The
Acanthocephala (
GreekGreek , an independent branch of the Indo-European family of languages, is the language of the Greeks. Native to the southern Balkans, it has the longest documented history of any Indo-European language, spanning 34 centuries of written records. In its ancient form, it is the language of classical...
ακανθος,
akanthos, thorn + κεφαλη,
kephale, head) is a phylum of parasitic worms known as
acanthocephales,
thorny-headed worms, or
spiny-headed worms, characterised by the presence of an evertable
proboscisIn general, a proboscis is an elongated appendage from the head of an animal, either a vertebrate or an invertebrate..-Etymology:...
, armed with spines, which it uses to pierce and hold the gut wall of its host. Acanthocephalans typically have
complex life cyclesParasite life cycles can take a variety of forms, all involving the exploitation of one or more hosts. Those that must infect more than one host species to complete their life cycles are said to have complex or indirect life cycles, while those that infect a single species have direct life...
, involving a number of hosts, including
invertebrateAn invertebrate is an animal without a vertebral column. The group includes 95% of all animal species — all animals except those in the Chordate subphylum Vertebrata ....
s,
fishA fish is any aquatic vertebrate animal that is typically ectothermic , covered with scales, and equipped with two sets of paired fins and several unpaired fins...
es, amphibians,
birdBirds are winged, bipedal, endothermic , vertebrate animals that lay eggs. There are around 10,000 living species, making them the most numerous tetrapod vertebrates. They inhabit ecosystems across the globe, from the Arctic to the Antarctic. Birds range in size from the Bee Hummingbird to the ...
s, and
mammalMammals are a class of vertebrate animals whose females are characterized by the possession of mammary glands while both males and females are characterized by sweat glands, hair, three middle ear bones used in hearing, and a neocortex region in the brain.Mammals are divided into three main...
s. About 1150
speciesIn biology, a species is:* a taxonomic rank or* a unit at that rank ....
have been described.
Morphological characteristics
There are several morphological characteristics that distinguish acanthocephalans from other phyla of parasitic worms.
Digestion
Acanthocephalans lack a
mouthThe mouth, buccal cavity, or oral cavity is the first portion of the alimentary canal that receives food and begins digestion by mechanically breaking up the solid food particles into smaller pieces and mixing them with saliva...
or alimentary canal. This is a feature they share with the
cestodaCestoda is a class of parasitic flatworms, commonly called tapeworms, that live in the digestive tract of vertebrates as adults and often in the bodies of various animals as juveniles...
(tapeworms), although the two groups are not closely related. Adult stages live in the
intestineIn anatomy, the intestine is the segment of the alimentary canal extending from the stomach to the anus and, in humans and other mammals, consists of two segments, the small intestine and the large intestine...
s of their host and uptake nutrients which have been
digestDigest can refer to any of the following:*A tax digest*Digestion of food** Digestophobia, the fear of eating something that may upset your stomach*Digest access authentication in HTTP, SIP and other computer network protocols...
ed by the host, directly, through their body surface.
Proboscis
The most notable feature of the acanthocephala is the presence of an anterior, protrudible
proboscisIn general, a proboscis is an elongated appendage from the head of an animal, either a vertebrate or an invertebrate..-Etymology:...
that is usually covered with spiny hooks (hence the common name: thorny 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 a
septum or
proboscis sheath. Traversing the cavity of the proboscis are
muscleMuscle is the contractile tissue of the body and is derived from the mesodermal layer of embryonic germ cells. Muscle cells contain contractile filaments that move past each other and change the size of the cell. They are classified as skeletal, cardiac, or smooth muscles. Their function is to...
-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.
http://parasites-world.com/acanthocephalns-drilling-acipenser-stellatus-intestine/
Phylogenetic relationships
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. Phylogenetic analysis of the 18S
ribosomalRibosomes are complexes of RNA and protein that are found in all cells. The ribosome is part of the mechanism that translates the DNA sequence into the protein sequence. Ribosomes from bacteria, archaea and eukaryotes , have significantly different structure and RNA...
gene has revealed that the Acanthocephala are most closely related to the
rotiferThe rotifers make up a phylum of microscopic and near-microscopic pseudocoelomate animals. They were first described by Rev. John Harris in 1696, and other forms were described by Anton van Leeuwenhoek in 1703...
s, or may even belong in that phylum. The two are included among the
PlatyzoaThe Platyzoa are a group of protostome animals proposed by Thomas Cavalier-Smith in 1998. Cavalier-Smith included in Platyzoa the Phylum Platyhelminthes or flatworms, and a new phylum, Acanthognatha, into which he gathered several previously described phyla of microscopic animals...
.
Size
The size of the animals varies greatly, from forms a few millimetres in length to
Gigantorhynchus gigas, which measures from 100 to 650 mm.
Skin
The body surface of the acanthocephala is peculiar. Externally, the skin has a thin
cuticleA cuticle , or cuticula, is any of a variety of tough but flexible, non-mineral outer coverings of an organism, or parts of an organism, that provide protection. Cuticles are non-homologous, differing in their origin, structure and chemical composition.-Human anatomy:In human anatomy, cuticle ...
covering the
epidermisIn anatomy, squamous epithelium is an epithelium characterised by its most superficial layer consisting of flat, scale-like cells called squamous cell...
, which consists of a
syncytiumIn biology, a syncytium is a large cell-like structure filled with cytoplasm containing many nuclei. Most cells in all organisms have a single nucleus; syncytia are specialized forms used by various organisms in normal tissue.- Formation :A syncytium can form in two ways...
with no
cell wallA cell wall is a tough, flexible and sometimes fairly rigid layer that surrounds some types of cells. It is located outside the cell membrane and provides these cells with structural support and protection, and also acts as a filtering mechanism. A major function of the cell wall is to act as a...
s. The syncytium is traversed by a series of branching
tubuleA tubule is a very small tube or fistular structure.A system of surface-connected membranes in muscle that enables a nerve impulse to travel to the interior of the muscle fibre.In anatomy, a tubule is a minute tube lined with glandular epithelium....
s containing fluid and is controlled by a few wandering,
amoeboidAmoeboids are single-celled life-forms characterized by an irregular shape."Amoeboid" and "amoeba" are often used interchangeably even by biologists, and especially refers to a creature moving by using pseudopods. Most references to "amoebas" or "amoebae" are to amoeboids in general rather than to...
nucleiIn cell biology, the nucleus , also sometimes referred to as the "control center", is a membrane-enclosed organelle found in eukaryotic cells. It contains most of the cell's genetic material, organized as multiple long linear DNA molecules in complex with a large variety of proteins, such as...
. Inside the syncytium is an irregular layer of circular muscle fibres, and within this again some rather scattered longitudinal fibres; there is no
endotheliumThe endothelium is the thin layer of cells that line the interior surface of blood vessels, forming an interface between circulating blood in the lumen and the rest of the vessel wall. Endothelial cells line the entire circulatory system, from the heart to the smallest capillary...
. In their micro-structure the muscular fibres resemble those of
nematodeThe "roundworms" or "nematodes" are the most diverse phylum of pseudocoelomates, and one of the most diverse of all animals. Nematode species are very difficult to distinguish; over 80,000 have been described, of which over 15,000 are parasitic. It has been estimated that the total number of...
s.
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 the
lemnisci 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.
Nervous system
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 a
retinaculum. In the male at least there is also a genital
ganglionIn anatomy, a ganglion is a biological tissue mass. Cells found in a ganglion are called ganglion cells, though this term is also sometimes used to refer specifically to retinal ganglion cells....
. Some scattered papillae may possibly be sense-organs.
"Brain-jacking"
Thorny-headed worms begin their life cycle inside invertebrates that reside in marine or freshwater systems.
Gammarus lacustris, a small crustacean that feeds near ponds and rivers, is one invertebrate that the thorny-headed worm may occupy. This crustacean is predated by ducks and hides by avoiding light and staying away from the surface. However, when infected by a thorny-headed worm it becomes attracted toward light and surfaces itself.
Gammarus lacustris 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. The duck is the definitive host for the acanthocephalan parasite. In order to be transmitted to the duck, the parasite's intermediate host (the gammarid) must be eaten by the duck. This modification of gammarid behavior by the acanthocephalan is thought to increase the rate of transmission of the parasite to its next host by increasing the susceptibility of the gammarid to predation.
It is thought that when
Gammarus lacustris is infected with a thorny-headed worm, the parasite causes
serotoninSerotonin is a monoamine neurotransmitter. It is found extensively in the gastrointestinal tract of animals, and about 80 to 90 percent of the human body's total serotonin is located in the enterochromaffin cells in the gut, where it is used to regulate intestinal movements...
to be massively expressed. Serotonin is a neurotransmitter involved in emotions and mood. Researchers have found that during mating
Gammarus lacustris expresses high levels of serotonin. Also during mating, the male
Gammarus lacustris clamps down on the female and holds on for days. Researchers have additionally found that blocking serotonin releases clamping. Another experiment found that serotonin also reduces the photophobic behavior in
Gammarus lacustris. Thus, it is thought that the thorny-headed worm physiologically changes the behavior of the
Gammarus lacustris in order to enter its final host, the bird.
Sex
The Acanthocephala are
dioeciousDioecious species are whose members can produce only one type of gamete; each individual organism belonging to a dioecious species is distinctly male or female . The majority of animal species are dioecious...
. There is a structure called the
genital ligament which runs from the posterior end of the proboscis sheath to the posterior end of the body. In the male, two testes lie on either side of this. Each opens in a
vas deferensThe vas deferens , also called ductus deferens, , is part of the male anatomy of some species; they transport sperm from the epididymis in anticipation of ejaculation.-Structure:...
which bears three diverticula or
vesiculae 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 a
penisThe penis is an external sexual organ of certain biologically male organisms, in both vertebrates and invertebrates....
which opens posteriorly.
In the female, the ovaries are found, like the testes, as rounded bodies along the ligament. From these masses of ova dehisce into the body cavity and float in its fluid. Here the eggs are fertilized and segment so that the young
embryoAn embryo is a multicellular diploid eukaryote in its earliest stage of development, from the time of first cell division until birth, hatching, or germination...
s are formed within their mother's body. The embryos escape into the
uterusThe uterus is a major female hormone-responsive reproductive sex organ of most mammals, including humans. It is within the uterus that the fetus develops during gestation. The term uterus is used consistently within the medical and related professions; the Germanic term, womb is more common in...
through the
uterine bell, a funnel like opening continuous with the uterus. At the junction of the bell and the uterus there is a second small opening situated
dorsallyIn anatomy, the dorsum is the upper side of animals that typically run, fly, or swim in a horizontal position, and the back side of animals that walk upright. In vertebrates the dorsum contains the backbone. The term dorsal refers to anatomical structures that are either situated toward or grow...
. The bell "swallows" the matured embryos and passes them on into the uterus, and from there, out of the body via the
oviductIn oviparous animals , the passage from the ovaries to the outside of the body is known as the oviduct. The eggs travel along the oviduct...
. Should the bell swallow any of the ova, or even one of the younger embryos, these are passed back into the body cavity through the second, dorsal, opening.
The embryo passes from the body of the female into the alimentary canal of the host and leaves this with the
fecesFeces, faeces, or fæces is a waste product from an animal's digestive tract expelled through the anus during defecation.-Etymology:...
.
Other features
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 acanthocephalans lack an excretory system, although some species have been shown to possess
flame cellright|frameA flame cell is a specialized excretory cell found in most "lower" freshwater invertebrates, not including Nematode, Platyhelminthes , flatworms, rotifers and nemerteans; these are the simplest animals to have a dedicated excretory system. Flame cells function like a kidney, removing...
s (protonephridia).
History
The earliest recognisable description of Acanthocephala - a worm with a proboscis armed with hooks - was made by Italian author
Francesco RediFrancesco Redi was an Italian physician.He is most well-known for his series of experiments, published in 1668 as Esperienze Intorno alla Generazione degl'Insetti which is regarded as one of the first steps in refuting "spontaneous generation" - a theory also known as Aristotelian...
(1684). In 1771 Koelreuther proposed the name Acanthocephala. Muller independetly called them Echinorhynchus in 1776.
RudolphiKarl Asmund Rudolphi was a Swedish-born naturalist, who is credited with being the "father of helminthology"....
in 1809 formally named them Acanthocephala.
Currently the phylum is divided into four classes - Palaeacanthocephala, Archiacanthocephala, Polyacanthocephala and Eoacanthocephala.
Life cycles
General patterns
Acanthocephalans have complex life cycles, involving a number of hosts, for both
developmentalMorphogenesis , is the biological process that causes an organism to develop its shape. It is one of three fundamental aspects of developmental biology along with the control of cell growth and cellular differentiation...
and resting stages. Complete life cycles have been worked out for only 25 species.
Having been expelled by the female, the acanthocephalan embryo is released along with the feces of the host. For development to occur, the embryo needs to be ingested by an invertebrate, almost always a
crustaceanCrustaceans are a very large group of arthropods, comprising almost 52,000 described species , and are usually treated as a subphylum . They include various familiar animals, such as crabs, lobsters, crayfish, shrimp, krill and barnacles...
(there is one known life cycle which uses a mollusc as a first intermediate host). Inside the intermediate host, the acanthocephalan penetrates the gut wall, moves into the body cavity, encysts, and begins transformation into the infective
cystacanth 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 a
paratenicIn parasitology, the term paratenic describes an intermediate host which is not needed for the development of the parasite, but nonetheless serves to maintain the life cycle of the parasite. Alaria americana may serve as an example: the so-called mesocercarial stages of this parasite reside in...
host, in which the parasite again forms a cyst. When consumed by a suitable final host, the cycstacant
excysts, 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 its
cement glands to plug the
vaginaThe vagina is a fibromuscular tubular tract leading from the uterus to the exterior of the body in female placental mammals and marsupials, or to the cloaca in female birds, monotremes, and some reptiles. Female insects and other invertebrates also have a vagina, which is the terminal part of the...
of the female, preventing subsequent matings from occurring. Embryos develop inside the female, and the life cycle repeats.
An example - Polymorphus spp.
Polymorphus spp. are parasites of seabirds, particularly the Eider Duck (
Somateria mollissima). Heavy infections of up to 750 parasites per bird are common, causing
ulcerationA peptic ulcer, also known as ulcus pepticum, PUD or peptic ulcer disease, is an ulcer of an area of the gastrointestinal tract that is usually acidic and thus extremely painful...
to the gut, disease and seasonal mortality. Recent research has suggested that there is no evidence of
pathogenA pathogen , infectious agent, or germ, is a biological agent that causes disease or illness to its host....
icity of
Polymorphus spp. to intermediate crab hosts. The cystacanth stage is long lived and probably remains infective throughout the life of the crab.
The life cycle of
Polymorphus spp. normally occurs between sea ducks (e.g. eiders and
scoterThe scoters are stocky seaducks in the genus Melanitta. The drakes are mostly black and have swollen bills. Females are brown.They breed in the far north of Europe, Asia and North America, and winter further south in temperate zones of those continents. They form large flocks on suitable coastal...
s) and small crabs. Infections found in commercial-sized
lobsterClawed lobsters compose a family of large marine crustaceans. Lobsters are economically important as seafood, forming the basis of a global industry that nets US$31.8 billion in trade annually....
s 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.
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