Joshua Brian: Parasites flex their mussels to influence ecosystem function

In this new post, Joshua Brian, a new ecological researcher working at King’s College London,  UK, discusses his paper: Parasitism dramatically alters the ecosystem services provided by freshwater mussels—recently shortlisted for the 2022 Haldane Prize for Early Career Researchers.

About the paper

The paired experimental mussels (one with trematodes, one without) ready to go back into the river! Will bitterling fish (another parasite) avoid mussels already parasitised by trematodes? (Spoiler: yes) (credit: Joshua Brian)

There is increasing awareness of the importance of parasites in ecosystem structure and function. However, there has been very little work done actually linking parasitism to ecosystem function. Specifically, most previous work looks at ‘one host, one parasite’ systems, which fits poorly with ecological reality—multiple species in a given location might be contributing to ecosystem function, and each of those species might be infected with multiple parasites. We used two species of freshwater mussel, Anodonta anatina and Unio pictorum (prodigious filter feeders) and two parasite species (a trematode and a bitterling fish) to explore how parasites can affect ecosystems: in our case, the proportion of daily river discharge successfully filtered by freshwater mussels.

Briefly, we found that both parasite species affect mussel filtration rates—the way that this scales up to the ecosystem level depends on environmental conditions. The parasites also interact with each other and have different distributions in the two host species. Basically, parasites affect ecosystem function in complex ways—you need to think about underlying environmental conditions, parasite-parasite interactions, parasite-host interactions, and host densities and distributions simultaneously! The overall conclusion was that parasites may have a major effect on ecosystems (changing proportional river filtration by up to 96%!), in ways that could not be predicted by looking at a single parasite or single host in isolation.

About the research

The thing I enjoyed most about conducting this research was the diversity of approaches required—we needed a combination of field experiments, lab experiments, natural history surveys and some mathematical simulations to get at our key questions. It was a (rewarding) challenge to really try and think through how all these moving parts could be brought together in a coherent fashion. This all came together quite quickly towards the end of my PhD—I had been wanting to explore these ideas for a while, but Covid-19 hit just after my second year. Fortunately, I had just finished a year-long observational sampling regime at that point, so I was still able to be productive in lockdown by writing up those results, but it did mean that the entirety of my PhD was observational up until that time. We carefully designed this study to make best use of the data we already had and carry out really targeted experiments to get at our key questions. It brought together the key themes of my thesis (parasite community ecology and host conservation) and I am really proud of the paper it became.

Set-up for the lab experiment – each bucket has its own mussel; some have parasites, some do not. Will parasites affect their filtration rates? (Another spoiler: yes) (credit: Joshua Brian)

About the author

My supervisor Professor David Aldridge (right) and I (left) proudly show off our incredibly sophisticated experimental design for part of the paper (namely, sticking some mussels in some garden pots in a river) (credit: Jasmine Yang)

I was born in Wellington, New Zealand and lived there for the first 24 years of my life. I think Wellington is great for encouraging awareness in the natural world—it is surrounded by sea on three sides (including the wild south coast), it has lots of rugged hills, and the world’s first urban ecosanctuary (Zealandia). Therefore, I suppose my location growing up was quite formative. I didn’t really consider ecology too much until the last year of high school though—before that I wanted to be an author (which I guess I am now!), and then a classicist. I actually did a degree in Classics alongside my undergrad degree in Marine Biology—I think it really developed by skills in critical thinking, which have been invaluable in ecology. In my spare time, I really enjoy running and have done the London Marathon a couple of times. I find it really helps me to relax and process the day. I also play lots of board games (a lockdown favourite was Great Western Trail), though I am dangerously competitive.

I complete the Cambridge Half Marathon. This photo makes me look like a much more competent runner than I am (credit: Saucony 2019 Cambridge Half Marathon photography)

I believe that the best thing about being an ecologist is being able to ask questions that interest you. Conversely, I think that the worst thing about being an ecologist is the difficulty of answering the myriad questions that you come up with! If I had to provide one piece of advice to other ECRs, don’t take yourself or your ideas too seriously! I guess another way of saying what I mean is: don’t be afraid to throw random ideas out there – most (nearly all…) of them won’t end up working, but they will start conversations. This paper was the product of many lockdown Zoom conversations between me and David (my PhD supervisor), which mostly consisted of one of us proposing an idea and the other promptly shooting it down.

Enjoyed the blogpost? Read the research here!

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