Creature Feature: Cooperative Trematode Parasites

As any true Star Wars groupie might know, you don’t want to mess with stormtroopers unless you come equipped to the battle armed with Jedi mind tricks, a hearty dose of bravado, and a shiny, shiny lightsaber. In earlier episodes of the franchise, stormtroopers tended to be portrayed as a villainous kamikaze foot soldiers for the Dark Side. If we were to try to explain the adaptive basis for their behaviors, we might be more forgiving of their hive-minded nature.

Kin selection has often been considered crucial to the evolution of eusociality in animals such as honey bees, ants, and naked mole-rats. À la Hamilton, the close genetic relatedness between colony-mates allowed for the independent evolution of the extreme division of labor and cooperation seen in these distantly-related taxa. If you sat through Lucas’s Attack of the Clones with gritted-teeth (why George, why?), you’ll know that the earliest stormtroopers were in fact clones of a bounty hunter named Jango Fett. This means the coefficient of relatedness, r, between each stormtrooper is 1 (r can range between 0 and 1, with identical twins and clones having the maximal r value). Even sisters in eusocial species tend to have r values close to ¾ due to haplodiploidy. Perhaps better model species to understand stormtrooper-like extreme cooperation are the parasitic trematodes recently discovered to have evolved caste formation [1]. That’s right folks, these parasitic flatworms exhibit an extreme form of cooperative division of labor via non-reproductive warriors and the reproductive baby-makers.

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Figure 1: Early versions of stormtroopers from the Star Wars universe were clones, thus they had a relatedness coefficient of r =1. This might explain the extreme degree of cooperation and division of labor seen in these foot soldiers. Caste formation in colony mates has more recently been described in some trematode parasites, with some morphs that are smaller than the others. (Figure modified from: Lloyd & Poulin, 2012 [5]).

“Aren’t you a little short to be a stormtrooper?”

Most parasites in the Trematode clade exhibit extreme life cycles that involve multiple host species, which may include snails, fish, crabs, birds, and mammals (read more about this here). One stage of the trematode life cycle requires clonal reproduction in an intermediate host; often these clones can constitute up to 40% of the body mass of their host, as observed in infected snails [2]. Snails with heterospecific infections (i.e. multiple trematode species) are often observed in the wild, although some have argued that this occurs less often than expected by chance. Trematode species like Himasthla sp. may have evolved a strategy to deal with both heterospecific and conspecific competitors to monopolize access to their intermediate host’s energy and resources. Researchers have independently discovered trematode species that form colonies that engage in division of labor [1], [3], [4]. The colonies consist of two morphologically-distinct clone types that either specialize in defense or reproduction. The smaller, larger-mouthed soldier clones are more active and were seen to actively engulf non-self clones of the same species as well clones of other trematode species [1]. These tiny stormtroopers are actually significantly smaller (see Figure 2 below). The larger morphs, on the other hand, attacked competitors much less frequently than the soldiers and were primarily involved in producing additional clones [1]. Researchers were also able to confirm that the soldier morphs were not temporally arrested in a non-reproductive stage, which means they are permanently engaged as foot soldiers on the front line of defense against non-kin.


Figure 2: Notice the extreme morphological differences between the tiny secondary or “soldier” morph compared to the large primary reproductive morph. Measurements of both morphs indicate that the reproductive morphs (in blue) consistently did not overlap in body width or length compared to the soldier morphs (in red), and their mouths were significantly smaller compared to their overall body size (Figure source: Hechinger et al., 2011 [1]).

Besides the obvious defensive role of these soldier morphs, some think that they may also be involved in nutrient exchange with the reproductive morphs or provide protection against microbes, although this has yet to be empirically confirmed [5]. So perhaps these soliders are also nourishing their colony mates, taking altruism to a whole other level. One study examined the fitness benefits in Philophthalmus sp., another trematode species that has also evolved division of labor [5]. The results indicated that in vitro co-infection with another trematode species (Maritrema novaezealandensis) did not always result in the engulfing behaviors previously described. Researchers suggest that it is possible the competitive pressures in a culture well-plate/lab setting may not have been high enough to elicit attack behaviors (see an example of this in the video below). However, they did observe consistently higher clonal reproduction when non-reproductive morphs were present, even when a competing trematode species was not present. In fact, colonies in which the two morphs were seen in contact, forming clumps, survived longer [5]. They suggest potential communication or nutrient exchange between morphs that might provide fitness benefits to the colony, although the mechanism for such a transfer is anything but a mystery.

Video 1: Note the larger Philophthalmus sp. cercaria consuming the cercarial larval stage of Maritrema novaezealandensis. (Video source: Leung & Poulin 2011, [4]).

More recent studies have examined whether trematodes actively adjust morph ratios depending on competitive regimes [6], [7]. For example, compelling results suggest that colonies may shift their investment in soldier morphs in response to the threat of infection by competing parasites. In other words, when there are more enemies, produce more stormtroopers. This fascinating social system provides researchers with fertile ground for questions about the evolution of cooperation and sociality. It is possible that cooperative division of labor is more widespread amongst trematode species, and even if it is relatively rare, what conditions are required for its emergence in distantly-related trematode taxa? And can we use these soldier morphs to reduce the burden caused by human diseases such as schistosomiasis and other trematode-mediated infectious diseases? [8]. Perhaps these mini-stormtroopers can be engaged in battles for a good cause…


  1. Hechinger, R. F., Wood, A. C., & Kuris, A. M. (2011). Social organization in a flatworm: trematode parasites form soldier and reproductive castes. Proceedings of the Royal Society of London B: Biological Sciences278(1706), 656-665.
  2. Hechinger, R. F., Lafferty, K. D., Mancini, F. T., Warner, R. R., & Kuris, A. M. (2009). How large is the hand in the puppet? Ecological and evolutionary factors affecting body mass of 15 trematode parasitic castrators in their snail host. Evolutionary Ecology23(5), 651.
  3. Miura, O. (2012). Social organization and caste formation in three additional parasitic flatworm species. Marine Ecology Progress Series465, 119-127.
  4. Leung, T. L., & Poulin, R. (2011). Small worms, big appetites: ratios of different functional morphs in relation to interspecific competition in trematode parasites. International journal for parasitology41(10), 1063-1068.
  5. Lloyd, M. M., & Poulin, R. (2012). Fitness benefits of a division of labour in parasitic trematode colonies with and without competition. International journal for parasitology42(10), 939-946.
  6. Lagrue, C., MacLeod, C. D., Keller, L., & Poulin, R. (2018). Caste ratio adjustments in response to perceived and realised competition in parasites with division of labour. Journal of Animal Ecology.
  7. Lloyd, M. M., & Poulin, R. (2014). Geographic variation in caste ratio of trematode colonies with a division of labour reflect local adaptation. Parasitology research113(7), 2593-2602.
  8. The Current Blog (UC Santa Barbara): Parasitic ‘Warrior Worms’ Discovered in Snails; UCSB Scientists See Possible Biomedical Applications (by Gail Gallessich) Link here.

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