"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

March 23, 2011

Rugogaster hydrolagi

Today's parasite is a strange worm found in a habitat that may shock and bewilder many readers on multiple levels. It's a worm that lives in the rectal gland of the spotted ratfish (Hydrolagus colliei). The spotted ratfish belongs to an enigmatic group known as the chimaera, an ancient group of cartilaginous fish that branched off from sharks almost 400 million years ago. The parasite itself is just as mysterious - it belongs to a group call Aspidogastrea (we have featured other species from this group on the blog before - Lobatostoma manteri and Aspidogaster conchicola), which made the evolutionary split with the far more diverse digenean trematodes probably also a few hundred million years ago (unfortunately, most parasites don't leave fossils). Rugogaster hydrolagi can grow up to 15 mm long (a bit over half an inch), and is so-called due its "rugae", which are the ridges on its body that give it a corrugated, accordion-like appearance. The life-cycle (like most aspecst of its ecology) is unknown, though like other aspidogastreans, it most likely features a mollusc intermediate host.

Photograph by Klaus Rohde


  1. Yuck! The Spotted Ratfish looks cool, though.

  2. Hello, I'm a fan of this site and all the parasites in the world.
    And thank you so much for this great selection of parasites I can enjoy everyday.
    I'm currently writing a book about parasites in korea,
    and I wonder If I could use some of the images in this site.
    Thank you

  3. Glad you like the site! As for the images, we only own a couple of these. Most come from wikipedia (where the agreement is usually that they only be used for other open sources) or from the Journal of Parasitology, where we were given permission to use them by the editor, but otherwise they are copyrighted by the Ameican Society of Parasitologists. My advice it to contact the individual sources of any images you would like to use. Thanks!