Originally posted May 17, 2009.

Acanthocephalan parasites are infamous for their ability to dramatically alter the behavior of their hosts.  These parasites often have multi-stage life cycles, meaning that they die unless the host that they’re currently residing in gets eaten by the next host in the cycle. Acanthocephalans are remarkably efficient at manipulating their intermediate hosts into ending up on the dinner plate of the next host in the cycle.

One well-known example of this manipulation involves the acanthocephalan parasitePolymorphus minutus and its crustacean intermediate host Gammarus roeseli.  The definitive host of P. minutus is a waterbird which scoops G. roeseli out of the water.  The usually defense by G. roeseli to avoid being consumed by the waterbird is to hunker down at the bottom of a streambed and hide in the rubble. The parasite is capable of counteracting this defensive mechanism, presumably by altering the concentration of serotonin in the crustacean’s nervous system. This manipulation causes the the host to swim to the water surface and clamp down on the surrounding vegetation.  In a closely related system (a different species of Gammarus and a different Polymorphus parasite), this behavior has been found to significantly increase consumption of infected gammarids by waterbirds.  In essence, the parasite causes its tiny host to swim up to where the predators are feasting and hang out until it becomes dinner.

Gammarus roeseli

The interests of G. roeseli and the parasite P. minutus are clearly not aligned when it comes to waterbird predation. There is, however, one thing that they can agree on.  Neither of them want to end up in the stomach of other predators (fishes, crustaceans, etc.). Non-waterbird predators represent a dead end for both members of the party.  The old addage “the enemy of my enemy is my friend” goes a long way here, and recent studies suggest that the parasite enhances the host’s ability to stay out of harm’s way when their interests converge.

Three-spined sticklebacks

Medoc et al. 2009 have shown that gammarids infected with P. minutus have a leg up on their uninfected counterparts when it comes to avoiding predation by three-spined sticklebacks.   Infected gammarids spent more time hiding in vegetation near the water surface and suffered much lower predation rates.  Additionally, another recent study found that infected gammarids are up to 35% faster when escaping a predatory crustacean.

This phenomenon has received little attention in the literature.  The few studies that have looked at whether or not parasites “help” their hosts escape from mutually unfavorable predators have reported mixed results.  Some studies have found that infection increased the host’s susceptibility to all predators, whether or not they’re included in the parasite’s life cycle.

The two studies decribed above showed that the gammarids could escape from predators if escape involved moving fast or hiding near the water surface. But, although the parasites have evolved an excellent host manipulation, it’s probably not perfect.

For example, in the above system where parasites make gammarids cling to water surface vegetation in order to be eaten by birds, it is likely that by doing so the gammarids become vulnerable to other predators. At that water level, there should be a number of other predators that can take advantage of the defenseless gammarids. How fine-tuned the parasite’s control of the gammarid can be is an interesting question deserving future study.

An acanthocephalan

5 Responses to “The Enemy of My Enemy is My Friend”

  1. Quick Links | A Blog Around The Clock

    […] The Enemy of My Enemy is My Friend […]

  2. Tweets that mention The Enemy of My Enemy is My Friend | Weinersmith --

    […] This post was mentioned on Twitter by Bora Zivkovic, thaitvnews. thaitvnews said: RT @BoraZ: The Enemy of My Enemy is My Friend […]

  3. Online Casino

    ha, I am going to experiment my thought, your post bring me some good ideas, it’s really amazing, thanks.

    – Thomas

  4. Poker zasady

    Literature is mostly about having sex and not much about having children. Life is the other way round.

  5. Anissa

    The proverb is sometimes phrased as “the enemy of mine enemy is my friend” or “my enemy’s enemy is my friend.

Leave a Reply

Your email address will not be published. Required fields are marked *

This site uses Akismet to reduce spam. Learn how your comment data is processed.


September 1st, 2018

Graduate student position available studying alternative reproductive tactics at BGSU

We seek a graduate student for a newly NSF-funded project examining the life history decisions made by male smallmouth bass. […]

June 14th, 2018

Part of that World

The other day I was singing “Part of your World” from The Little Mermaid, but was changing some of the […]

June 13th, 2018

Parasite manipulation of host behavior in pop culture

I’m going to be giving a talk at the sure-to-be-amazing Zombie Apocalypse Medicine Meeting. The meeting celebrates all things zombie-related, and […]

June 4th, 2017

Soonish giveaway on Goodreads!

Five copies of the advance reader version of Zach and my new book Soonish are up for grabs on Goodreads! Click […]

March 7th, 2017

Soonish: Ten Emerging Technologies That’ll Improve and/or Ruin Everything

Zach and I wrote a book! Soonish: Ten Emerging Technologies That’ll Improve and/or Ruin Everything explores 10 emerging technologies, and discusses the roadblocks […]

January 26th, 2017

Tales from the Crypt: a parasitoid manipulates the behaviour of its parasite host

I have a new paper out with Dr. Scott Egan, Dr. Andrew Forbes, and Sean Liu! The paper is Open Access […]

May 30th, 2016

Postdoc with Dr. Ryan Hechinger (and me!)

We’re looking for a postdoc! See below! —————— Postdoctoral Opportunity with the Marine Biology Research Division at SIO Postdoctoral Scholar […]

May 7th, 2016

Science…sort of Episode 240: Moon Rocks Don’t Glow

I co-hosted an episode of Science…sort of recently. I pasted the show notes below, but you’ll have to head over […]

February 24th, 2016

Books on parasites

I’m often asked by students to suggest books they can read about parasites. Below is a list of books that […]

August 22nd, 2015

Great Adaptations – A kid’s book about evolution

Zach Weinersmith and I contributed to Tiffany Taylor’s children’s book about evolution. Tiffany worked with scientists to create Seuss-style stories […]