Contex-dependent symbiont dispersal: my favorite symbiont ecology paper of 2015!

For symbionts, transmission is dispersal. When free-living species (e.g., lions, aphids, and ducks) disperse, we expect them to have dispersal strategies that have been favored by natural selection; they should leave habitats where fitness prospects are low and preferentially disperse to habitats where fitness prospects are high, as long as the fitness benefits outweigh the costs. Logically, symbionts should also move from low quality to high quality habitats, where the “habitats” are hosts or locations on the host. However, we almost always assume that symbiont transmission/dispersal is a random process with a fixed rate. That is, we assume that host quality or site quality on the host doesn’t matter. But guess what? IT DOES MATTER. And you can read all about it in my favorite symbiont ecology paper from 2015! I’ll summarize it for you here:

The branchiobdellidan-crayfish symbiosis is one of my favorite symbiont-host systems, so I’ve blogged about it several times previously (e.g., here and here). In contrast to the Chaetogaster-snail system that I talked about last week, it’s relatively easy to measure branchiobdellidan fitness, because the branchiobdellidans lay cocoons on their crayfish hosts. Adult branchiobdellidans stay nearby and tend their little cocoon gardens (adorable!), so it’s easy to quantify each worm’s reproductive output.

In a field survey of branchiobdellidans on crayfish, Skelton et al. (2015) found that branchiobdellidan reproduction depended on crayfish size and the microhabitat on the host; some microhabitats favored branchiobdellidan reproduction, while cocoons were never found in other microhabitats. Also, there was a limit to the number of worms found in any given microhabitat, where some microhabitats on the crayfish could support more worms than others. And here’s something even more awesome: branchiobdellidans weren’t found in the suboptimal microhabitats unless the better microhabitats were already full. Ideal free distribution, anyone? SO. COOL.

But it gets better. Using the field survey data, Skelton et al. (2015) built a symbiont fitness-based dispersal model that incorporated crayfish size and microhabitat occupancy and quality, where there was some fitness threshold below which worms would disperse from donor to receiver crayfish. Then they ran a lab experiment where they put donor crayfish (with worms) in tanks with receiver crayfish (without worms), and counted how many worms dispersed and where on the hosts the worms ended up. Skelton et al. (2015) didn’t know what the worm fitness threshold was, so they used a model fitting procedure to figure out which threshold produced the best fits to the experimental data. The resulting fitness-based dispersal model could predict whether worms would disperse with 95% accuracy. 95% ACCURACY!! And 67% of the time, the model predicted the exact number of worms that dispersed. In contrast, the model that assumed a fixed rate of dispersal – with no influence of host size or microhabitat occupancy – couldn’t predict dispersal any better than a null model. When’s the last time an ecologist predicted something with 95% accuracy?!

So, symbiont dispersal not only depends on symbiont fitness prospects, but knowing which factors influence symbiont fitness can allow us to predict symbiont transmission/dispersal with incredible accuracy – much better than if we assumed a fixed rate. This has huge implications for the way that we model symbiont transmission! Go check out the paper. It’s beautiful.



Skelton, J., R.P. Creed, and B.L. Brown. 2015.  A symbiont’s dispersal strategy: condition-dependent dispersal underlies predictable variation in direct transmission among hosts. Proceedings of the Royal Society B 282: 20152081.

The coextinction of parasites, commensals, and mutualists – a call for more natural history studies!

By definition, mutualists, commensals, and parasites (hereafter “affiliates” in this post) depend on their hosts for resources or services. Therefore, if a host species goes extinct, the affiliates associated with that host may go extinct, too. And in fact, coextinction events like this should be as common as – or even more common than – extinctions of hosts, because we know that every host species has many mutualists, commensals, and parasites. Just think about the mites living in your eyebrows, the bacteria living in your intestines, and that one time that you had lice in third grade. If humans disappeared, all of those affiliate species might also go extinct!

Of course, you might not believe in the intrinsic value of all species; you might be wondering why you should care if some tiny species that you’ve never heard of goes extinct. Extinctions of mutualistic species – such as the gut microbes that help you digest food and the pollinators that keep our agricultural systems running – have obvious implications for our economy and health. But parasites, too, play important roles in our lives. For instance, they regulate populations of wildlife host species, and they may prevent you from having allergic reactions to things that you shouldn’t be allergic to. And of course, species exist in intricate webs of interactions, and by accidentally (or purposely!) adding or removing species from ecosystems, we have often learned that one species can have huge impacts on ecological communities.

So, coextinctions of affiliates are important, and these coextinctions should be common. That means that we have documented tons of these coextinction events, right? Actually, we haven’t! There are very few examples of documented coextinctions (Dunn 2009, Colwell et al. 2012), and some of those are not entirely open and shut cases. But why?

Say that you document the extinction of a particular host species: Host A. Should every affiliate associated with Host A also go extinct? Because some affiliates likely use multiple host species, some of the affiliates of Host A probably survived on other host species. Also, even an affiliate that historically only used Host A might be able to continue existing if it can switch to a new host species. For instance, maybe Host B, a close relative of Host A, is a suitable alternative host.

Now imagine that you’re trying to document affiliate coextinctions as Host A disappears. What evidence might you use to figure out which affiliates have also disappeared? There might be published accounts of some of the affiliates of Host A, but there are very few host species (if any) for which every affiliate species has been documented. Therefore, the loss of one host species means that several unnamed and undescribed invertebrate species will be lost before ever being documented by humans. Even if you had a perfect list of every affiliate species, it might be really difficult to confirm whether each affiliate was now extinct. That’s because we rarely (if ever) have perfect lists of every host species used by a given affiliate species. So, if one affiliate species frequently uses three host species, but you think it is a specialist on Host A, you might think the affiliate has gone extinct, only to find it happily hanging out on Hosts B and C when you survey those species three decades later.

To summarize, we predict many coextinctions of affiliates to occur as hosts go extinct, but we have hardly documented any such coextinctions. It may be that that affiliate species are much less vulnerable than we expect due to the use of multiple host species or host species switching as a primary host goes extinct, and/or it may be that we are just very poorly equipped to observe and document these coexinctions. Clearly, if we’re going to get better estimates of affiliate coextinction rates, we need more data! Specifically, we need:

  • Better understanding of the natural histories of these systems. We need complete lists of affiliates for each host species, complete lists of host species for each affiliate species, preserved specimens of affiliates for genetic identification, and information on the strengths of the interactions between each affiliate and host species.
  • Better estimates of how frequently affiliates shift host species, and whether jumps to new host species are associated with declines in the availability of the current species. In other words, how often do we expect affiliates to sink with the ship versus swimming to safety? (For further reading about this, see Kiers et al. 2010.)

(It’s been too long since my last pirate worm cartoon….)


Some related reading:

Conservation – save the parasites along with the hosts?

Are pubic lice going extinct?


Colwell, R.K., R.R. Dunn, N.C. Harris, and D.J. Futuyme. 2012. Coextinction and Persistence of Dependent Species in a Changing World. Annual Review of Ecology Evolution and Systematics 43: 183-203.

Dunn, R.R., N.C. Harris, R.K. Colwell, L.P. Koh, and N.S. Sodhi. 2009. The sixth mass coextinction: are most endangered species parasites and mutualists? Proc. R. Soc. B 276: 3037–3045.

Kiers, E.T., T.M. Palmer, A.R. Ives, J.F. Bruno, and J.L. Bronstein. 2010. Mutualisms in a changing world: an evolutionary perspective. Ecology Letters 13(12): 1459-1474.

Pinchy Has an Itch

Last week, I told you that small crayfish groom off their branchiobdellids.  Intermediate-sized crayfish try to groom off their branchiobdellids, but the crayfish can’t reach all of the worms.  Specifically, they can’t reach the worms that hang out on that one place on their dorsal carapaces.  Have you ever had an itch on your back that you couldn’t reach?  Yeah, that’s the story of the intermediate-sized crayfish’s life.

pinchy itch

Pirate Worms

So, you know how sometimes you see another scientist’s work and you get ‘system envy’?  How you suddenly realize that everything about their system is amazing, and you want to switch to work in their system right now?  Well, today I’m going to introduce you guys to a really, really cool system, and then I’m going to do a mini-series of posts about some of the recent work that I’ve seen regarding this system.

OK, READY?  There aren’t many relevant videos on the internet, but I want you to click this link to go see some really cool worms on youtube.  And if you want to see an even more amazing video, you can click here to download it. (It is seriously worth it.)  And for pictures, check these out.

You’re looking at branchiobdellidans.  They’re annelid worms that live as ectosymbionts on crayfish.  There are 150 species and 21 genera of branchiobdellidans in the world, and they are all thought to be obligate crayfish symbionts, meaning that they can’t survive and reproduce if they aren’t on a crayfish host.  Some of the worm species will hang out anywhere on the crayfish body, while others specialize on the crayfish gill chamber or the crayfish chelae.

An ectosymbiont is just an organism that lives on another organism.  The term doesn’t imply anything about the nature of the relationship between the branchiobdellidans and crayfish.  We can assume that branchiobdellidans benefit from the relationship because they get a place to live and lay their eggs, and they also graze on the smaller organisms that live on the crayfish exoskeletons and in the crayfish gill chambers.  But what about the crayfish?  Do they benefit from the relationship?  If branchiobdellidans increase crayfish fitness, then the relationship is mutually beneficial (=mutualism).  If crayfish don’t benefit from the relationship but aren’t harmed either, then the relationship is a commensalism.  And if crayfish fitness is reduced by the relationship, then branchiobdellids are parasitic.  Or, if you’d rather see that in cartoons:

HMS Crayfish 2

Are branchiobdellids mutualists, like passengers who pay for their voyage? Are they commensalists, which get a free-ride but don’t harm their hosts, like stowaways? Or are they pirates, which board the crayfish ship by force and loot the crayfish booty? And perhaps more importantly, are there support groups for people obsessed with dressing worms up in fancy costumes?

Historically, scientists thought that branchiobdellidans were parasites or commensalists, because they are known to consume crayfish gill tissue.  In fact, the more branchiobdellidans a crayfish has, the more scars the crayfish has in the gill tissue.  So, that’s sounds rather parasite-ish!  But the branchiobdellids also clean the gill chamber, which is good for crayfish respiration.  And so the question is whether branchiobdellidans have a net positive or net negative effect on crayfish.  Next time, I’ll tell you about a paper that shows that the net effect of branchiobdellidans on crayfish depends on branchiobdellid density and ‘context’, where branchiobdellidans are more beneficial to crayfish when crayfish are in environments where their gills are more likely to be colonized by bacteria and other organisms.

Until then, check out this review about branchiobdellidans that just came out in Freshwater Science!


Skelton, J., K.J. Farrell, R.P. Creed, B.W. Williams, C. Ames, B.S. Helms, J. Stoekel, and B.L. Brown. 2013. Servants, scoundrels, and hitchhikers: current understanding of the complex interactions between crayfish and their ectosymbiotic worms (Branchiobdellida). Freshwater Science 32(4): 1345-1357.