We’ve spent the past few tutorials going over different types of Cnidarians,
and with the most popular classes covered, it’s time to wrap things up by looking at some
particularly interesting species. The myxozoans, or “slime animals,” were first discovered in the
1880s and originally classified as protozoans since all of them are obligate parasites,
many are single-celled, and at least one, Henneguya salminicola, a common salmon parasite,
is incapable of aerobic respiration. So the argument was that since they do not meet all the
characteristics of animals, they are not animals. This made sense, except that myxozoans have “polar
capsules” which closely resemble nematocysts, organelles that are found in no other creatures
except cnidarians. This was originally attributed to convergent evolution, where two unrelated
organisms develop similar characteristics, like wings evolving in both birds and bats. However,
further genetic analyses revealed that the myxozoans are actually highly derived Cnidarians
that evolved into single cells from free-swimming, jellyfish-like ancestors, making them animals.
Myxozoans have complex parasitic life cycles that involve an intermediate, or secondary host,
usually a fish, but sometimes an amphibian, reptile, bird, or mammal, and a definitive,
or primary host, usually worms or bryozoans. Myxozoans inside infected intermediate hosts
are characterized by the “cell-in-cell state”, where the daughter cells develop directly inside
the mother cells. These parasitic cells divide and spread, via the nervous or circulatory system,
and eventually produce myxospores that leave the intermediate host.
The myxospores are then consumed by the definitive host where they attach to the lining of the gut
epithelium using their nematocysts, or polar filaments, and multiply. They eventually produce
sexual stages, and release spiny actinospores that exit the definitive host, usually by the anus,
and float in the water column. If the actinospores come into contact with skin or gills
of a suitable intermediate host they penetrate into the epithelium to cause a new infection.
Though most myxozoan parasites do not harm their intermediate hosts, some are increasingly damaging
wild fish stocks. They are perhaps most infamous due to their economic impact on commercially
valuable fishes. Myxobolus cerebralis, for example, causes whirling disease in both farmed
and wild salmon and trout. Whirling disease causes skeletal deformities and neurological damage
that forces infected fish to “whirl” in an awkward corkscrew-like pattern, making feeding
difficult or impossible. Other myxozoans target the muscle tissue, creating “slime-like” cysts.
Living fish do not show symptoms of infection, but after death their flesh becomes extremely soft,
slimy, and inedible due to myoliquefaction. Other diseases caused by myxozoans include
proliferative kidney disease, enteromyxosis, proliferative gill disease, and many others.
The sister taxon of myxozoa is polypodiozoa, which was discovered in 1871 by professor Ovsjannikov
in the eggs of the Volga sterlet, Acipenser ruthenus, and by now is found in almost all
species of the order Acipenseriformes where it severely compromises reproduction capacities.
Unlike myxozoa, the only known species of polypodiozoa, Polypodium hydriforme,
is multicellular and found in freshwater. Its life cycle consists of a parasitic
and free-living stage. A unique cnidarian, it is adapted to intracellular parasitism
inside the oocytes, or immature egg cells, of Acipenseriform fish such as sturgeons
and paddlefish. Inside the infected oocytes, the binucleated parasitic cell, which has two nuclei,
one small and one large, forms a two-cell stage. This process of endocytokinesis happens when the
small nucleus inside the binucleate cell becomes surrounded with a tiny portion of cytoplasm.
The smaller internal cell divides, passes a morula stage, and then becomes an inverted planula and
stolon with tentacles on the inside. The external region has the ability to consume the cells of its
host and is called a trophamnion. Development of polypodiozoa within the oocytes of its host
is strongly correlated with the progress of oogenesis in the host-fish and is triggered by it.
This means that the development of the parasite depends on the sexual development of its host.
Finally, right before the host fish spawns, the parasitic stolon inside the infected egg turns
inside out, and just after spawning begins its free-living, non-parasitic life stage.
The long stolon asexually splits into numerous fragments, yielding free specimens that “walk”
on the substrate using their tentacles and form sexually reproductive gonads to infect new hosts.
And with that, we conclude our study of the phlyum Cnidaria, which was probably quite
a bit more involved than you anticipated. As fascinating as this was, we have only scratched
the surface on kingdom Animalia, so let’s continue working our way through the major animal phyla.