"So, naturalists observe, a flea has smaller fleas that on him prey; and these have smaller still to bite ’em; and so proceed ad infinitum."
- Jonathan Swift

August 6, 2010

August 6 - Paronatrema sp.

On the whole, digenean trematodes aren’t very common in sharks. Parasitologists learn early that tapeworms rule the sharks, and trematodes rule the fishes. There are a few exceptions, like the giant flat gorgoderids that live in the body cavities of sting rays, and today’s parasite, Paronatrema, which is a member of a very poorly known group called the syncoeliids. Paronatrema and the only syncoeliid that infects sharks – Otiotrema – are unusual in another way: they are ectoparasites. Nearly ALL trematodes are endoparasites; even the ones that live on the skin (e.g. Transversotrematids) are technically endoparasites because they are under the first layer of skin. Not syncoeliids; these are bold enough to choose a totally different host group and to live free and open on the surfaces of the gills and branchial cavity of sharks. How do they do this? No one really knows, because they have hardly been studied at all. Given that you have to put your hand in a tiger shark’s mouth to get one, perhaps its no surprise…

Contributed by Al Dove.


  1. Wow, what an amazing critter! Just about everything about this worm spells "out of character" for a digenean!

    The diversity of digeneans and their life cycles never ceases to amaze me.

  2. It’s super interesting how this digenean trematode is uncommon in so many ways. Not only is it one of the only trematodes to be involved with sharks, but it is also an ectoparasite! I love the humor in this post, I can relate to the urge not to put my hand in a shark’s mouth!