This blog post is provided Tamara Layden with edits by Dan Preston and tells the #StoryBehindThePaper for the article “Trematode parasites exceed aquatic insect biomass in Oregon stream food webs”, which was recently published in Journal of Animal Ecology.

As far as animals go, it is probably safe to say that parasites, such as trematodes, fall into the “lesser loved” category. Why? First, they are intentionally cryptic, literally hiding inside the body of another (and some are so translucent that you can see right through them); second, even the ones you can see are hardly ever considered cute (unless you have a fondness for their little eyespots, like I do); and third, they by nature exemplify, as you’d put it colloquially: a “toxic relationship” (a.k.a. parasitism, where only one species benefits and the other is harmed in return). So why give these creatures any love and attention? They’re weird, they’re cool, and they actually play significant roles in both aquatic and terrestrial ecosystems, impacting host population dynamics and community structure and, as we found in our study, the flow of energy. So, although our human inclination is often to avoid these high “ick factor” animals, the more we understand, the more we are able to appreciate these innovative creatures. Or maybe this might just make your skin crawl even more; read on and see.

First, let’s get into the weirdness of our focal parasitic group: Trematodes. Trematodes or “flukes” are wormlike parasites with both male and female reproductive organs. They are also considered “digenean” parasites, meaning they undergo sexual reproduction in a definitive host (often a vertebrate) and asexual reproduction in an intermediate host (usually an aquatic snail). Our study focused primarily on the latter, dissecting thousands of snail hosts to uncover the parasites within. In our case, the trematode invaders were particularly malicious, effectively castrating their snail host while using its resources to replicate and generate motile and free-living (i.e. not host dependent) larvae or “cercariae”. On the surface, the snail appears unimpacted, but zoom in using a backlit dissecting scope and you might see the surface of your petri dish swirl and shimmer. Although initially this made my eyes itch (note to readers, do not look into human eye parasites, they are way less fun), this is also when our research became visible. Thousands of cercariae came pouring out of the infected snail, like fish from a very full bucket, searching for a downstream host to penetrate percutaneously (through the skin). This process must occur quickly as the free-living stage is short-lived. After leaving the snail, the trematode cercariae often infect an aquatic invertebrate, fish, or amphibian as a second intermediate host. This host is then eaten by the final host, with the parasites being trophically transmitted, along with the meal. Adult trematodes attach to host tissue using their unique suckers: one near its head and the other on its underside. This allows for optimal nutrient absorption to produce eggs that their host then passes into the aquatic environment, so the cycle can begin again anew. Pretty weird, huh?

Now, let’s get into their cool factor. Although they are pretty ruthless in their life cycle for snails and other macroinvertebrates and vertebrate predators, they are actually pretty ingenuous for such seemingly simple creatures. Remember those eyespots? Some species (e.g., Metagonimoides oregonensis) have conspicuous (and very cute) pigmented granules that may aid in light cues, while others have long retractable tails to aid in movement that can extend longer than the length of their body (e.g., Microphalloidea). Adding to their coolness is their important role in the ecosystem. Because parasites, such as trematodes, are so heavily reliant on a host for survival, they rarely kill the host and instead, must maintain a delicate balance of nutrient sequestration while avoiding overexploitation. Naturally, this has other impacts on the host populations, which can include changes in host demographics, morphology, and behavior. Meanwhile, because the trematode life cycle is just that, a cycle, this process is capable of moving nutrients, carbon, and energy through an ecosystem. Energy is challenging to measure directly, so we focused on biomass in this study. As a result, we found the highest recorded biomass of trematodes, exceeding the combined biomass of all aquatic insects – comparable to the biomass of crayfish inhabiting the same stream system (can you say, trematode Étouffée?)! This suggests trematodes likely assume important functional roles equivalent to some of their more noticeable neighbors. So cool.

Although as humans we would probably prefer there to be zero parasites on the planet, it turns out there are likely way more parasites than we generally recognize inhabiting all ecosystems. Meanwhile, being human-centric, this knowledge may be troubling considering there are parasites that infect humans and our beloved pets; however, these cryptic creatures also have the potential to significantly contribute to food web and ecosystem dynamics in underappreciated ways. Thus, perhaps it’s time we challenge ourselves and hold these two dichotomies together – acknowledging that we should probably cook our food thoroughly and keep an eye on our dogs drinking stream water, while also recognizing that each snail, crayfish, and aquatic insect on the stream benthos may be a vessel, a home, to some other cool critters we cannot see. These hidden parasites, although rarely accredited beyond their role in causing disease, can be important to the functioning of the entire ecosystem, whether we like it or not (so we might as well learn to love ‘em).