Time to use some mussel: Exploring drivers of parasite community structure

This blog post is provided by Joshua Brian and tells the #StoryBehindThePaper for the article “Abundance data applied to a novel model invertebrate host sheds new light on parasite community assembly in nature“, which was recently published in Journal of Animal Ecology.

As much as we might try to avoid thinking about this, parasites are everywhere! However, the last couple of decades have seen a major shift in the way we think about parasites. While they can still cause serious diseases in humans (for example, schistosomiasis or Chagas disease), they are also recognised as a crucial component of ecosystems (Hudson et al., 2006), and they now even have a dedicated global conservation plan! (Carlson et al., 2020).

However, we still generally have quite a poor understanding of how parasite communities (all the different parasites that live in a single host organism) come together. For example, what is the relative importance of parasite dispersal, host characteristics or parasite-parasite interactions in determining parasite distribution across hosts? Another problem is the current coverage of host species. Most in-depth studies on parasite communities have been on vertebrates (particularly mammals), while invertebrates and especially aquatic invertebrates have been neglected.

In our paper, we saw an opportunity to address some of these research gaps. We used a single species of freshwater mussel (Anodonta anatina) in our study, which we thought was an interesting and useful choice for several reasons. First, they have a simple body plan and a large range of parasites such as trematodes, mites, nematodes and oligochaetes, which meant that we could get not only presence-absence but also abundance data for a diverse parasite community. Second, freshwater mussels are actually one of the most endangered animal groups in the world (don’t worry though – the species we used is very common!). Despite their threatened status, little is known about their parasite communities, or the risks of parasites for their conservation (Brian and Aldridge, 2019). This study therefore provided a window into freshwater mussel parasites.

I proudly display the first mussels of our year of sampling. Credit: David Aldridge

There is one more advantage to studying mussels – fieldwork is really easy! Unlike many other hosts, mussels don’t run away, so to collect mussels we simply waded into the river and hand-sampled from the mud. However, this wasn’t always pleasant: we wanted to sample all through the year, and so several collections involved smashing through a layer of river ice in order to find our quarry! Once we had found enough mussels, we took them back to the lab and I dissected every one to count all their parasites, in some cases utilizing techniques I had developed earlier in my PhD (Brian and Aldridge, 2020).

A rather chilly day for sampling… Credit: Joshua Brian

Using the results of these dissections alongside a range of modelling techniques, we found some really interesting things about parasite communities! For example, factors at a range of scales influence parasite community structure: the time of year, the length and reproductive status of the mussel, and competition between parasites all played an important role in determining what the parasite communities of different mussels looked like. However, their relative importance varied between different parasite species, suggesting that you can’t apply generalities to parasite community construction. There is hope though – we found that lots of this variation could be explained by parasite life history traits, such as which part of the host they lived in, and whether they used other hosts to complete their life cycle. Therefore, incorporating parasite life history traits into future studies should be a good way of understanding more about parasite communities. Finally, we also found that, if you just look at whether the parasite is there or not (and ignore the number of that parasite), as is common in parasite studies, it can give misleading results with potentially serious consequences. For example, if we had only used presence-absence data, we would have missed the fact that a trematode was castrating our mussels when it was highly abundant!

Some common parasites in mussels. All scale bars 250 µM. (a) Sporocyst with developing cercariae of the digenean trematode Rhipidocotyle campanula. (b) Metacercaria of the digenean trematode Echinoparyphium recurvatum. (c) The mussel-specific parasitic mite Unionicola intermedia. (d) A dorylaimid nematode. (e) Ciliates (Tetrahymena sp.) that are found on the gills. (f) A chironomid larva. Credit for all images: Joshua Brian

Our work is the most in-depth study to date of parasite communities in freshwater mussels, and introduces them as an interesting system to study parasite community structure. We think that by taking some of our conclusions on board, the field of parasite community ecology can really start to nail down some key concepts. This can improve our understanding of these crucial systems, facilitate effective conservation of ecosystem function, and mitigate disease spread in animal and human populations.

References

Hudson, P. J., Dobson, A. P., & Lafferty, K. D. (2006). Is a healthy ecosystem one that is rich in parasites?. Trends Ecol. Evol., 21, 381-385.
Carlson, C. J., Hopkins, S., Bell, K. C., Doña, J., Godfrey, S. S., Kwak, M. L., … & Wood, C. L. (2020). A global parasite conservation plan. Biol. Conserv., 250, 108596.
Brian, J. I., & Aldridge, D. C. (2019). Endosymbionts: An overlooked threat in the conservation of freshwater mussels?. Biol. Conserv., 237, 155-165.
Brian, J. I., & Aldridge, D. C. (2020). An efficient photograph-based quantitative method for assessing castrating trematode parasites in bivalve molluscs. Parasitology, 147, 1375-1380.

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