Spooky amphipods

Happy (almost) Halloween! In honor of this wonderful holiday, I’m blogging about pumpkin-colored zombies.

BUT FIRST – I don’t think I’ve blogged about amphipods before, so I need to spend a minute gushing over how cool they are. If you’ve ever visited a beach, you’re probably familiar with “sand fleas” – which are not, in fact, fleas. They’re terrestrial amphipods. And that’s just the tip of the amphipod iceberg. There are thousands more amphipod species living in freshwater and marine environments around the world, where they play important roles in food webs. They can be beautiful and adorable, like this Coral Hopper Amaryllis (photo credit Dave Harasti):


Or this stinkin’ cute ladybug amphipod (photo credit Yury Ivanov):

Cyproideidae amphipod

They can also be really cool parasites. (Or parasitoids? Or predators? I’m a bit unclear on the life histories.) For instance, species in the hyperiid suborder slice open gelatinous plankton (e.g., salps), eat out their insides, and then hang out and lay eggs within the remaining body of the dead host. Aren’t they awesomely creepy?! (Photo credit Tara Oceans.)


And then of course there are the whale “lice.” Look closely…


In addition to being cool parasites, amphipods can be infected by cool parasites. That brings me to a recent paper by Johnson and Heard (2017), which is open access and available here. The paper has some great cartoons and photos in the life cycle and other diagrams – kudos for SciArt! – so you should take a look! There’s also an article about this paper over at ScienMag, including funny author quotes. You should check it out! But here’s my brief summary and some cartoons:

The salt marsh amphipod Orchestia grillus is brown and wary of bird predators when it is not infected by trematodes. But when infected by metacercariae of the trematode Levinseniella byrdi, the amphipod turns conspicuously orange and stumbles into more open habitats where predation by the definitive bird host might be more likely. Johnson and Heard (2017) tracked the density of these uninfected and infected amphipods in Massachusetts salt marsh sites where nutrients have been artificially added for years to understand the effects of eutrophication on salt marsh ecosystems. They found that eutrophication predictability increased the density of all amphipods in the salt marsh. And whereas reference sites had few infected amphipods, nutrient enrichment increased the prevalence of trematode infection to ~30%, creating an orange amphipod apocalypse. So the biomass of both hosts and parasites increased in nutrient enriched systems, similar to this other study regarding snails and their trematode parasites. Pretty cool stuff! But again, the paper is better than my summary, so go take a look!



Johnson, D. S., and R. Heard2017Bottom-up control of parasitesEcosphere 8(10).

Is zombie transmission transmission frequency or density dependent?

For reasons that I cannot explain, visitors have started to stumble upon my blog by googling the question, “is zombie infection frequency or density dependent?” Maybe there’s a really awesome educator out there using that example in class. Or maybe the zombie apocalypse has started and people are secretly beginning to plan for the end. Either way, this is a neat question that I’m willing to speculate about!

First, we need to decide what kinds of zombies we are talking about. Let’s assume we’re looking at World War Z type zombies, where infection is transmitted via bites/saliva/fluid transfer. Let’s say that the zombies are highly mobile and thus the human and zombie populations are well-mixed. Also, let’s assume that zombies don’t really have a contact structure, like humans do, because they’ve lost any kind of social system that they had a humans.

Given those assumptions, I would expect disease-relevant contacts to increase with host density. So, if I had to pick between density dependent and frequency dependent transmission, I’d expect density dependent transmission. But don’t forget that there are nonlinear contact functions, too. Those might work better, because even a tireless biting machine can only bite so many people per day.

When might zombie transmission be frequency dependent? FD transmission would be appropriate if larger populations covered larger areas, so that host density was constant. I suppose that could happen if humans were dispersing as much as possible and running away from zombie-packed areas. What do you think?

The Last of Us

I thought that we’d do a quick, just-for-fun post today.  ABOUT ZOMBIES.  Now, I like me some zombie movies.  Sometimes their plots even have cool disease ecology components, like competition between the “zombie virus” and other host pathogens.  On the other hand, there are many, many biological inaccuracies involved in the popular zombie idea, as Neil deGrasse Tyson explains.  

But if you’ve been following this blog or popular science for any length of time, you know that in a way, Neil deGrasse Tyson is wrong in saying that if zombies exist, they only exist on other planets.  There are “zombies” on Earth: parasite zombies!  That is, some parasites can dramatically alter their host’s behavior, so that the host is effectively just a vehicle for the parasite.  The point of this manipulation is to get the host to behave in such a way as to increase the parasite’s probability of transmission to the next host.  Usually, this involves the host getting eaten by the next host, like when infected ants hang out at the top of blades of grass, where they are likely to be eaten by cows or sheep.  But that’s not always the case.  For instance, with rabies – the pathogen most similar to the classic idea of a zombie virus –  the virus makes (some) animals behave aggressively, and this increases the probability that the virus will be transmitted to new hosts via bites.  

Now, thankfully, there aren’t any parasites that re-animate dead corpses…yet.  But there are parasites that use corpses as points of transmission.  For instance, Cordyceps fungus makes ants leave their normal routines to go bite onto leaves above major areas of ant traffic.  Then the fungus sprouts a fruiting body out of the ant’s corpse and rains spores of death down on the ant’s extended family.  

Don’t you think that a Coryceps fungus apocalypse would be a cool video game plot?  Well, actually, it already is a cool video game plot!  The Last of Us has been out for a while on PS3, and a remastered version was just released on PS4.  From Wiki:

“In 2013, Joel (Troy Baker) is a single father living near Austin, Texas with his twelve-year-old daughter Sarah (Hana Hayes). One night, an outbreak of a mutant Cordyceps fungus ravages the United States, which transforms its human hosts into cannibalistic monsters…”

Now, real Cordyceps doesn’t turn insects into cannibals, but I’m willing to overlook this error in biology because – WAIT FOR IT – they’re going to make The Last of Us into a movie, too!  That’s right.  Cordyceps is coming to the big screen.  Awwyisss.     

Nematomorphs and the Role of Parasite Manipulation in Ecosystems


In the past year, I have had so much fun sharing really cool parasite ecology with you guys.  Many thanks to everyone who has visited, followed, shared, commented, and/or given me feedback!  You’re awesome! For this birthday post, I’m writing about one of my favorite parasite ecology papers, which is about one of my favorite parasites: nematomorphs!

Now, I could spend at least 3000 words telling you every awesome thing that I know about nematomorphs, which are also called Gordian worms and hairworms.  Instead, I’m going to urge you to google nematomorphs.  They have an especially interesting life cycle, and you should click through to see some of the related photos and videos.  The larval nematomorphs  infect an aquatic invertebrate (like a damselfly larva or a snail); that invertebrate then ends up on land, dies, and the invertebrate’s tissue and the larval nematomorphs are consumed by a terrestrial invertebrate like a cricket; the nematomorph grows into an adult within the cricket and then it manipulates the cricket’s behavior to cause the insect to jump into a body of water; the adult nematomorph emerges from the cricket in the water and swims around looking for an adult of the opposite sex; finally, the adults mate and the females lay eggs.

In an earlier paper, Sato et al. (2011) found that crickets infected with nematomorphs were 60% more likely jump in Japanese headwater streams than uninfected crickets.  And these crickets were an important part of the diet of endangered trout.  In fact, the trout ate so many crickets during the nematomorph breeding season that the trout ate significantly fewer benthic invertebrates than usual.  This raised an interesting question: does the manipulation of crickets by nematomorphs have cascading effects on energy flow in stream ecosystems?

Sato et al. (2012) then performed an experiment in headwater streams that typically have few crickets where they 1) added dead crickets to the streams as a resource subsidy, 2) prevented just crickets from jumping in the streams, or 3) prevented all terrestrial invertebrates from entering the streams.  They found that both big and little trout ate more crickets when they were added to the stream.  And little trout then ate fewer benthic invertebrates in those streams, just like they saw in Sato et al. (2011).  Furthermore, in the streams where crickets were added, there was less benthic algae and faster leaf decomposition because there were three times more benthic invertebrates in those streams.  So, it does look like the added cricket resources can have far-reaching effects on headwater streams!  I can’t wait to see the diffeq model of this… it’s coming, right?

Sato et al. (2012) point out that there is also some evidence from other systems where parasite manipulation has far-reaching effects on energy flow in ecosystems.   I expect we’ll be seeing a lot more research on this topic in the future!

I couldn’t make these crickets not look sinister…

I couldn’t make these crickets not look sinister…

For a very recent study about nematomorphs, see the Parasite of the Day‘s post about Sato’s 2014 paper.


Sato, T., K. Watanabe, M. Kanaiwa, Y. Niizuma, Y. Harada, and K.D. Lafferty. 2011. Nematomorph parasites drive energy flow through a riparian ecosystem. Ecology 92: 201–207.

Sato, T., T. Egusa, K. Fukushima, T. Oda, N. Ohte, N. tokuchi, K. Watanabe, M. Kanaiwa, I. Murakami, and K.D. Lafferty. 2012. Nematomorph parasites indirectly alter the food web and ecosystem function of streams through behavioural manipulation of their cricket hosts. Ecology Letters 15: 786–793.

Zombie Crabs

I just can’t resist telling you guys about parasites that alter host behavior.  Today, the hosts are zombie crabs, and the parasite is the Sacculina barnacle.

The larval female barnacle finds a crab and infects it.  They specifically take up residence on the crab abdomen, where the crab would carry its own eggs (if it were female).  The barnacle will infect male and female crabs, and if it infects a male, the male’s hormones will be manipulated so that the male’s morphology and behavior are altered to be more like female morphology and behavior.  NEAT!

Later, larval male barnacles come fertilize the female’s eggs.  The host crab will then care for the parasite eggs!!  

I highly recommend doing some googling for photos/videos.  

The blobby thing on the abdomen is the Sacculina barnacle. Photo from wikimedia.

The Oatmeal – Parasite Zombies

Are you getting tired of parasite zombie posts yet?  I hope not, because I’ve got a lot more where these have come from!  Starting with this really awesome (I mean, REALLY AWESOME) cartoon by The Oatmeal.   

Dicrocoelium dendriticumthe lancet liver fluke, is a parasite of livestock (e.g., cows, sheep).  Of course, Parasite of the Day has written about this awesome parasite before.  I highly recommend popping over there for more details.  And of course, check out the full cartoon from The Oatmeal!


A taste of The Oatmeal‘s brilliant cartoon about the lancet fluke.

Parasite Ecology Papers in Your Easter Basket

Happy Easter, Everyone!  Since we all know that chocolate (delicious, delicious chocolate) rots your teeth, I got you something else for Easter:  Parasite Papers!  MMMMM!


Parasite Resurrection:

Remember when I talked about the dilution effect recently?  I said that having hosts species with low competency would reduce disease transmission because low competency hosts reduce encounter rates between infectious agents and high competency hosts.  ‘Decoy hosts’ (aka ‘dead end hosts’) are resistant ‘hosts’; they have a competency of 0, or close to it.  So, they should be sucking up parasites without transmitting them, and therefore they should reduce the parasite population.  But wait… what if parasites don’t die when they try to infect a decoy host?  What if they get a second chance to find a susceptible host?  Dun, dun, dun!  (Ecology and Evolution is open access.  You could read this paper RIGHT NOW, if you want.  I’m sure the suspense is killing you.)

King, K.C.,  S.K.J.R. Auld, P.J. Wilson, T. James, and T.J. Little. 2013. The bacterial parasite Pasteuria ramosa is not killed if it fails to infect: implications for coevolution. Ecology and Evolution 3(2): 197-203.

Protective Symbionts Come at a Cost:

This is one of my favorite disease ecology topics: symbionts that reduce infection!  But are disease-reducing symbioses free?  Not in this aphid disease system!  (Again, this is open access.)

Vorburger, C., P. Ganesanandamoorthy, and M. Kwiatkowski. 2013. Comparing constitutive and induced costs of symbiont-conferred resistance to parasitoids in aphids. Ecology and Evolution 3(3): 706-713.

Potassium Increases Disease Epidemics:

How about some abiotic factors?  Potassium increases Daphnia and fungus populations, thereby increasing epidemics in this aquatic system.  I like this paper because its very thorough.  They did field work, modelling, and lab and mesocosm experiments.

Civitello, D.J., R.M. Penczykowski, J.L. Hite, M.A. Duffy, and S.R. Hall. 2013. Potassium stimulates fungal epidemics in Daphnia by increasing host and parasite reproduction. Ecology 94:380–388.

Paired Papers: Cane Toad Invasion!

Cane toads are still spreading throughout Australia.  Last year, I read a cool paper about how   there is some “enemy release” at the cane toad range edge, because lung worm parasite infections lag behind the invasion front by roughly two years.  The Ecology Letters papers used a common garden experiment to demonstrate selection for better parasite transmission at the range edge.  This year, their JAE paper explores the evolution of the toads; the rate of range expansion is increasing, and toads at the range edge are bigger and grow faster than non-edge toads.

Kelehear, C., G. P. Brown, and R. Shine. 2012. Rapid evolution of parasite life history traits on
an expanding range-edge. Ecology Letters 15: 329–337.

Brown, G.P., C. Kelehear, and R. Shine. 2013. The early toad gets the worm: cane toads at an invasion front benefit from higher prey availability. Journal of Animal Ecology.

Dilution Effect: a Meta-Analysis

Back to the dilution effect.  Does host biodiversity broadly reduce infection?  This paper says no, its more about community composition.

Daniel J Salkeld, D.J., K.A. Padgett, and J.H. Jones. 2013. A meta-analysis suggesting that the relationship between biodiversity and risk of zoonotic pathogen transmission is idiosyncratic.  Ecology Letters.


Paper 1:  I have to get some parasite manipulation stuff in here.  So, here’s a neat paper about parasites making brine shrimp aggregate more than if they were uninfected.  “EAT US ALL; WE’RE INFECTED.”

Paper 2:  Oh, there’s more.  Cestode-infected brine shrimp hang out at the surface of the water more than uninfected brine shrimp.  Infected hosts alter their resource use, too.  (Sadly, they don’t switch to eating brains.)  That altered resource use changes the isotopic signature of the host, which could in turn effect energy flow in food webs.  NEAT!

Nicolas O. Rode, N.O., E.J.P. Lievens, E. Flaven, A. Segard, R. Jabbour-Zahab, M.I. Sanchez, T. Lenormand. 2013. Why join groups? Lessons from parasite-manipulated Artemia. Ecology Letters 16(4): 493-501.

Sanchez, M.I., N. Varo, C. Matesanz, C. Ramo, J.A. Amat, and A.I. Green. 2013. Cestodes change the isotopic signature of brine shrimp, Artemia, hosts: Implications for aquatic food webs.  International Journal of Parasitology 43(1): 73-80.  (PDF LINK)

Which paper (or pair of papers) are you most interested in?

Why did the chicken cross the road? Because it was a PARASITE ZOMBIE.

This month, I’ve done a lot of posting about parasites turning hosts (like snails and insects) into “zombies.”  But I never addressed one very important question:  why did the chicken cross the road?  BECAUSE IT WAS A ZOMBIE.  (Thank you, Saturday Morning Breakfast Cereal, for making my life complete.)

Zombie Snails

Today is my one month blog anniversary!  Yay!  In honor of this occasion, I decided to post about something very fun:  ZOMBIE SNAILS.

Animals Animated Gif on Giphy

(GIF from giphy)

The parasite in question is the trematode Leucochloridium paradoxum, which is also called the green-banded broodsac.  The adult trematode lives in birds, and parasite eggs are shed in the birds’ feces.  Snails consume the eggs, and get infected with the intermediate parasite stages.  If you’re interested in trematode life cycles, you’ll find the details of the sporocyst to cercariae stages BIZARRE.  Check out this blog, if you’re interested.

Anyways, the parasite does two things to manipulate the snail.  First, it makes the snail hang out in the open, exposed to predation.  Second, it turns the snail’s antennae into psychedelic, pulsating, caterpillar-like beacons of yum for birds.  Birds nip off the antennae full of cercariae (infective parasite stages), get infected, and the life cycle continues.  Awwwwwesome.

You really need to watch this short video about zombie snails before you can consider your life complete.

So far, I haven’t gotten any comments on my blog.  My goal this month is to get some feedback from my wonderful audience.  To break the ice, I’m going to start with a poll.  So, how do you feel about zombie snails?

Zombie Ants

Instead of talking about a specific paper today, I’m going to talk about ZOMBIE ANTS <cue horror music>.  Parts of this will count as self-plagiarism, because I’ve blogged about this in other places.  Let the post cannibalism begin!

I attended the annual meeting of the American Society of Parasitologists last summer, and it was awesome.  David Hughes gave a really awesome talk called “Zombie Ants: The Precise Manipulation of Social Insect Behavior by a Fungal Parasite.”  I’m sure you’re totally hooked already, but just in case you’re not seeing how cool this is, let me reel you in with some photos.  (These aren’t ants, but you’ll see those later.)

Photo from here.

Photo from here.   For more cool fungus pics, just google “Cordyceps fungus.”

Parasites often manipulate their host’s behavior in order to increase the probability that they (the parasites) will be successfully transmitted to their next host.  In the case of the parasitic fungus of these ants, the fungus wants the ant to go hang out somewhere that will result in the fungus being able to rain spores down on other ants.  It does this by making the ants climb up to a leaf above an area of a high ant traffic (areas that Hughes calls “killing fields”). The ant is then “forced” to bite down on the main vein of that leaf, and then its mandibles get stuck that way, so that the ant is attached to the underside of the leaf forever.  Then the fungus sprouts out of the ant’s head and does its whole raining spores of death thing.

Since David Attenborough explains things better than I do, I’m going to link you to David Hughes’ website, where he has more info and some sexy videos (including Attenborough).

As a final note, Hughes is looking into using this parasitic fungus as a form of biocontrol for pest ants, like on farms.  In other words, YOU TOO could have your very own ZOMBIE ANTS.

How do you feel about using parasites as biocontrols?  That’s a huge can of worms, I know, but I’d like to hear your opinions.