Wrapping things up with phylum Rotifera, we reach the acanthocephalans. These are obligate
endoparasites, meaning they are only found within the bodies of other animals, as they cannot
survive outside of their host for long. The clade was once considered the sister phylum to Rotifera,
but more recent analyses have placed them instead as a major clade within the rotifer phylum, though
their exact relationship to other rotifer clades is still contested. As of 2022, modern cladograms
place acanthocephalans as a subclass or clade alongside the bdelloid and seisonid rotifers.
Acanthocephala get their namesake from their most distinctive feature, a cylindrical, eversible
proboscis that bears rows of curved spines. This proboscis, when everted, meaning turned inside
out, is used to anchor the parasite to the host’s intestinal wall. Like the free-living rotifers,
some spiny-headed worms are considered to be cosmopolitan, meaning they can be found all
over the world, and they include between 1,100 and 1,500 described species. Their total species
count is currently contested as they are the subject of numerous ongoing phylogenic studies.
Species range in size from less than 2 millimeters to up to 1 meter in length. Females are generally
slightly larger than males and both sexes are bilaterally flattened with numerous wrinkles
throughout the body that aid in increasing the animal’s surface area. Since acanthocephalans
completely lack a mouth, they absorb nutrients from their host directly across their body walls.
Up to 80% of their mass is devoted to their unique lacunar system of fluid-filled canals.
These animals have no heart, respiratory, digestive, or true circulatory system, but the
muscles of the body wall form tubes that connect to the lacunar system. This forms a network of
contractible tubes so that the body-wall muscles collectively function similarly to a heart.
The animal is then able to pump lacunar fluid throughout the body by squeezing these muscles.
This system essentially acts as a rudimentary circulatory system. Paired with an excretory
system comprised of a pair of flame cells containing protonephridia, it is responsible for
moving fluids into, within, and out of the animal. The acanthocephalan nervous system consists of a
central ganglion within the proboscis receptacle and nerves through the proboscis and body. Though
they do have sensory endings in the proboscis and genital area, the majority of the nervous system
and sensory organs are greatly reduced as is common in obligate endoparasites.
All known acanthocephalans are dioecious, meaning they possess either male or female reproductive
organs. However, complete life cycles have only been worked out for a handful of species,
as there are currently over 1,000 species only known from their adult phase.
Adult spiny-headed worms mate within their final vertebrate host. Once an egg is fertilized, it
matures and subsequently leaves the female's body through her oviduct, where it then passes into the
host’s alimentary canal and is expelled from the host within the feces. In order for the egg to
develop from this stage, it must be consumed by an arthropod, usually a crustacean. Inside the
intermediate host, the egg develops into an acanthella. It then 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 except the reproductive ones. The
parasite is released when the first intermediate host is ingested. If this new host is a suitable
final host, such as a vertebrate, the cystacanth develops into a mature adult. However, if the
host is unsuitable, like a larger crustacean, the parasite will instead reform a cyst. It then feeds
within the final host, where it grows and develops its sexual organs. Mating will then occur.
Luckily, human infection by spiny-headed worms is extremely rare. The species
Moniliformis moniliformis is the most common parasitic acanthocephalan found in humans,
but since it requires its primary host to consume raw infested beetles or cockroaches,
it very rarely passes to humans. Another potential human parasite are individuals
of the genus Macracanthorhynchus, otherwise known as the “giant thorny-headed worm of swine” which
occur throughout the world in the small intestine of pigs and very rarely in humans. In most cases,
there is very little inflammation, and the parasite can co-exist within
its host peacefully. However, in rare cases infection can cause great pain, lacerations,
ulcers, and even death due to multiple infections. And with that gruesome image,
we conclude our study of the parasitic clade acanthocephala and the phylum Rotifera in general.
Let’s move forward and introduce the second most biodiverse phylum on the planet, Mollusca.