I was at the annual ESA meeting in Minneapolis last week and had an opportunity to attend lots of interesting talks. One of my favorites was about crayfish and their symbiotic crayfish worm (check out the link here and here for abstracts). The talk by Petipa stood out to me because he used a classic succession framework to predict community assemblage of symbiotic worm species on crayfish host. Plus, his PowerPoint slides were very pretty.
After I came back from ESA, I Googled their work a bit and came across this interesting paper, which is the topic of this blog post.
First, let me tell you briefly the biology of symbiosis between crayfish and worms (Branchiobdellid). Crayfish worms clean the surface of crayfish by removing parasites and old tissues, increasing crayfish growth and survival. Crayfish, in turn, provide food and shelter for worms. This is considered as a classic client-cleaner symbiosis relationship.
Using this cool study system, they tested the prediction that crayfish growth would increase with increasing worm density, though benefits would reach an asymptote at high worm densities (i.e. diminishing return).
They conducted three independent field experiments at different time and locations. They added different number of worms on crayfish (between 0 and 12 per crayfish) and followed crayfish growth for a few months. They found just what they predicted: Worms are increasingly beneficial at medium density (4 or 6) but actually become harmful to crayfish at density of 12. Cray fish with 12 worms grew less than the ones with no worms.
So what happens at density of 12? Apparently, worms start to eat the host! In fact they quantified the gill scars on crayfish and they found about 6 times more scars in the 12 worm treatment than on no-worm treatment.
So, the take home message is that the worms in moderation are good for a crayfish but having too much of a good thing can be bad. I think it’s a really cool empirical work demonstrating that species interactions can be fluid (or context dependent, technically speaking). In other words, the interactions cannot always be categorized as negative (e.g. competition, predation) or positive (mutualism, commensalism), but the outcome of the interactions depends on many factors… like the density of symbionts as this study, but can also depends on available resource and presence or absence of other organisms. This phenomenon is sometimes referred as “mutualism-parasitism continuum” (e.g. Johnson et al. 1997, Karst et al. 2004). I’ve been very fascinated by mutualism-parasitism continuum ever since I started working on symbiosis between plants and rhizobia (soil microbes that convert atmospheric nitrogen to ammonia for plants to utilize). I think this study wonderfully demonstrated the mutualism-parasitism continuum using a relatively simple experimental design. Now I’m going to look for more cool papers from this team!
Brown BL, RP Creed, J Skelton, MA Rollins, KJ Farrell. 2012. The fine line between mutualism and parasitism: complex effects in a cleaning symbiosis demonstrated by multiple field experiments. Oecologia. 170(1):199-207.