Many animal species have fascinatingly complex social systems, but the pinnacle of sociality is relatively rare: the reproductive division of labor. Taxa that have separate castes of reproductive and non-reproductive individuals include the hymenopteran insects (ants, bees, and wasps), gall-forming aphids, termites, ambrosia beetles, sponge-dwelling shrimp, naked mole rats, and – because this is a Parasite Ecology blog – trematode parasites.
You’ve probably seen photos of the insane phenotypic differences between castes in some species; for instance, the difference between a queen fire ant and a worker fire ant (amazing photo by Alex Wild):
Or between queen, worker, and soldier termites (photo from here):
Trematode rediae have equally obvious caste differences, where reproductive rediae are huge and full of developing offspring, whereas soldier rediae are tiny with relatively large pharynxes (photo from here). And they don’t just look different; these castes are also spatially segregated, and they have unique behaviors. Reproductive individuals tend to hang out in the host snail’s gonads, while soldier trematodes tend to hang out in the mantle. And reproductive rediae rarely attack rediae from other trematode species, whereas soldier rediae readily attack invading species.
But despite these differences between reproductive and soldier rediae, the reproductive division of labor in first intermediate host trematode colonies wasn’t discovered until a few years ago. And until January (Garcia-Vedrenne et al. 2017), soldier rediae had only been documented in one trematode superfamily: the Echinostomatoidea.
It would not have been surprising if echinostomoids were the only trematodes to have soldier rediae, because echinostomoids are known for their ability to “fight” other trematode species. For instance, in a well-studied salt marsh system, echinostomoids sit at the top of a trematode dominance hierarchy, where they can successfully invade and conquer a California horn snail infected by a different trematode species, and they can successfully fight off invasions of their snail by other trematode species. But we now know that at least four species of heterophyid trematodes, which fall in the middle of that dominance hierarchy, also have a distinct soldier caste (Garcia-Vedrenne et al. 2017)!
This is a pretty big addition to our existing knowledge of these systems, and it makes one wonder how many other trematode species have undocumented soldier castes. Check out the paper to learn more!
Garcia-Vedrenne, A.E., A.C.E. Quintana, A.M. DeRogatis, C.M. Dover, M. Lopez, A. Kuris, and R.F. Hechinger. 2017. Trematodes with a reproductive division of labour: heterophyids also have a soldier caste and early infections reveal how colonies become structured. International Journal for Parasitology, 47(1): 41-50.