| Heptamegacanthus | |
|---|---|
Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Acanthocephala |
| Class: | Archiacanthocephala |
| Order: | Oligacanthorhynchida |
| Family: | Oligacanthorhynchidae |
| Genus: | Heptamegacanthus Spencer-Jones, 1990[1] |
| Species: | H. niekerki |
| Binomial name | |
| Heptamegacanthus niekerki Spencer-Jones, 1990 | |
  H. niekerki | |
Heptamegacanthus is a genus ofacanthocephalans (thorny-headed or spiny-headedparasitic worms) containing a single species,Heptamegacanthus niekerki, that is aparasite of the endangeredgiant golden mole. It is found only in isolated forests in theTranskei and nearEast London, both inSouth Africa. The worms are about 4 mm long and 2 mm wide with minimalsexual dimorphism. An individual's body consists of a short, widetrunk and a tubular feeding and sucking organ called theproboscis which is covered with hooks. The hooks are used to pierce and hold therectal wall of its host. There are 40 to 45 of these hooks arranged in rings surrounding the proboscis. They are notradially symmetrical. There are seven largeanterior hooks; the hooks in the anterior ring are twice as large as those in the second ring and the remaining hooks decrease progressively in sizeposteriorly.
The life cycle ofH. niekerki remains unknown; however, like other acanthocephalans, it likely involvescomplex life cycles with at least two hosts. Although theintermediate host forHeptamegacanthus is not definitively identified, it is presumed to be anarthropod such as an insect. In this host, thelarvae develop into an infectious stage known as a cystacanth. These are then ingested by thedefinitive host, where they mature and reproduce sexually within theintestines. The resulting eggs are expelled and hatch into new larvae.
Heptamegacanthus is amonotypic genus ofacanthocephalans (also called thorny-headed or spiny-headed parasitic worms). The genus and speciesHeptamegacanthus niekerki wasformally described in 1990 by Mary E. Spencer Jones,curator at theNatural History Museum, London, from specimens (5 females and 5 males) collected live, preserved, and sent from South Africa. The genus nameHeptamegacanthus refers to the seven large hooks on itsproboscis and thespecific nameniekerki derives from Jan van Niekerk, who collected the species in the field.[1]
The classification of the genusHeptamegacanthus within the familyOligacanthorhynchidae is supported by six distinctmorphological features. Firstly, its proboscis is more or less spherical, and equipped with hooks that feature ananterior strong base (manubrium) and a robust root for attachment. Additionally,sensory papillae on the proboscis enhance its sensory capabilities. The proboscis receptacle, a complex structure for housing the proboscis when retracted, consists of a thin, non-muscular outer layer and a thicker, muscular inner layer, which is pierceddorsally by muscles that retract the proboscis. Thelemnisci, which are long, flattened structures containing several giant nuclei, play a role in the worm's sensory system. The presence of eightcement glands, each with a large central nucleus, is another distinguishing feature; these glands produce a substance used in the reproductive process. Finally, the eggs ofHeptamegacanthus have an oval shape and distinctly textured outer shell. It differs from other worms in the family Oligacanthorhynchidae mainly by its smaller size, and the number and greater size of hooks in the anterior ring.[1] TheNational Center for Biotechnology Information does not indicate that anyphylogenetic analysis has been published onHeptamegacanthus that would confirm its position as a unique genus in the family Oligacanthorhynchidae.[2]
Heptamegacanthus niekerki consists of a proboscis covered in hooks, a proboscis receptacle, and a trunk with a length twice that of the width.Sexual dimorphism is minimal, with the male being 3.49–4.23 mm long by 1.42–2.14 mm wide, only slightly larger than female at 3.15–3.59 mm long by 1.53–1.94 mm wide. This is unusual for acanthocephalans where the female is usually much larger than the male, but has been found in other acanthocephalans includingCorynosoma.Heptamegacanthus is also very small for a member of the family Oligacanthorhynchidae. Although acanthocephalans are able to survive inaccidental hosts without completing their development rendering them smaller than normal, thisdwarfism has been ruled out forHeptamegacanthus as the samples collected were mature and females contained eggs indicating that thegiant golden mole (Chrysospalax trevelyani) is adefinitive and not aparatenic (organism that harbors the sexually immature parasites) or accidental host.[1]
The proboscis is nearly spherical being 256–381 μm long and 435–604 μm wide in males and 281–381 μm long and 416–635 μm wide in females. It is armed with 40 to 45 hooks that are notradially symmetrical, with seven large anterior hooks. The hooks come in four distinct types: the first two rings of hooks have large bases (manubria) and roots, with the hooks in the very front (243–275 μm long in males and 256–297 μm long in females) being twice the size of those in the second row (81–180 μm long in males 103–199 μm long in females). Hooks towards the rear are spine-like, tapering to smaller sizes (43–84 μm to 28–52 μm long in males and 34–71 μm to 31–58 μm long in females long in males). One sensory papilla is present on each side of neck and on the apex of the proboscis. The proboscis receptacle originates on the body wall at about one quarter of body-length from the anterior end of the worm. The proboscis receptacle is single walled, having a small, non-muscular layer and inner large, muscular wall with well-developed retractor muscles penetrating the receptacle wall dorsally. The brain is located in the mid-region of the proboscis receptacle. The lemnisci (bundles ofsensory nerve fibers) are long, extending nearly half the body length, flat, and contain several giant nuclei (0.832–1.635 mm long and 89–333 μm wide in the female). The worm does not haveprotonephridia, which are found in other acanthocephalan species forexcretion and water regulation.[1]
Thetestes of the male are large and tilted, 409 to 832 μm long and 204 to 525 μm wide, and are positioned towards the front half of the body before the central (pre-equatorial) dividing line. Duringcopulation, the male injectssemen from its seminal vesicle into itscopulatory bursa (a fluid-filled sac). TheSaefftigen's pouch (a muscular sac 832–877 μm long and 236–365 μm wide) then contracts, ejecting fluid causing theeversion of the copulatory bursa. The shape of the copulatory bursa grips the female during copulation. There are eight pre-equatorial cement glands that are 281–404 μm long and 243–436 μm wide, which are used to temporarily close theposterior end of the female after copulation, each with a single giant nucleus.[1]
Females have a short, muscular reproductive system, including a uterine bell (a funnel like opening continuous with theuterus) that is large and short (179–218 μm long to 140–173 μm wide), a uterus (396–461 μm long and 166–224 μm wide), and avagina (102–186 μm long and 32–173 μm diameter). Females produce numerous oval eggs that are 56–96 μm long and 43–52 μm wide with highly sculptured outer shells, stored in a space called thepseudocoel.[1]
The distribution ofH. niekerki is determined by that of its host, the giant golden mole.Heptamegacanthus niekerki has been found in the Nqadu Forest,Transkei,South Africa, thetype locality. The giant golden mole's range is very limited (it is anendangered species),[3] and consists only of isolated forest ranges in Pirie Forest and Komgha nearKing William's Town,East London, andPort St. Johns in the Transkei.[1] The giant golden mole, and thusH. niekerki, is threatened primarily by theurbanization of the East London area causingfragmentation of forests and subsequenthabitat loss. The forests are also being degraded by exploitation for firewood, bark, timber harvesting and livestock overgrazing/trampling.[3]
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The specificlife cycle ofHeptamegacanthus is unknown, but the life cycle of thorny-headed worms, or acanthocephala, in general unfolds in three distinct stages. It begins when an egg develops into an infective form known as anacanthor. This acanthor is released with thefeces of its definitive host, typically a vertebrate, and must be ingested by anintermediate host, anarthropod such as an insect, to continue its development.[7]Although the specific intermediate hosts for the genusHeptamegacanthus are unidentified, it is generally accepted that insects serve as the primary intermediaries for the broader orderOligacanthorhynchida to which it belongs.[7]
Once inside the intermediate host, the acanthor sheds its outer layer in a process calledmolting, transitioning into its next stage, the acanthella.[5] At this stage, whenH. niekerki measures between 38–60 μm in length and 19–26 μm in width, it burrows into the host'sintestinal wall and continues to grow.[1] The life cycle culminates in the formation of a cystacanth, alarval stage that retains juvenile features (differing from the adult only in size and stage of sexual development) and awaits ingestion by the definitive host to mature fully. Once inside the definitive host, these larvae attach themselves to the intestinal walls, mature into sexually reproductive adults, and complete the cycle by releasing new acanthors into the host's feces.[5]
Heptamegacanthus niekerki has been found attached to the wall of therectum in the giant golden mole.[1] There are no known paratenic hosts whereHeptamegacanthus might reside without undergoing further development or reproduction. There are no reported cases ofH. niekerki infesting humans in the English-language medical literature.[5]