giribet, Gonzalo; Edgecombe, Gregory D. (2020).The invertebrate tree of life. Princeton University Press.ISBN978-0-691-17025-1.
^Luo, C.; Parry, L. A.; Boudinot, B. E.; Wang, S.; Jarzembowski, E. A.; Zhang, H.; Wang, B. (2025). "A Jurassic acanthocephalan illuminates the origin of thorny-headed worms".Nature:1–7.doi:10.1038/s41586-025-08830-5.
^Sielaff, M.; Schmidt, H.; Struck, T. H.; Rosenkranz, D.; Mark Welch, D. B.; Hankeln, T; Herlyn, H. (March 2016). "Phylogeny of Syndermata (syn. Rotifera): Mitochondrial gene order verifies epizoicSeisonidea as sister to endoparasiticAcanthocephala within monophyleticHemirotifera".Molecular Phylogenetics and Evolution.96:79–92.Bibcode:2016MolPE..96...79S.doi:10.1016/j.ympev.2015.11.017.PMID26702959.
^abBethel, William M.; Holmes, John C. (1973). "Altered Evasive Behavior and Responses to Light in Amphipods Harboring Acanthocephalan Cystacanths".The Journal of Parasitology.59 (6):945–956.doi:10.2307/3278623.ISSN0022-3395.JSTOR3278623.
^Maynard, Barbara J.; DeMartini, Laura; Wright, William G. (1996). "Gammarus lacustris Harboring Polymorphus paradoxus Show Altered Patterns of Serotonin-like Immunoreactivity".The Journal of Parasitology.82 (4):663–666.doi:10.2307/3283801.ISSN0022-3395.JSTOR3283801.PMID8691384.
^Itämies, J.; Valtonen, E. T.; Fagerholm, H. P. (1980). "Polymorphus minutus (Acanthocephala) infestation in eiders and its role as a possible cause of death".Ann. Zool. Fenn.17 (4):285–289.
^Tada, I; Otsuji, Y; Kamiya, H.; Mimori, T.; Sakaguchi, Y.; Makizumi, S (February 1983). "The first case of a human infected with an acanthocephalan parasite,Bolbosoma sp".The Journal of Parasitology.69 (1):205–8.doi:10.2307/3281300.JSTOR3281300.PMID6827437.
Amin, O. M. (1987). "Key to the families and subfamilies of Acanthocephala, with erection of a new class (Polyacanthocephala) and a new order (Polyacanthorhynchida)".Journal of Parasitology.73 (6):1216–1219.doi:10.2307/3282307.JSTOR3282307.PMID3437357.
Lühe, M. (1904). "Geschichte und Ergebnisse der Echinorhynchen – Forschung bis auf Westrumb (1821)".Zoologischer Annalen.1:139–250.
