The Life Cycle of Rhabdias ranae, An Evolutionary Link Between Free-Living and Parasitic Nematodes

    Rhabdias ranae is a nematode of Rana pipiens  which incorporates a free-living phase as well as a parasitic phase, both exemplified by separate adults.  The parasitic phase is a protandrous hermaphrodite which begins as a male and later transforms into a female, using stored sperm from its initial sexual existence to fertilize eggs from its later sexual form. The resultant egg capsules produced by fertilization are delivered by the now-female in the lungs, coughed up and swallowed.  The egg capsules hatch in the intestine and result in rhabditiform larvae.  Defecation delivers the larvae to the outside where they undergo four molts, developing into free-living adult males and females.  After copulation, the free-living females produce egg capsules (fertilized eggs), a few of which hatch within the female, consuming the tissues of their mother.  These larvae emerge as filariforms and which penetrate the skin of suitable hosts such as frogs.  The larvae make their way to the lungs where they develop into the protandrous hermaphroditic adults.  This life cycle demonstrates the relationship and possible evolution of parasitic nematodes from free-living ones suggesting there may have been transitional stages between free living and parasitic existences.

PROCEDURE

1.  Sacrifice a number of Rana pipiens and excise the lungs.  Tease the lungs apart in saline and examine the parasitic females (protrandrous hermaphrodites).  Also, excise the rectum and examine it for egg capsules and rhabditiform larvae. 
2.  The adults can be fixed and mounted using the glycerine mounting technique.
3. Scrape the lining of the rectum of infected frogs to obtain large numbers of egg capsules and place the scrapings in a depression slide or Syracuse dish and cover with some filtered aquarium or pond water. 
4. Cover the dish and examine it 5-7 days later.  Replace water as it evaporates, keeping the specimen moist.
5.  Observe the free-living adults relative to morphology and copulation.
6.  Mount the free-living adults using the glycerine technique and compare the free-living adults to parasitic adult.