"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 11, 2015

Crassicauda magna

During this blog's first year back in 2010, we featured a parasitic nematode (roundworm) that lives in the placenta of sperm whales of all places. Today, we're featuring a study on another nematode which lives in the sperm whale's cousin - the much smaller and more enigmatic pygmy sperm whale Kogia breviceps.
Photo of C. magna in whale tissue from Fig. 1 of the paper

Crassicauda magna is a parasites that really gets under the skin of the pygmy sperm whale. While most worms in the Crassicauda genus live in the urogential and renal system of whales, C. magna just had to be different from the rest of the pack. Instead of living in the whale's plumbing system, it had opt for a life being sandwiched between layers of blubber and muscle, living snugly under the whale's subcutaneous tissue.

While it can be a tight fit in there, C. magna can grow quite large -the largest known fragment is 3.7 m (about 12 feet) long, but due to where they are found in the body and the relatively cryptic nature of its host, no fully intact C. magna has ever been retrieved. The original species description for C. magna was published in 1939, and was based upon fragmentary remains from the front half of the worm, as the rest of the parasite not recovered.

Even though this parasite appears to have a global distribution (like its host), very little is actually known about it. Only a few anatomical details have been recorded, pieced together from worm fragments which had been collected over the years, and until the publication of the present study, there were no genetic data for C. magna. This is not too surprising considering that much of what is known about the pygmy sperm whale itself (let alone C. magna) had about from examining stranded individuals - which is not exactly a routine occurrence.

The C. magna specimens which were the subject of this new study were retrieved from a dead pygmy sperm whale which was beached at Moreton Bay, Queensland. Most importantly, from a taxonomist's perspective, the research team involved was able to retrieve parts of the tail from male worms. The reason why this was kind of a big deal is that one of the key features used to identify different species of nematodes are the needle-like structures on the male genitalia call copulatory spicules. The male worms use these spicules to pry apart the female worm's vulva for sperm transfer, and it just so happened that each species have distinctively shaped spicules, which can be used to tell them apart.

The researchers were able to compare the worms collected for this study with other specimens of Crassicauda stored at the South Australian Museum, the Natural History Museum in London, and the Muséum national d'Histoire naturelle in Paris. They noted that the spicules on C. magna are remarkable similar to those found on another species that was described in 1966 call Crassicauda duguyi - which was also collected from the neck muscle of a pygmy sperm whale (in this case, it was stranded on the west coast of France). Their conclusion was the C. duguyi is most likely just C. magna instead of being a different species, but the taxonomist who described it was not able make the match because the original species description of C. magna did not have information on the male genitalia.

Unlike previous studies, the researchers responsible for the current one also managed to extract some genetic material from the worms they collected. They sequence a section of the worm's ribosomal DNA which was used to reassess the classification of C. magna in relation to other parasitic nematodes. With such a genetic marker at hand, it can be used in the future to find out more about this enigmatic parasite and its equally cryptic host.

Jabbar, A., Beveridge, I., & Bryant, M. S. (2015). Morphological and molecular observations on the status of Crassicauda magna, a parasite of the subcutaneous tissues of the pygmy sperm whale, with a re-evaluation of the systematic relationships of the genus Crassicauda. Parasitology Research 114: 835-841

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