Eusociality in Parasites: Raiders and Basestars

I was told a story once by my Parasitology professor (Valerie McKenzie) at the University of Colorado, it involved brave soldiers, pregnant damsels, and predatory invaders. No, the story wasn’t a fairy tale, it was Parasitology. More specifically, it was about the trematode Himasthla sp. and a biological phenomenon known as: eusociality.

Let’s start with the basics before we start talking about the battle… Eusociality (eu is the greek word for ‘good’, by the way) is the highest level of social organization a species can attain. It’s recognized by three main characteristics: shared responsibilities for offspring, overlapping generations (many adults from born in different times), and a caste system (usually simply reproductive and non-reproductive). The first thing that probably pops into your head is ants (especially if you read my ant rant) and you’d be right! Ants, bees, wasps, and termites are all great examples of eusocial species. You may also think that this description also sounds like lions, tigers, and bears (oh, my!) and you’d be right about that too, however, meerkats and naked mole rats are the most common examples of eusocial mammals.

For a long time people thought these were the main classes (Insecta and Mammalia) of eusocial animals until an undergraduate and his lab decided to challenge this thinking. After cracking open hundreds (if not thousands) of smelly California horn snails he kept noticing two very distinct body types in the parasites he was counting. For a long time these different morphs were regarded as just a difference in life stage (one a couple days older than the other), however, the undergraduate began to question this as they acted very differently, looked extremely distinct, and there were never any that looked in between the two morphs. A shocking hypothesis was suggested, the primary morph may be a reproductive version of the parasite and the secondary morph may be a non-reproductive soldier version.

To test this hypothesis the body size, body shape, and mouth parts were measured showing that the physical characteristics of each morph were consistent and very distinct from one another, furthermore, it was also found that secondary morphs (the soldiers) had extremely large mouths (better to kill you with, my dear).

(Ryan F. Hechinger et, al)

(Ryan F. Hechinger et al, 2011)

Besides physical characteristics it was also found that the secondary morphs were much more active that primary morphs, in fact they moved five times more often. This is most likely because they are extremely small where as the primary morphs are large, sluggish, and well… really pregnant. Thus the secondary morphs are more able to be active and defensive. This was further supported when it was found that secondary morphs were much more likely to attack other species of parasites that were around them.

(Ryan F. Hechinger et al)

(Ryan F. Hechinger et al, 2011)

Here are some pictures of the secondary morphs attacking a variety of other species of parasites…


(Ryan F. Hechinger et al, 2011)

The final nail in the coffin for this theory was the discovery that secondary morphs (soldiers) cannot reproduce (much like worker ants). With this new theory established many other researchers began to examine eusociality in parasites and have found that it is extremely common through out this class as well.

I was lucky enough to be able to see this amazing system in action in my Parasitology lab and snap some awesome pictures and videos of the parasite cage match.

Pygidiopsoides sp. fighting Himasthla rhigedana

Pygidiopsoides sp. fighting Himasthla rhigedana

Image 002

Also, on a really nerdy note… The primary and secondary morphs reminded me a lot of cylon raiders and basestars in Battlestar Galactica… Anyone else?



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