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.

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.