Guard Crab Functional Diversity

Continuing my theme of Odes to Awesome systems, I want to tell you guys about one of my favorite animals: guard crabs.  These crabs live on corals and deter starfish predators from attacking the corals by pinching the starfish.  The guard crab cute factor is out of this world:

Photo credit: Bryan Mayes

I’ve been fascinated by guard crabs ever since I read the really thorough studies of Peter Glynn from the 1970’s and 1980’s.  He did all kinds of work showing that guard crabs protect corals from predation by Ancanthaster starfish.  Not all coral species have guard crabs, and Peter Glynn showed that populations of unprotected coral species are decimated during Ancanthaster outbreaks, while protected species suffer few losses.  Furthermore, protected corals sometimes indirectly defend unprotected corals, because starfish won’t cross barriers of protected corals.

Previously, it seemed like some guard crab species were a lot better at protecting corals than others.  This is true in other defensive symbiont systems, too (e.g., ants that protect Acacia trees).  But I just read a really cool study by McKeon and Moore (2014) that shows that the slacker symbionts may not be as lazy as we thought!  Specifically, small guard crabs aren’t very good at protecting corals from starfish predation.  However, small guard crabs are good at protecting corals from smaller corallivores, like predatory snails.  Furthermore, big crabs are no good at protecting against those smaller corallivores.  (This is a cool parallel to the paper I talked about last week, where hydroids protect hermit crabs from limpets but not blue crabs.)

So, corals need functionally diverse guard crab communities – communities with large and small guard crabs – to be protected from multiple corallivore species.  Awwwwesome!

Reference:     

McKeon CS, Moore JM (2014) Species and size diversity in protective services offered by coral guard-crabs. PeerJ 2:e574.

How do parasitoids respond to defended hosts?

Last week, I talked about the new Godzilla movie and how I thought that the MUTOs should have been parasitoids.  This week, let’s talk about some awesome, real life parasitoids: parasitoid wasps (Aphidius ervi).

Quickly, the life cycle works like this: the female wasp finds an aphid nymph, she stabs the aphid with her ovipositor, and then she typically lays one egg inside the aphid.  After one day, the egg hatches into a larval parasitoid, and the larva hangs out inside the aphid while eating the aphid’s innards.  After about one week of this, the aphid dies.  Actually, the aphid’s corpse becomes a “mummy,” and the larva pupates inside the mummy before eventually emerging as an adult parasitoid.  Mating happens, and then the female wasps go off to infect more aphids.

But here’s an interesting complication: some aphids are protected by bacterial symbionts (Hamiltonella defensa).  The degree to which aphids are protected varies with the strain of H. defensa, but the take-home message is that when a wasp lays an egg inside an aphid, the egg is much less likely to survive to adulthood if the aphid has H. defensa symbionts (Oliver et al. 2012).  However, if a wasp lays two eggs inside an aphid with H. defensa symbionts – which is not what wasps usually do – then one larva is more likely to survive than it would have been if it had been a single egg.  In other words, only one larva is going to make it out of there alive, even when two eggs are laid, but one of the larvae is better off than it would have been if it were a single egg.

You might be thinking, “Why, what an interesting tidbit.  Who cares?”  NATURAL SELECTION CARES.  Just kidding, natural selection isn’t sentient, but natural selection should favor any wasp strategies that increase wasp fitness.  And wasp fitness is higher when more wasp eggies turn into wasp larvae and then adult wasps.  And wasp larvae are less likely to die in aphids with H. defensa if two eggs are laid in the aphid, instead of the typical single egg.  See where I’m going with this?

Yes, wasps can differentiate between aphids with and without H. defensa.  And when aphids have H. defensa, wasps are much more likely to lay two eggs in those defended aphids than they are to lay two eggs in undefended aphids.  And that, my friends, is amazing.  While wasps still probably have reduced fitness when infecting defended aphids, the superinfecting tactic (=laying two eggs) likely compensates for some of the reduced fitness.

 

(Yes, sometimes aphids have conversations in my head, and I write them down. You’re welcome.)

Check out the open access paper to learn about the mechanism behind the success of superinfection:

Reference:

Oliver, K.M., K. Noge, E.M. Huang, J.M. Campos, J.X. Becerra, and M.S. Hunter. 2012. Parasitic wasp responses to symbiont-based defense in aphids. BMC Biology 10:11.