50 Shades of Symbionts

When we “sell” our science to journals and policy makers and even the general public, we often pitch our work in broad, abstract strokes (“trait-mediated indirect effects of…”) and/or in a highly applied context (“acid runoff tolerance of two functionally important species”).  I’m not saying that’s wrong.  But I think that most scientists – and most non-scientists – fall in love with ecology because the systems (the actual plants/animals/etc.) are cool, and then we have to gloss over the insanely awesome systems that we study in order to talk about the general applicability of our results.  Well, no more brushing cool systems under the rug!  Parasite Ecology is taking action by doing a few weeks of Odes to Awesome Systems.

The best way to prove that you have an awesome study system is to graphically illustrate the unquestionable adorableness of your study species.  EXHIBIT A – Hermit Crabs with Pink Afros:

Photo from here.

Did you know that hermit crabs have over 500 symbiont species?  More than 100 of those symbionts are obligate symbionts, meaning that they are only found on/in/with hermit crabs.  I learned that while perusing a heartwarming tale entitled, “”The Not So Lonely Lives of Hermit Crabs: Studies on Hermit Crab Symbionts.

One of those obligate symbionts is the pink afro (also called “snail fur”) in the photos above.  Those colonial hydroids (genus Hydractinia) are found exclusively on gastropod shells, and especially shells that are occupied by hermit crabs.  Unsurprisingly, scientists who go out and find hermit crabs with pink afros just have to ask this question:  do the hydroids affect their hermit crab hosts?

As I’ve blogged about before (here and here), some symbionts protect their hosts from natural enemies.  Buckley and Ebersole (1994) wondered if the hydroids could protect hermit crabs from being eaten by blue crabs.  They found that blue crabs were just as likely to attack hermit crabs with or without hydroids, so the hydroids didn’t have any effect on predator preference.  However, blue crabs were much more successful when attacking hermit crabs with hydroids.  Having hydroids actually made hermit crabs more susceptible to predation!

BUT… gastropod shell strength wasn’t associated with the presence of hydroids.  So what was it about hydroids that made it easier for blue crabs to successfully attack hermit crabs?  Well, a second, parasitic symbiont – shell-boring Polydoran worms – decreased shell strength, and those worms were more likely to be present if the shells had hydroids.  So, one symbiont mediated the occurrence of a second symbiont, which in turn mediated blue crab predation success.  Nuts!

Hermits

This could be the part of the story where we conclude that hydroids decrease hermit crab fitness.  But remember how most symbioses are context-dependent, where the strength and even the sign of the interaction depends on environmental and ecological conditions?  Well, it turns out that hydroids protect hermit crabs from a different enemy: ectoparasitic slipper limpets.  Therefore, Buckley and Ebersole (1994) suggest that the relationship between hydroids and hermit crabs changes throughout the year, depending on whether blue crabs and/or limpets are abundant.  That really emphasizes the importance of studying symbioses across broad time scales and under varying ecological and environmental conditions.

So, there you have it.  You can’t figure out hermit crab ecology without thinking about hermit crab symbionts.   Pink afros are more than just fashion statements.

Reference:

Buckley WJ, Ebersole JP (1994) Symbiotic organisms increase the vulnerability of a hermit crab to predation. J Exp Mar Bio Ecol 182:49–64.

3 thoughts on “50 Shades of Symbionts

  1. Pingback: Guard Crab Functional Diversity | Parasite Ecology

  2. Pingback: Predator vs. Parasite vs. Parasitoid vs. Mutualist– A Simple Classification Scheme | Parasite Ecology

  3. Pingback: The parasite ecology papers that got away: Part III | Parasite Ecology

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