BY ALPER ÖZKAN (MSN/PhD)
d_ozkan@ug.bilkent.edu.tr
Biology is the science of exceptions. Something always runs contrary to the rule, no matter how widely encompassing that rule may be—there are anaerobic animals and multicellular bacteria, there are parasites that sometimes benefit their hosts and predators that are the mutualists of their prey. Herbivores will eat meat when they can help themselves to it; a carnivore in desperate times may turn herbivorous. So too is the case with cnidarians: most cycle through two life stages, the clonal polyp and the gametogenic medusa, but there are exceptions—corals, sea anemones and the ever-popular Hydra don’t produce medusae, while others, such as the Narcomedusae, do not settle down, instead developing directly from larva to jellyfish. But these at least are jellyfish-like enough that they can be recognized as such, while another exception is weird enough that it took some time for it to be identified as an animal—let alone a cnidarian.
The life cycle of a myxozoan begins with a spore, and you know you’re in for a fun ride when the first thing you read about an animal is that it reproduces by spores. These spores are equipped with what are called polar capsules, each of which can launch a polar filament to penetrate the scaly hide of the parasite’s intended host—a fish. The myxozoan then injects itself through the injury it has created (a fantastic way of infecting a host—see also Sacculina and Haptoglossa, which do largely the same) and travels to various tissues to reproduce—Myxobolus cerebralis prefers the bone and cartilage of the skull and Tetracapsuloides bryosalmonae goes for the kidneys, while Ceratomyxa shasta is fine with most internal organs. The parasite may either remain benign or cause severe disease, depending on the host: commercial fish tend to be
affected severely, while native species often resist its influence. Either way, once the host dies, the myxozoan infects an annelid or bryozoan through a second type of spore produced in the fish, and once there, resumes producing fish-infecting spores.
This is the conventional life cycle of myxozoans, but sometimes a sporulating jellyfish just doesn’t cut it. Enter Buddenbrockia, the sporulating jellyfish that recombines into a complex, multicellular animal—this particular myxozoan, you see, isn’t content with just producing spores, and instead aggregates its cells (cells that, I should stress, even differentiate into germ layers during the process) to create a worm within its host, which then generates spores, as per the course for myxozoans, until its body ruptures to release its progeny. There is also Polypodium, the intracellular parasite jellyfish (not the only animal to live in the cells of another, though—Trichinella does the same, and can infect humans to boot) that infects caviar and begins its life as an inside-out animal, with its “skin” on the inside and stomach cavity on the outside. When it is ready to emerge, the parasite turns itself right side out, internalizing the remaining yolk of the sturgeon egg during the process.
As far as humanity is concerned, however, jellyfish can create a much more pressing problem—I mentioned in my last column that ctenophores, introduced from other waters, now pretty much rule the Black Sea, and overfishing has led to jellyfish doing much the same elsewhere. These jellies not only keep fish populations from recovering, but may also attack nuclear power plants, blooming in such numbers that they clog water intakes and block cooling systems. Among the invading jellyfish species are the monstrous Nomura’s jelly, which is about the size of a full-grown man, and the immortal jelly Turritopsis dohrnii (once included in T. nutricula, but now reclassified), which can cheat death by reverting to its polyp stage (the polyp-to-medusa transition is typically a one-way street). Jellyfish stings are no laughing matter, either—box jellyfish in particular are famous for their venom, as well as for the near invisibility of their tentacles, which makes it particularly easy to run into a waiting jelly in the treacherous waters of the Death Continent (i.e., Australia, where they swarm in great numbers alongside other benign, peaceful marine friends such as stonefish, cone shells and saltwater crocodiles).
Another potent sting belongs to the Portuguese man o’ war, Physalia physalis, which is a siphonophore—a colony of tiny animals that each perform a specific function. Cnidarians have a lot of colony animals among their number: corals, sea pens and some hydrozoans (such as fire corals, which are not truly corals) are also colonial, but none show the level of specialization exhibited by siphonophores. These marine behemoths are moved through the water by specialized medusae called nectophores, while feeding is accomplished through digestive polyps called gastrozooids, and reproduction is likewise the task of another type of polyp, the gonozooid—all of which bud from a central stalk formed by the protozooid, the very first individual of the colony. Defensive polyps are also common, and the entire structure is topped with a gas-filled float called the pneumatophore, which forms the distinctive “sail” of the man o’ war (Physalia lacks nectophores and relies entirely on the wind to move).
And that’s it for this week—but not for cnidarians, since I intend to return to the topic at a later date. But not next time, for that will be a lesson in history.