Parasitic Jellyfish?!?!?! Meet Polypodium hydriforme!

Polypodium hydriforme...a parasitic cnidarian!

I had the great fortune to attend a great regional parasite conference this past weekend.  The RockyParasitophile in bold, white lettering above the name of the conference, with the hosting field station's logo on the back. The shirts themselves were black and over 2/3rds of the order sold in the three days of the conference!)  At one point, I wound up sitting at a table with a fellow PhD student and two well-known parasitologists who teach at major universities.  During what was an exciting conversation (in which I was mostly trying to listen instead of speaking), one of the professors mentioned a parasitic cnidarian! I was so stunned I didn't pipe up before the conversation was on to other things.  (I was also a bit slow since this was around 2am.) Luckily, I was able to catch one of the professors the next day and ask for the scientific name of the parasite.  So, without further ado, meet the most uniquely awesome jellyfish of all time...Polypodium hydriforme!
Mountain Conference of Parasitologists is a great way to learn more about parasites and to meet fellow parasitophiles.  (Actually, I was able to help design a RMCP t-shirt for this year's conference...it had the word

Taxonomy

Diagram of a cnidocyst.

Unlike every other parasite I've ever blogged about here on Parasitophilia, this parasite belongs to phylum Cnidaria.  Cnidaria is the phylum in which you find animals that are diploblastic (unlike the other things we've talked about, which are all triploblastic).  This group includes animals like jellyfish, sea anemones, tiny freshwater guys like Hydra and Obelia.  All members have tentacles with stinging cells called cnidocytes.  The cnidocytes are equipped with nematocysts (stinging capsules) that are fired at prey items when the cnidarians are ready to feed.  P. hydriforme has these structures, just like other cnidarians.  The parasite also belongs to the class Polypodiozoa and the family Polypodiidae.  This family only contains one species (that's right, this little guy has a whole family to itself)!  Obviously, it is the only member of this genus as well.

Recent molecular studies have brought the conventional taxonomic categorization for this organism into question.  There is some evidence that this parasite may be more closely related to myxozoans than to cnidarians.  However, the presence of cnidocytes still makes this organism a cnidarian in my classically-trained, organismally-based, little biologist eyes.

Normal, black eggs among infected eggs. The arrows show
abnormal eggs and the circle denotes a mature stolon.

Life Cycle
The life cycle of this parasite begins with the emergence of the parasite from the eggs of its host organism, a type of sturgeon or paddlefish.  The parasite emerges in a life stage known as a "stolon" from the eggs in a fresh water ecosystem.  The stolon fragments into a bunch of tiny little medusa-like stages (when you think "medusa" stages....think of the morphology of what you normally see when you look at jellyfish in the zoo).  These little medusoid forms go on to multiply by splitting in half and then growing sexual organs. Eventually, the parasites release their gametes, which mate in the water to form an embryo.  The embryo develops into a planuliform larva, which then infects the bodies of the appropriate host fish.  Within the bodies, the parasite infects the oocytes, where it lives the majority of its life. The larvae develop into an inside-out stolon and waits until the fish is ready to spawn.  Just before the fish releases its eggs, the stolon everts itself to reveal its tentacles (within the egg).  After being released, the eggs become the source of food for the parasite for a time before the stolon emerges from the eggs. (This is the best interpretation of the life cycle that I could understand, if you see something that isn't accurate, please let me know!)

A mature stolon with everted tentacles.

Economic Importance
One of the things that makes this parasite especially important (beside the fact that it's really cool!), is that it infects the eggs of fish that have culinary significance.  One of the biggest problems is that sturgeons contract these parasites.  Sturgeons, for those of you who don't know, produce eggs that many people eat as a delicacy....a very expensive delicacy...it's caviar...caviar is sturgeon eggs.  These eggs are normally small and black, but when infected by P. hydriforme the eggs become enlarged and take on a gray appearance.  If this parasite gets into a farm that raises sturgeons to harvest caviar, it can wreck the farm's production levels.  Many wealthy connoisseurs would be distraught without their caviar...though personally, I'm not a fan of sturgeon eggs.  I really couldn't care less if the caviar industry died off.  However, I would be very sad if that meant the end of the sturgeon.  Sturgeons don't deserve to go extinct.  Then again, I doubt that if the market for caviar disappeared that the fish would disappear alongside it.  They might be doing just fine in the wild.  (I can't confirm that though, you'll have to ask an ichthyologist.)

Another mature, free-living stolon.

Moral of the Story 
What it all comes down to is this...we have an awesomely unique parasite that we know a little more about now!  It impacts the caviar industry, but not so heavily that it stops caviar production. I suppose having less caviar means they can charge more for it anyway (because caviar isn't expensive enough as it is).  Perhaps the parasite is good for sturgeon egg farmers needing an excuse to boost their prices?!  Or perhaps the parasite is just awesome for being a parasitic cnidarian/myxozoan (depending on your taxonomic perspective).  Either way, Polypodium hydriforme is one amazing parasite that every parasitophile should know a little something about!